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+Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Life-Story of Insects
+
+Author: Geo. H. Carpenter
+
+Release Date: August 1, 2005 [EBook #16410]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE LIFE-STORY OF INSECTS ***
+
+
+
+
+Produced by Justin Kerk, Laura Wisewell and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+                 The Cambridge Manuals of Science and
+                              Literature
+
+
+
+                      THE LIFE-STORY OF INSECTS
+
+
+
+                      CAMBRIDGE UNIVERSITY PRESS
+                      London: FETTER LANE, E.C.
+                          C.F. CLAY, MANAGER
+
+                            [Illustration]
+
+                    Edinburgh: 100, PRINCES STREET
+             London: H.K. LEWIS, 136, GOWER STREET, W.C.
+            WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
+                       Berlin: A. ASHER AND CO.
+                       Leipzig: F.A. BROCKHAUS
+                     New York: G.P. PUTNAM'S SONS
+             Bombay and Calcutta: MACMILLAN AND CO., LTD.
+
+
+
+
+[Illustration: _Frontispiece._ Transformation of a Gnat (_Culex_).
+    Magnified 5 times.
+A.  Larva. (The head is directed downwards and the tail-siphon with
+    spiracle points upwards to the surface of the water.)
+B.  Pupal Cuticle from which the Imago is emerging. (The pair of
+    'respiratory trumpets' on the thorax of the pupa are conspicuous. The
+    wings of the Imago are crumpled, and the hind feet are not yet
+    withdrawn.)
+C.  Adult Gnat. Female.]
+
+
+
+                            [Illustration]
+
+
+
+                            THE LIFE-STORY
+
+                              OF INSECTS
+
+
+
+                                  BY
+
+                          GEO. H. CARPENTER
+
+                  Professor of Zoology in the Royal
+                      College of Science, Dublin
+
+                              Cambridge:
+                       at the University Press
+                              New York:
+                          G.P. Putnam's Sons
+                                 1913
+
+
+                              Cambridge:
+                      PRINTED BY JOHN CLAY, M.A.
+                       AT THE UNIVERSITY PRESS
+
+
+     With the exception of the coat of arms at the foot, the design on
+     the title page is a reproduction of one used by the earliest known
+     Cambridge printer John Siberch 1521
+
+
+
+
+PREFACE
+
+The object of this little book is to afford an outline sketch of the
+facts and meaning of insect-transformations. Considerations of space
+forbid anything like an exhaustive treatment of so vast a subject, and
+some aspects of the question, the physiological for example, are almost
+neglected. Other books already published in this series, such as Dr
+Gordon Hewitt's _House-flies_ and Mr O H. Latter's _Bees and Wasps_, may
+be consulted with advantage for details of special insect life-stories.
+Recent researches have emphasised the practical importance to human
+society of entomological study, and insects will always be a source of
+delight to the lover of nature. This humble volume will best serve its
+object if its reading should lead fresh observers to the brookside and
+the woodland.
+
+G.H.C.
+
+DUBLIN,
+
+_July_, 1913.
+
+
+
+
+CONTENTS
+
+CHAP.                                               PAGE
+
+   I.  Introduction                                     1
+
+  II.  Growth and Change                                8
+
+ III.  The Life-stories of some Sucking Insects        16
+
+  IV.  From Water to Air                               23
+
+   V.  Transformations, Outward and Inward             35
+
+  VI.  Larvae and their Adaptations                    49
+
+ VII.  Pupae and their Modifications                   79
+
+VIII.  The Life-story and the Seasons                  89
+
+  IX.  Past and Present--the Meaning of the Story     105
+
+       Outline Classification of Insects              122
+
+       Table of Geological Systems                    123
+
+       Bibliography                                   124
+
+       Index                                          129
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+Stages in the Transformations of a Gnat    _Frontispiece_
+
+FIG                                                  PAGE
+ 1. Stages of the Diamond-back Moth (_Plutella          3
+    cruciferarum_)
+
+ 2. Head of typical Moth                                5
+
+ 3. Head of Caterpillar                                 5
+
+ 4. Common Cockroach (_Blatta orientalis_)             12
+
+ 5. Nymph of Locust (_Schistocera americana_)          13
+
+ 6. _Aphis pomi_, winged and wingless females          19
+
+ 7. Mussel Scale-Insect (_Mytilaspis pomorum_)         21
+
+ 8. Emergence of Dragon-fly (_Aeschna cyanea_)      29-31
+
+ 9. Nymph of May-fly (_Chloeon dipterum_)              33
+
+10. Imaginal buds of Butterfly                         39
+
+11. Imaginal buds of Blow-fly                          43
+
+12. Carrion Beetle (_Silpha_) and larva                51
+
+13. Larva of Ground-beetle (_Aepus_)                   52
+
+14. Willow-beetle (_Phyllodecta_) and larva            53
+
+15. Cabbage-beetle (_Psylliodes_) and larva            54
+
+16. Corn Weevil (_Calandra_) and larva                 55
+
+17. Ruby Tiger Moth (_Phragmatobia fuliginosa_)        61
+
+18. Larvae and Pupa of Hive-bee (_Apis mellifica_)     65
+
+19. Larva of Gall-midge (_Contarinia nasturtii_)       68
+
+20. Crane-fly (_Tipula oleracea_) and larva            69
+
+21. Maggot of House-fly (_Musca domestica_)            71
+
+22. Ox Warble-fly (_Hypoderma bovis_) with egg,
+    larva, and puparium                                75
+
+23. Pupa of White Butterfly (_Pieris_)                 85
+
+
+
+
+CHAPTER I
+
+INTRODUCTION
+
+
+Among the manifold operations of living creatures few have more strongly
+impressed the casual observer or more deeply interested the thoughtful
+student than the transformations of insects. The schoolboy watches the
+tiny green caterpillars hatched from eggs laid on a cabbage leaf by the
+common white butterfly, or maybe rears successfully a batch of silkworms
+through the changes and chances of their lives, while the naturalist
+questions yet again the 'how' and 'why' of these common though wondrous
+life-stories, as he seeks to trace their course more fully than his
+predecessors knew.
+
+[Illustration: Fig. 1. _a_, Diamond-back Moth (_Plutella
+cruciferarum_); _b_, young caterpillar, dorsal view; _c_, full-grown
+caterpillar, dorsal view; _d_, side view; _e_, pupa, ventral view.
+Magnified 6 times. From _Journ. Dept. Agric. Ireland_, vol. I.]
+
+Everyone is familiar with the main facts of such a life-story as that of
+a moth or butterfly. The form of the adult insect (fig. 1 _a_) is
+dominated by the wings--two pairs of scaly wings, carried respectively
+on the middle and hindmost of the three segments that make up the
+_thorax_ or central region of the insect's body. Each of these three
+segments carries a pair of legs. In front of the thorax is the head on
+which the pair of long jointed feelers and the pair of large,
+sub-globular, compound eyes are the most prominent features. Below the
+head, however, may be seen, now coiled up like a watch-spring, now
+stretched out to draw the nectar from some scented blossom, the
+butterfly's sucking trunk or proboscis, situated between a pair of short
+hairy limbs or palps (fig. 2). These palps belong to the appendages of
+the hindmost segment of the head, appendages which in insects are
+modified to form a hind-lip or _labium_, bounding the mouth cavity below
+or behind. The proboscis is made up of the pair of jaw-appendages in
+front of the labium, the _maxillae_, as they are called. Behind the
+thorax is situated the _abdomen,_ made up of nine or ten recognisable
+segments, none of which carry limbs comparable to the walking legs, or
+to the jaws which are the modified limbs of the head-segments. The whole
+cuticle or outer covering of the body, formed (as is usual in the group
+of animals to which insects belong) of a horny (chitinous) secretion of
+the skin, is firm and hard, and densely covered with hairy or scaly
+outgrowths. Along the sides of the insect are a series of paired
+openings or spiracles, leading to a set of air-tubes which ramify
+throughout the body and carry oxygen directly to the tissues.
+
+[Illustration: Fig. 2. A. Head of a typical Moth, showing proboscis
+formed by flexible maxillae (_g_) between the labial palps (_p_); _c_,
+face; _e_, eye; the structure _m_ has been regarded as the vestige of a
+mandible. B. Basal part (_b_) of maxilla removed from head, with
+vestigial palp (_p_). Magnified.]
+
+Such a butterfly as we have briefly sketched lays an egg on the leaf of
+some suitable food-plant, and there is hatched from it the well-known
+crawling larva[1] (fig. 1 _b, c, d_) called a caterpillar, offering in
+many superficial features a marked contrast to its parent. Except on the
+head, whose surface is hard and firm, the caterpillar's cuticle is as a
+rule thin and flexible, though it may carry a protective armature of
+closely set hairs, or strong sharp spines. The feelers (fig. 3 _At_) are
+very short and the eyes are small and simple. In connection with the
+mouth, there are present in front of the maxillae a pair of _mandibles_
+(fig. 3 _Mn_), strong jaws, adapted for biting solid food, which are
+absent from the adult butterfly, though well developed in cockroaches,
+dragon-flies, beetles, and many other insects. The three pairs of legs
+on the segments of the thorax are relatively short, and as many as five
+segments of the abdomen may carry short cylindrical limbs or pro-legs,
+which assist the clinging habits and worm-like locomotion of the
+caterpillar. No trace of wings is visible externally. The caterpillar,
+therefore, differs markedly from its parent in its outward structure, in
+its mode of progression, and in its manner of feeding; for while the
+butterfly sucks nectar or other liquid food, the caterpillar bites up
+and devours solid vegetable substances, such as the leaves of herbs or
+trees. It is well-known that between the close of its larval life and
+its attainment of perfection as a butterfly, the insect spends a
+period as a _pupa_ (fig. 1 _e_) unable to move from place to place, and
+taking no food.
+
+[1] The term _larva_ is applied to any young animal which differs
+markedly from its parent.
+
+[Illustration: Fig. 3. Head of Caterpillar of Goat-moth (_Cossus_) seen
+from behind. _At_, feeler; _Mn_, mandible; _Mx_, maxilla; _Lm_, labium,
+spinneret projecting beyond it. Magnified. After Lyonet from Miall and
+Denny's _Cockroach_.]
+
+Such, in brief, is the course of the most familiar of insect
+life-stories. For the student of the animal world as a whole, this
+familiar transformation raises some startling problems, which have been
+suggestively treated by F. Brauer (1869), L.C. Miall (1895), J. Lubbock
+(1874), R. Heymons (1907), P. Deegener (1909) and other writers[2]. To
+appreciate these problems is the first step towards learning the true
+meaning of the transformation.
+
+[2] The dates in brackets after authors' names will facilitate reference
+to the Bibliography (pp. 124-8).
+
+The butterfly's egg is absolutely and relatively of large size, and
+contains a considerable amount of yolk. As a rule we find that young
+animals hatched from such eggs resemble their parents rather closely and
+pass through no marked changes during their lives. A chicken, a
+crocodile, a dogfish, a cuttlefish, and a spider afford well-known
+examples of this rule. Land-animals, generally, produce young which are
+miniature copies of themselves, for example horses, dogs, and other
+mammals, snails and slugs, scorpions and earthworms. On the other hand,
+metamorphosis among animals is associated with eggs of small size, with
+aquatic habit, and with relatively low zoological rank. The young of a
+starfish, for example, has hardly a character in common with its parent,
+while a marine segmented worm and an oyster, unlike enough when adult,
+develop from closely similar larval forms. If we take a class of
+animals, the Crustacea, nearly allied to insects, we find that its more
+lowly members, such as 'water-fleas' and barnacles, pass through far
+more striking changes than its higher groups, such as lobsters and
+woodlice. But among the Insects, a class of predominantly terrestrial
+and aerial creatures producing large eggs, the highest groups undergo,
+as we shall see, the most profound changes. The life-story of the
+butterfly, then, well-known as it may be, furnishes a puzzling exception
+to some wide-reaching generalisations concerning animal development. And
+the student of science often finds that an exception to some rule is the
+key to a problem of the highest interest.
+
+During many centuries naturalists have bent their energies to explain
+the difficulties presented by insect transformations. Aristotle, the
+first serious student of organised beings whose writings have been
+preserved for us, and William Harvey, the famous demonstrator of the
+mammalian blood circulation two thousand years later, agreed in
+regarding the pupa as a second egg. The egg laid by a butterfly had not,
+according to Harvey, enough store of food to provide for the building-up
+of a complex organism like the parent; only the imperfect larva could be
+produced from it. The larva was regarded as feeding voraciously for the
+purpose of acquiring a large store of nutritive material, after which it
+was believed to revert to the state of a second but far larger egg, the
+pupa, from which the winged insect could take origin. Others again,
+following de R�aumur (1734), have speculated whether the development of
+pupa within larva, and of winged insect within pupa might not be
+explained as abnormal births. But a comparison of the transformation of
+butterflies with simpler insect life-stories will convince the enquirer
+that no such heroic theories as these are necessary. It will be realised
+that even the most profound transformation among insects can be
+explained as a special case of growth.
+
+
+
+
+CHAPTER II
+
+GROWTH AND CHANGE
+
+
+The caterpillar differs markedly from the butterfly. As we pursue our
+studies of insect growth and transformation we shall find that in some
+cases the difference between young and adult is much greater--as for
+example between the maggot and the house-fly, in others far less--as
+between the young and full-grown grasshopper or plant-bug. It is
+evidently wise to begin a general survey of the subject with some of
+those simpler cases in which the differences between the young and
+adult insect are comparatively slight. We shall then be in a position to
+understand better the meaning of the more puzzling and complex cases in
+which the differences between the stages are profound.
+
+In the first place it is necessary to realise that the changes which any
+insect passes through during its life-story are essentially
+accompaniments of its growth. The limits of this little book allow only
+slight reference to features of internal structure; we must be content,
+in the main, to deal with the outward form. But there is an important
+relation between this outward form and the underlying living tissues
+which must be clearly understood. Throughout the great race of
+animals--the Arthropoda--of which insects form a class, the body is
+covered outwardly by a _cuticle_ or secretion of the underlying layer of
+living cells which form the outer skin or _epidermis_[3] (see fig. 10
+_ep_, _cu_, p. 39). This cuticle has regions which are hard and firm,
+forming an _exoskeleton_, and, between these, areas which are relatively
+soft and flexible. The firm regions are commonly segmental in their
+arrangement, and the intervening flexible connections render possible
+accurate motions of the exoskeletal parts in relation to each other,
+the motions being due to the contraction of muscles which are attached
+within the exoskeleton.
+
+[3] The term 'hypodermis' frequently applied to this layer is
+misleading. The layer is the true outer skin--ectoderm or epidermis.
+
+Now this jointed exoskeleton--an admirably formed suit of armour though
+it often is--has one drawback: it is not part of the insect's living
+tissues. It is a cuticle formed by the solidifying of a fluid secreted
+by the epidermal cells, therefore without life, without the power of
+growth, and with only a limited capacity for stretching. It follows,
+therefore, that at least during the period through which the insect
+continues to grow, the cuticle must be periodically shed. Thus in the
+life-story of an insect or other arthropod, such as a lobster, a spider,
+or a centipede, there must be a succession of cuticle-castings--'moults'
+or _ecdyses_ as they are often called.
+
+When such a moult is about to take place the cuticle separates from the
+underlying epidermis, and a fluid collects beneath. A delicate new
+cuticle (see fig. 10 _cu'_) is then formed in contact with the
+epidermis, and the old cuticle opens, usually with a slit lengthwise
+along the back, to allow the insect in its new coat to emerge. At first
+this new coat is thin and flabby, but after a period of exposure to the
+air it hardens and darkens, becoming a worthy and larger successor to
+that which has been cast. The cuticle moreover is by no means wholly
+external. The greater part of the digestive canal and the whole
+air-tube system are formed by inpushings of the outer skin (ectoderm)
+and are consequently lined with an extension of the chitinous cuticle
+which is shed and renewed at every moult.
+
+In all insects these successive moults tend to be associated with change
+of form, sometimes slight, sometimes very great. The new cuticle is
+rarely an exact reproduction of the old one, it exhibits some new
+features, which are often indications of the insect's approach towards
+maturity. Even in some of those interesting and primitive insects the
+Bristle-tails (Thysanura) and Spring-tails (Collembola), in which wings
+are never developed, perceptible differences in the form and arrangement
+of the abdominal limbs can be traced through the successive stages, as
+R. Heymons (1906) and K.W. Verhoeff (1911) have shown for Machilis. But
+the changes undergone by such insects are comparatively so slight, that
+the creatures are often known as 'Ametabola' or insects without
+transformation in the life-history. Now there are a considerable number
+of winged insects--cockroaches and grasshoppers for example--in which
+the observable changes are also comparatively slight. We will sketch
+briefly the main features of the life-story of such an insect.
+
+[Illustration: Fig. 4. Common Cockroach (_Blatta orientalis_). _a_,
+female; _b_, male; _c_, side view of female; _d_, young. After Marlatt,
+_Entom. Bull._ 4, _U.S. Dept. Agric._]
+
+The young creature is hatched from the egg in a form closely resembling,
+on the whole, that of its parent, so that the term 'miniature adult'
+sometimes applied to it, is not inappropriate. The baby cockroach (fig.
+4 _d_) is known by its flattened body, rounded prothorax, and stiff,
+jointed tail-feelers or cercopods; the baby grasshopper by its strong,
+elongate hind-legs, adapted, like those of the adult, for vigorous
+leaping. During the growth of the insect to the adult state there may be
+four or five moults, each preceded and succeeded by a characteristic
+instar[4]. The first instar differs, however, from the adult in one
+conspicuous and noteworthy feature, it possesses no trace of wings. But
+after the first or the second moult, definite wing-rudiments are visible
+in the form of outgrowths on the corners of the second and third
+thoracic segments. In each succeeding instar these rudiments become more
+prominent, and in the fourth or the fifth stage, they show a branching
+arrangement of air-tubes, prefiguring the nervures of the adult's wing
+(fig. 5). After the last moult the wings are exposed, articulated to the
+segments that bear them, and capable of motion. Having been formed
+beneath the cuticle of the wing-rudiments of the penultimate instar, the
+wings are necessarily abbreviated and crumpled. But during the process
+of hardening of the cuticle, they rapidly increase in size, blood and
+air being forced through the nervures, so that the wings attaining their
+full expanse and firmness, become suited for the function of flight.
+
+[4] The convenient term 'instar' has been proposed by Fischer and
+advocated by Sharp (1895) for the form assumed by an insect during a
+stage of its life-story. Thus the creature as hatched from the egg is
+the _first instar_, after the first moult it has become the _second
+instar_, and so on, the number of moults being always one less than the
+number of instars.
+
+[Illustration: Fig. 5. Nymph of Locust (_Schistocera americana_) with
+distinct wing-rudiments. After Howard, _Insect Life_, vol. VII.]
+
+The changes through which these insects pass are therefore largely
+connected with the development of the wings. It is noteworthy that in an
+immature cockroach the entire dorsal cuticle is hard and firm. In the
+adult, however, while the cuticle of the prothorax remains firm, that of
+the two hinder thoracic and of all the abdominal segments is somewhat
+thin and delicate on the dorsal aspect. It needs not now to be
+resistant, because it is covered by the two firm forewings, which shield
+and protect it, except when the insect is flying. There are, indeed,
+slight changes in other structures not directly connected with the
+wings. In a young grasshopper, for example, the feelers are relatively
+stouter than in the adult, and the prothorax does not show the
+specifically distinctive shape with its definite keels and furrows.
+Changes in the secondary sexual characters may also be noticed. For
+instance, in an immature cockroach both male and female carry a pair of
+jointed tail-feelers or cercopods on the tenth abdominal segment, and a
+pair of unjointed limbs or stylets on the ninth. In the adult stage,
+both sexes possess cercopods, but the males only have stylets, those of
+the female disappearing at the final moult.
+
+Reviewing the main features of the life-story of a grasshopper or
+cockroach, we notice that there is no marked or sudden change of form.
+The newly-hatched insect resembles generally its parent, except that it
+has no wings. Wing-rudiments appear, however, in an early instar as
+visible outgrowths on the thoracic segments, and become larger after
+each moult. All through its various stages the immature insect--_nymph_
+as it is called--lives in the same kind of situations and on the same
+kind of food as its parent, and it is all along active and lively,
+undergoing no resting period like the pupal stage in the transformation
+of the butterfly.
+
+One interesting and suggestive fact remains to be mentioned. There are
+grasshoppers and cockroaches in which the changes are even less than
+those just sketched, because the wings remain, even in the adult, in a
+rudimentary state (as for example in the female of the common kitchen
+cockroach, _Blatta orientalis_, see fig. 4 _a_), or are never developed
+at all. Such exceptional winglessness in members of a winged family can
+only be explained by the recognition of a life-story, not merely in the
+individual but in the race. We cannot doubt that the ancestors of these
+wingless insects possessed wings, which in the course of time have been
+lost by the whole species or by the members of the female sex. It is
+generally assumed that this loss has been gradual, and so in many cases
+it probably may have been. But there are species of insects in which
+some generations are winged and others wingless; a winged mother gives
+birth to wingless offspring, and a wingless parent to young with
+well-developed wings. Such discontinuity in the life-story of a single
+generation forces us to recognise the possibility of similar sudden
+mutations in the course of that age-long process of evolution to which
+the facts of insect growth, and indeed of all animal development, bear
+striking testimony.
+
+
+
+
+CHAPTER III
+
+THE LIFE-STORIES OF SOME SUCKING INSECTS
+
+
+We may now turn our attention to some examples of the remarkable
+alternation of winged and wingless generations in the yearly life-cycle
+of the same species, mentioned at the end of the last chapter.
+Cockroaches and grasshoppers belong to an order of insects, the
+Orthoptera[5], characterised by firm forewings and biting jaws; in all
+of them the change of form during the life-history is comparatively
+slight. A great contrast to those insects in the structure of the
+mouth-parts is presented by the Hemiptera, an order including the bugs,
+pond-skaters, cicads, plant-lice, and scale-insects. These all have an
+elongated, grooved labium projecting from the head in form of a beak,
+within which work, to and fro, the slender needle-like mandibles and
+maxillae by means of which the insect pierces holes through the skin of
+a leaf or an animal, and is thus enabled to suck a meal of sap or blood,
+according to its mode of life. In many Hemiptera--the various families
+of bugs both aquatic and terrestrial, for example--the life-history is
+nearly as simple as that of a cockroach. It is the family of the
+plant-lice (Aphidae) that affords typical illustrations of that
+alternation of generations to which reference has been made.
+
+[5] See outline classification of insects, p. 122.
+
+The yearly cycle of the common Aphids of the apple tree has been lately
+worked out in detail by J.B. Smith (1900) and E.D. Sanderson (1902). In
+late autumn tiny wingless males and females are found in large numbers
+on the withered leaves. The sexes pair together, and the females lay
+their relatively large, smooth, hard-coated black eggs on the twigs;
+these resistant eggs carry the species safely over the winter. At
+springtide, when the leaves begin to sprout from the opening buds the
+aphid eggs are hatched, and the young insects after a series of moults,
+through which hardly any change of form is apparent, all grow into
+wingless 'stem-mothers' much larger than the egg-laying females of the
+autumn. The stem-mothers have the power, unusual among animals as a
+whole, but not very infrequent in the insects and their allies, of
+reproducing their kind without having paired[6] with a male. Eggs
+capable of parthenogenetic development, produced in large numbers in the
+ovaries of these females, give rise to young which, developing within
+the body of the mother, are born in an active state. Successive broods
+of these wingless virgin females (fig. 6 _a_) appear through the spring
+and summer months, and as the rate of their development is rapid, often
+the whole life-story is completed within a week. The aphid population
+increases very fast. Later a generation appears in which the thoracic
+segments of the nymphs are seen to bear wing-rudiments like those of the
+young cockroach, and a host of winged females (fig. 6_b_) are produced;
+these have the power of migrating to other plants. We understand that
+wings are not necessary to the earlier broods whose members have plenty
+of room and food on their native shoots, but that when the population
+becomes crowded, a winged brood capable of emigration is advantageous to
+the race.
+
+[6] Such virgin reproduction is termed 'parthenogenesis.'
+
+Many generations of virgin female aphids, some wingless, others winged
+when adult, succeed each other through the summer months. At the close
+of the year the latest brood of these bring forth young, which develop
+into males and egg-laying females; thus the yearly cycle is completed.
+Variations in points of detail may be noticed in different species of
+aphids. The autumn males and egg-laying females are, for example,
+frequently winged, and the same species may have constantly recurring
+generations of different forms adapted for different food-plants, or for
+different regions of the same food-plant. But taking a general view of
+the life-story of aphids for comparison with the life-story of other
+insects, three points are especially noteworthy. Virgin reproduction
+recurs regularly, parthenogenetic broods being succeeded by a single
+sexual brood. A winged parent brings forth young which remain always
+wingless, and wingless adults produce young which acquire wings. The
+wings are developed, as in the cockroach, from outward and visible
+wing-rudiments.
+
+[Illustration: Fig. 6. Apple Aphid (_Aphis pomi_), virgin females, _a_,
+wingless; _b_, winged. Magnified 20 times.]
+
+A family of Hemiptera, related to the Aphidae and equally obnoxious to
+the gardener, is that of the Coccidae or scale-insects. These furnish an
+excellent illustration of features noticeable in certain insect
+life-histories. In the first place, the newly-hatched young differs
+markedly from the parent in the details of its structure. A young coccid
+(fig. 7 _c_) is flattened oval in shape, has well-developed feelers
+(fig. 7 _d_) and legs, and runs actively about, usually on the leaves or
+bark of trees and shrubs, through which it pierces with its long jaws,
+so that it may suck sap from the soft tissues beneath. After a time it
+fixes itself by means of these jaws and the characteristic scale or
+protective covering, composed partly of a waxy secretion and partly of
+dried excrement, begins to grow over its body. The female loses legs and
+feelers, and never acquires wings, becoming little more than a sluggish
+egg-bag (fig. 7 _e_). The male on the other hand passes into a second
+larval stage in which there are no functional legs, but rudiments of
+legs and of wings are present on the epidermis beneath the cuticle, as
+shown by B.O. Schmidt for Aspidiotus (1885). The penultimate instar of
+this sex in which the wing-rudiments are visible externally lies
+passively beneath the scale, its behaviour resembling that of a
+butterfly pupa. The adult winged male (fig. 7 _a_) leads a short, but
+active life.
+
+[Illustration: Fig. 7. Mussel Scale-insect (_Mytilaspis pomorum_). _a_,
+male; _b_, foot of male; _c_, larva, ventral view; _d_, feeler of larva;
+_e_, female, ventral view. After Howard, _Yearbook U.S. Dept. Agric._
+1904. Magnified, _a, c, e_ x 20; _b, d_ x 120.]
+
+Another family allied to the Aphidae is that of the Cicads, hardly
+represented in our fauna but abundant in many of the warmer regions of
+the earth. Here also the young insect differs widely from its parent in
+form, living underground and being provided with strong fore-legs for
+digging in the soil. After a long subterranean existence, usually
+extending over several years, the insect attains the penultimate stage
+of its life-story, during which it rests passively within an earthen
+cell, awaiting the final moult, which will usher in its winged and
+perfect state.
+
+In the life-histories of cicads and coccids, then, there are some
+features which recall those of the caterpillar's transformation into the
+butterfly. The newly-hatched insect is externally so unlike its parent
+that it may be styled a larva. The penultimate instar is quiescent and
+does not feed. But while the caterpillar shows throughout its life no
+outward trace of wings, external wing-rudiments are evident in the young
+stages of the cicad. In the male coccid we find a late larval stage with
+hidden wing-rudiments, the importance of which, for comparison with the
+caterpillar, will be appreciated later.
+
+
+
+
+CHAPTER IV
+
+FROM WATER TO AIR
+
+
+Insects as a whole are preeminently creatures of the land and the air.
+This is shown not only by the possession of wings by a vast majority of
+the class, but by the mode of breathing to which reference has already
+been made (p. 2), a system of branching air-tubes carrying atmospheric
+air with its combustion-supporting oxygen to all the insect's tissues.
+The air gains access to these tubes through a number of paired air-holes
+or spiracles, arranged segmentally in series.
+
+It is of great interest to find that, nevertheless, a number of insects
+spend much of their time under water. This is true of not a few in the
+perfect winged state, as for example aquatic beetles and water-bugs
+('boatmen' and 'scorpions') which have some way of protecting their
+spiracles when submerged, and, possessing usually the power of flight,
+can pass on occasion from pond or stream to upper air. But it is
+advisable in connection with our present subject to dwell especially on
+some insects that remain continually under water till they are ready to
+undergo their final moult and attain the winged state, which they pass
+entirely in the air. The preparatory instars of such insects are
+aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
+caddis-flies, many beetles and two-winged flies, and a few moths thus
+divide their life-story between the water and the air. For the present
+we confine attention to the Stone-flies, the May-flies, and the
+Dragon-flies, three well-known orders of insects respectively called by
+systematists the Plecoptera, the Ephemeroptera and the Odonata.
+
+In the case of many insects that have aquatic larvae, the latter are
+provided with some arrangement for enabling them to reach atmospheric
+air through the surface-film of the water. But the larva of a stone-fly,
+a dragon-fly, or a may-fly is adapted more completely than these for
+aquatic life; it can, by means of gills of some kind, breathe the air
+dissolved in water.
+
+The aquatic young of a stone-fly does not differ sufficiently in form
+from its parent to warrant us in calling it a larva; the life-history is
+like that of a cockroach, all the instars however except the final
+one--the winged adult or _imago_--live in the water. The young of one of
+our large species, a Perla for example, has well-chitinised cuticle,
+broad head, powerful legs, long feelers and cerci like those of the
+imago; its wings arise from external rudiments, which are conspicuous in
+the later aquatic stages. But it lives completely submerged, usually
+clinging or walking beneath the stones that lie in the bed of a clear
+stream, and examination of the ventral aspect of the thorax reveals six
+pairs of tufted gills, by means of which it is able to breathe the air
+dissolved in the water wherein it lives. At the base of the tail-feelers
+or cerci also, there are little tufts of thread-like gills as J.A.
+Palm�n (1877) has shown. An insect that is continually submerged and has
+no contact with the upper air cannot breathe through a series of paired
+spiracles, and during the aquatic life-period of the stone-fly these
+remain closed. Nevertheless, breathing is carried on by means of the
+ordinary system of branching air-tubes, the trunks of which are in
+connection with the tufted hollow gill-filaments, through whose delicate
+cuticle gaseous exchange can take place, though the method of this
+exchange is as yet very imperfectly understood. When the stone-fly nymph
+is fully grown, it comes out of the water and climbs to some convenient
+eminence. The cuticle splits open along the back, and the imago, clothed
+in its new cuticle, as yet soft and flexible, creeps out. The spiracles
+are now open, and the stone-fly breathes atmospheric air like other
+flying insects. But throughout its winged life, the stone-fly bears
+memorials of its aquatic past in the little withered vestiges of gills
+that can still be distinguished beneath the thorax.
+
+The adult dragon-fly (fig. 8 _d_) is specialised in such a way that it
+captures its prey--flies and other small insects--on the wing, swooping
+through the air like a hawk and feeding voraciously. The head is
+remarkable for its large globular compound eyes, its short bristle-like
+feelers, and its very strong mandibles which bite up the bodies of the
+victims. The thorax bears the two pairs of ample wings, firm and almost
+glassy in texture, and its segments are projected forward ventrally, so
+that all six legs, which are armed with rows of sharp, slender spines,
+can be held in front of the mouth, where they form an effective
+fly-trap. The abdomen is very long and usually narrow.
+
+A female dragon-fly after a remarkable mode of pairing, the details of
+which are beside our present subject, drops her eggs in the water, or
+lays them on water-weeds, perhaps cutting an incision where they can be
+the more safely lodged, or even goes down below the surface and deposits
+them in the mud at the bottom of a pond. From the eggs are hatched the
+aquatic larvae which differ in many respects from the imago. The
+dragon-fly larva has the same predaceous mode of life as its parent, but
+it is sluggish in habit, lurking for its prey at the bottom of the pond,
+among the mud or vegetation, which it resembles in colour. The thoracic
+segments have not the specialisation that they show in the imago; the
+abdomen is relatively shorter and broader. The larval head has, like
+that of the imago, short feelers, and the eyes are somewhat large,
+though far from attaining the size of the great globular eyes of the
+dragon-fly. But the third pair of jaws, forming the labium, are most
+remarkably modified into a 'mask,' the distal central portion (mentum)
+being hinged to the basal piece (sub-mentum) which is itself jointed
+below the head. The mentum carries at its extremity a pair of lobes with
+sharp fangs. Thus the mask can be folded under the head when the larva
+lurks in its hiding place, or be suddenly darted out so as to secure any
+unwary small insect that may pass close enough for capture. Dragon-fly
+larvae walk, and also swim by movements of the abdomen or by expelling a
+jet of water from the hind-gut. The walls of this terminal region of the
+intestine have areas lined with delicate cuticle and traversed by
+numerous air-tubes, so that gaseous exchange can take place between the
+air in the tubes and that dissolved in the water. The larvae of the
+larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
+mostly in this way. Those of the slender and delicate 'Demoiselles'
+(Agrionidae) are provided with three leaf-like gill-plates at the tail,
+between whose delicate surfaces numerous air-tubes ramify. These
+gill-plates are at times used for propulsion. Thus air supply is ensured
+during aquatic life. But occasionally, when the water in which the
+larva lives is foul and poor in oxygen, the tail is thrust out of the
+water so that air can be admitted directly into the intestinal chamber.
+The aquatic life of these insects lasts for more than a year, and F.
+Balfour-Browne (1909) has observed from ten to fourteen moults in
+Agrion. Outward wing-rudiments are early visible on the thoracic
+segments; when these have become conspicuous the insect, beginning in
+some respects to approach the adult condition, is often called a nymph.
+In an advanced dragon-fly nymph, H. Dewitz (1891) has shown that the
+thoracic spiracles are open, and, as the time for its final moult draws
+near, the insect may thrust the front part of its body out of the water,
+and breathe atmospheric air through these. Thus before the great change
+takes place the nymph has foretastes of the aerial mode of breathing
+which it will practise when the perfect stage shall have been attained.
+The emergence of the dragon-fly from its nymph-cuticle has been
+described by many naturalists from de R�aumur (1740) to L.C. Miall
+(1895) and O.H. Latter (1904). The nymph climbs out of the water by
+ascending some aquatic plant, and awaits the change so graphically
+sketched by Tennyson:
+
+    A hidden impulse rent the veil,
+    Of his old husk, from head to tail,
+    Came out clear plates of sapphire mail.
+
+'From head to tail,' for the nymph-cuticle splits lengthwise down the
+back, and the head and thorax of the imago are freed from it (fig. 8
+_a_), then the legs clasp the empty cuticle, and the abdomen is drawn
+out (fig. 8 _b, c_). After a short rest, the newly-emerged fly climbs
+yet higher up the water-weed, and remains for some hours with the
+abdomen bent concave dorsalwards (fig. 8 _d_), to allow space for the
+expansion and hardening of the wings. For some days after emergence the
+cuticle of the dragon-fly has a dull pale hue, as compared with the dark
+or brightly metallic aspect that characterises it when fully mature. The
+life of the imago endures but a short time compared with the long
+aquatic larval and nymphal stages. After some weeks, or at most a few
+months, the dragon-flies, having paired and laid their eggs, die before
+the approach of winter.
+
+[Illustration: Fig. 8 _a, b_. Dragon-fly (_Aeschna cyanea_). Two stages
+in emergence of fly from nymph-cuticle. From Latter's _Natural
+History_.]
+
+[Illustration: Fig. 8 _c_. Dragon-fly emerged, wings
+expanding. From Latter's _Natural History_.]
+
+[Illustration: Fig. 8 _d_. Dragon-fly (_Aeschna cyanea_) with
+expanded wings.]
+
+The life-story of a may-fly follows the same general course as that just
+described for the dragon-flies, but there are some suggestive
+differences. In the first place, we notice a wider divergence between
+the imago and the larva. An adult may-fly is one of the most delicate
+of insects; the head has elaborate compound eyes, but the feelers are
+very short, and the jaws are reduced to such tiny vestiges that the
+insect is unable to feed. Its aquatic larva is fairly robust, with a
+large head which is provided with well-developed jaws, as the larval and
+nymphal stages extend over one or two years, and the insects browse on
+water-weeds or devour creatures smaller and weaker than themselves. They
+breathe dissolved air by means of thread-like or plate-like gills
+traversed by branching air-tubes, somewhat resembling those of the
+demoiselle dragon-fly larva. But in the may-fly larva, there is a series
+of these gills (fig. 9_b_) arranged laterally in pairs on the abdominal
+segments, and C. B�rner (1909) has recently given reasons, from the
+position and muscular attachments of these organs, for believing that
+they show a true correspondence to (in technical phraseology are
+homologous with) the thoracic legs. One feature in which the larva often
+agrees with the imago is the possession on the terminal abdominal
+segment of a pair of long jointed cerci, and in many genera a median
+jointed tail-process (see fig. 9) is also present, in some cases both in
+the larva and the imago, in others in the larva during its later stages
+only. The prolonged larval life in may-flies often involves a large
+series of moults; Lubbock (1863) has enumerated twenty-one in the
+life-history of Chloeon. In the second year of aquatic life
+wing-rudiments (fig. 9 _a_) are visible, and the larva becomes a nymph.
+When the time for the winged condition approaches the nymphs leave the
+water in large swarms. The vivid accounts of these swarms given by
+Swammerdam (1675), de R�aumur (1742) and other old-time observers are
+available in summarised form for English readers in Miall's admirable
+book (1895). May-flies are eagerly sought as food by trout, and the rise
+of the fly on many lakes ushers in a welcome season to the angler.
+
+The nymph-cuticle opens and the winged insect emerges. But this is not
+the final instar; may-flies are exceptional among insects in undergoing
+yet another moult after they have acquired wings which they can use for
+flight. The instar that emerges from the nymph-cuticle is a sub-imago,
+dull in hue, with a curious immature aspect about it. A few hours later
+the final moult takes place, a very delicate cuticle being shed and
+revealing the true imago. Then follow the dancing flight over the calm
+waters, the mating and egg-laying, the rapid death. The whole winged
+existence prepared for by the long aquatic life may be over in a single
+evening; at most it lasts but for a few days.
+
+[Illustration: Fig. 9. Nymph of May-fly (_Chloeon dipterum_) showing on
+right side wing-rudiment (_a_), on left tracheal gills (_b_). Magnified
+4 times. [Feelers and legs are cut short.] From Miall and Denny after
+Vayssi�re.]
+
+In the development of the may-flies, then, we notice not only a
+considerable divergence between larva and imago, both in habitat and
+structure; we see also what is to be observed often in more highly
+organised insects--a feeding stage prolonged through the years of larval
+and nymphal life, while the winged imago takes no food and devotes its
+energies through its short existence to the task of reproduction. Such
+division of the life-history into a long feeding, and a short breeding
+period has, as will be seen later, an important bearing on the question
+of insect transformation generally, and the dragon-flies and may-flies
+afford examples of two stages in its specialisation. The sub-imaginal
+instar of the may-fly furnishes also a noteworthy fact for comparison
+with other insect histories. In two points, however, the life-story of
+these flies with their aquatic larvae recalls that of the cockroach. All
+the larval and nymphal instars are active, and the wing-rudiments are
+outwardly visible long before the final moult.
+
+
+
+
+CHAPTER V
+
+TRANSFORMATIONS,--OUTWARD AND INWARD
+
+
+We are now in a position to study in some detail the transformation of
+those insects whose life-story corresponds more or less closely with
+that of the butterfly, sketched in the opening pages of this little
+book. In the case of some of the insects reviewed in the last three
+chapters, the may-flies and cicads for example, a marked difference
+between the larva and the imago has been noticed; in others, as the
+coccids, we find a resting instar before the winged condition is
+assumed, suggesting the pupal stage in the butterfly's life-story.
+
+The various insect orders whose members exhibit no marked divergence
+between larva and imago (the Orthoptera for example) are often said to
+undergo no transformation, to be 'Ametabola.' Those with life-stories
+such as the dragon-flies' are said to undergo partial transformation,
+and are termed 'Hemimetabola.' Moths, caddis-flies, beetles, two-winged
+flies, saw-flies, ants, wasps, bees, and the great majority of insects,
+having the same type of life-story as the butterfly, are said to undergo
+complete transformation and are classed as 'Metabola' or 'Holometabola.'
+Wherein lies the fundamental difference between these Holometabola on
+the one hand and the Hemimetabola and Ametabola on the other? It is not
+that the larva differs from the imago or that there is a passive stage
+in the life-history; these conditions are observable among insects with
+a 'partial' transformation as we have seen, though the resting instar
+that simulates the butterfly pupa is certainly exceptional. It has been
+pointed out by Sharp (1899) that the most important indication of the
+difference between the two modes of development is furnished by the
+position of the wing-rudiments. In all Ametabola and Hemimetabola these
+are visible externally long before the penultimate instar has been
+reached; in the Holometabola they are not seen until the pupal stage.
+
+Attention has already been drawn to the contrast in outward form between
+a butterfly and its caterpillar. As in the case of dragon-fly or
+may-fly, the larval period is essentially a time for feeding and growth,
+and during this period the larval cuticle is cast four or five, in some
+species even seven or eight times. After each moult some changes in
+detail may be observable, for example in the proportions of the
+body-segments or their outgrowths, in the colour or the closeness of the
+hairy or spiny armature. But in all main features the caterpillar
+retains throughout its life the characteristic form in which it left
+the egg. From the tiny, newly-hatched larva to the full-fed caterpillar,
+possibly several inches in length, there is all along the same crawling,
+somewhat worm-like body, destitute of any outward trace of wings. When
+however the last larval cuticle has split open lengthwise along the
+back, and has been worked off by vigorous wriggling motions of the
+insect, the pupa thus revealed shows the wing-rudiments conspicuous at
+the sides of the body, and lying neatly alongside these are to be seen
+the forms of feelers, legs, and maxillae of the imago prefigured in the
+cuticle of the pupa (fig. 1 _e_). The pupa thus resembles the imago much
+more closely than it resembles the larva; even in the proportions of the
+body a relative shortening is to be noticed, and the imago of any insect
+with complete transformation is reduced in length as compared with the
+full-fed larva. Now these wings and other structures characteristic of
+the imago, appear in the pupa which is revealed by the shedding of the
+last larval cuticle. From these facts we infer that the wing-rudiments
+must be present in the larva, hidden beneath the cuticle; and until the
+last larval instar, not beneath the cuticle only, but growing in
+such-wise that they are hidden by the epidermis. For if they were
+growing outwardly the new cuticle would be formed over them, so that
+they would be apparent after the next moult. But it is clear that only
+in the pupa, forming beneath the cuticle of the last larval instar, can
+they grow outwards.
+
+Anatomical study of the caterpillar at various stages verifies the
+conclusions just drawn from superficial observation. A hundred and fifty
+years ago P. Lyonet in his monumental work (1762) on the caterpillar of
+the Goat Moth (Cossus) detected, in the second and third thoracic
+segments, four little white masses buried in the fat-body, and, while
+doubtful as to their real meaning, he suggested that their number and
+position might well give rise to the suspicion that they were rudiments
+of the wings of the moth. But it was a century later that A. Weismann in
+his classical studies (1864) on the development of common flies, showed
+the presence in the maggot of definite rudiments of wings, and other
+organs of the adult--rudiments to which he gave the name of _imaginal
+discs_. We will recur later to these transformations of the Diptera. For
+the present, we pursue our survey of changes in the life-history of the
+Lepidoptera and can take to guide us the excellent researches of J.
+Gonin (1894).
+
+Careful study of the imaginal discs of the wings in a caterpillar (fig.
+10) made by examining microscopically sections cut through them, shows
+that the epidermis is pushed in to form a little pouch (_C, p_) and that
+into this grows the actual wing-rudiment. Consequently the whitish disk
+which seems to lie within the body-wall of the larva, is really a
+double fold of the epidermis, the outer fold forming the pouch, the
+inner the actual wing-bud. Into the cavity of the latter pass branches
+from the air-tube system. In its earliest stage, the wing-bud is simply
+an ingrowing mass of cells (fig. 10 _A_) which subsequently becomes an
+inpushed pouch (_B_). Until the last stage of larval life the wing-bud
+remains hidden in its pouch, and no cuticle is formed over it. When the
+pupal stage draws near the bud grows out of its sheath, and projecting
+from the general surface of the epidermis becomes covered with cuticle
+to be revealed, as we have seen, after the last larval moult, as the
+pupal wing. Thus all through the life of the humble, crawling
+caterpillar, 'it doth not yet appear what it shall be,' but there are
+being prepared, hidden and unseen, the wondrous organs of flight, which
+in due time will equip the insect for the glorious aerial existence that
+awaits it.
+
+[Illustration: Fig. 10. A, B, C, Sections through epidermis and cuticle,
+showing three stages in growth of the imaginal disc (_w_) of a wing in
+the caterpillar of a White Butterfly (_Pieris_). _ep_, epidermis; _cu_,
+cuticle; _t_, air-tube, whence branches pass into the developing wing.
+In C, _cu'_ represents the new cuticle forming beneath the old one, and
+(_p_) the pouch within which the wing-disc (_w_) lies. Highly magnified.
+After Gonin, _Bull. Soc. Vaud._ XXX.]
+
+As mentioned above, this hidden growth of the wing-rudiments, in
+butterflies, beetles, flies, bees, and the great majority of the winged
+insects, has been emphasised by Sharp (1899) as a character contrasting
+markedly with the outward and visible growth of the wing-rudiments in
+such insects as cockroaches, bugs, and dragon-flies. The divergence
+between the two modes of development is certainly very striking, and a
+conceivable method of transition from the one to the other is not easy
+to explain. Sharp has expressed the divergence by the terms
+_Endopterygota_, applied to all the orders of insects with hidden
+wing-rudiments (the 'Metabola' or 'Holometabola' of most
+classifications) and _Exopterygota_, including all those insects whose
+wing-rudiments are visible throughout growth ('Hemimetabola' and
+'Ametabola'). Those curious lowly insects, belonging to the two orders
+of the Collembola and Thysanura, none of whose members ever develop
+wings at all, form a third sub-class, the _Apterygota_ (see
+Classificatory Table, p. 122).
+
+Not the wings only, but other structures of the imago, varying in extent
+in different orders, are formed from the imaginal discs. For example, de
+R�aumur and G. Newport (1839) found that if the thoracic leg of a
+late-stage caterpillar were cut off, the corresponding leg of the
+resulting butterfly would still be developed, although in a truncated
+condition. Gonin has shown that in the Cabbage White butterfly (_Pieris
+brassicae_) the legs of the imago are represented, through the greater
+part of larval life, only by small groups of cells situated within the
+bases of the larval legs. After the third moult these imaginal discs
+grow rapidly and the proximal portion of each, destined to develop into
+the thigh and shin of the butterfly's leg, sinks into a depression at
+the side of the thorax, while the tip of the shin and the
+five-segmented foot project into the cavity of the larval leg. Hence we
+understand that the amputation of the latter by the old naturalists
+truncated only and did not destroy the imaginal limb. In the blow-fly
+maggot, Weismann, B.T. Lowne (1890) and J. Van Rees (1888) have shown
+that the imaginal discs of the legs (fig. 11--1, 2, 3) grow out from
+deep dermal inpushings. Simple at first, these outgrowths by partial
+splitting, become differentiated into thigh and shin.
+
+[Illustration: Fig. 11. Front region of Maggot of Blow-fly
+(_Calliphora_) showing diagrammatically the imaginal discs, which are
+shaded. _e_, eye; _f_, feeler; _W_, fore-wing; _w_, hind-wing; 1, 2, 3,
+legs. _H_ is the 'cephalic vesicle,' which becomes everted at the close
+of the metamorphosis, so as to bring the feelers and eyes to the front,
+the brain (_B_) moving forwards at the same time. After Van Rees, _Zool.
+Jahrb._ 1894, and Lowne's _Blow-fly_.]
+
+Similarly the feelers and jaws of the butterfly are developed from
+imaginal discs, and this fact explains how it comes to pass that they
+differ so widely from the corresponding structures in the caterpillar.
+The larval feelers (fig. 3 _At_) are short and stumpy, those of the
+butterfly long and many-jointed. The maxilla of the larva (fig. 3 _Mx_)
+consists of a base carrying two short jointed processes; in the
+butterfly a certain portion of the maxilla, the hood or galea, is
+modified into a long, flexible grooved process, capable of forming with
+its fellow the trunk through which the insect sucks its liquid food
+(fig. 2). Nothing but some such provision as that of the imaginal discs
+could render possible the wonderful replacement of the caterpillar's
+jaws, biting solid food, into those of the butterfly sipping nectar from
+flowers.
+
+A curious segmental displacement of the imaginal discs with regard to
+the larva is noticeable in some Diptera. In the larva of the
+harlequin-midge (Chironomus) as described by Miall and Hammond (1900)
+the brain is situated in the thorax, and the imaginal discs for the
+head, eyes, and feelers of the adult lie in close association with it,
+though they arise from inpushings of the larval head. These rudiments do
+not appear until the last larval stage has been reached. In the gnats
+Culex and Corethra, on the other hand, the imaginal discs for the
+head-appendages retain their normal position within the larval head, and
+appear in an early stage of larval life. Among the flies of the
+bluebottle group (Muscidae) the brain (fig. 11 _B_) is situated, as in
+Chironomus, in the thoracic region of the legless maggot, which is the
+larva of an insect of this family, and the imaginal discs for eyes and
+feelers (fig. 11 _e_, _f_) lie just in front of it. Here, the imaginal
+buds of the legs (fig. 11--1, 2, 3) and wings (fig. 11 _W_, _w_) are
+deeply inpushed, retaining their connection with the skin only by means
+of a thread of cells. As the larva is legless and headless its outer
+form is not affected by the discs and it is not surprising to learn that
+they appear early. It has indeed been suggested that the pharyngeal
+region of the larva, in connection with which the imaginal head-discs
+are developed, should be regarded, though it lies in the thorax, as an
+inpushed anterior section of the larval head. In any case this region is
+pushed out during the formation of the pupa within the final larval
+cuticle, so that the imaginal head with its contained brain, its
+compound eyes, and its complex feelers, takes its rightful place at the
+front end of the insect.
+
+The mention of the brain suggests a few brief remarks on the changes in
+the internal organs during insect transformation. There are no imaginal
+discs for the nervous system; the brain, nerve-cords and ganglia of the
+butterfly or bluebottle are the direct outcome of those of the
+caterpillar or maggot. More than seventy years ago, Newport (1839)
+traced the rapid but continuous changes, which, during the early pupal
+period, convert the elongate nerve-cord of the caterpillar with its
+relatively far-separated ganglia into the shortened, condensed
+nerve-cord of the Tortoise-shell butterfly (_Vanessa urticae_) with
+several of the ganglia coalesced. In many Diptera, on the other hand,
+the nervous system of the larva is more concentrated than that of the
+imago.
+
+The tubular heart also of a winged insect is the directly modified
+survival of the larval heart.
+
+Similarly the reproductive organs undergo a gradual, continuous
+development throughout an insect's life-story. Their rudiments appear in
+the embryo, often at a very early stage; they are recognisable in the
+larva, and the matured structures in the imago are the result of their
+slow process of growth, the details of which must be reckoned beyond the
+scope of this book. For a full summary of the subject the reader is
+referred to L.F. Henneguy's work (1904) containing references to much
+important modern literature, which cannot be mentioned here.
+
+On the other hand, the digestive system of insects that undergo a
+metamorphosis, passes through a profound crisis of dissolution and
+rebuilding. This is not surprising when we remember that there is often
+a great difference between larva and imago in the nature of the food.
+The digestive canal of a caterpillar runs a fairly straight course
+through the body and consists of a gullet, stomach (mid-gut),
+intestine, and rectum; it is adapted for the digestion of solid food. In
+the butterfly there is one outgrowth of the gullet in the head--a
+pharyngeal sac adapted for sucking liquids; and another outgrowth at the
+hinder end of the gullet (which is much longer than in the larva)--a
+crop or food-reservoir lying in the abdomen. The intestine of the
+butterfly also is longer than that of the larva, being coiled or
+twisted. Towards the end of the last larval stage, the cells of the
+inner coat (epithelium) lining the stomach begin to undergo
+degeneration, small replacing cells appearing between their bases and
+later giving rise to the more delicate epithelium that lines the mid-gut
+of the imago. The larval cells are shed into the cavity of the stomach
+and become completely broken down. J. Anglas (1902), describing these
+microscopic changes in the transformations of wasps and bees, has shown
+that the tiny replacing cells can be recognised in sections through the
+digestive canal of a very young larva; they may be regarded as
+representing imaginal buds of the adult gastric epithelium. In the
+transformations of two-winged flies of the bluebottle group, A.
+Kowalevsky (1887) has shown that these replacing cells are aggregated in
+little masses scattered at different points along the stomach and thus
+corresponding rather closely to the imaginal discs of the legs and
+wings.
+
+The gullet, crop, and gizzard of an insect, which lie in front of the
+stomach, are lined by cells derived from the outer skin (ectoderm) which
+is pushed in to form what is called the 'fore-gut.' Similarly the
+intestine and rectum, behind the stomach, are lined with ectodermal
+cells which arise from the inpushed 'hind-gut.' The larval fore- and
+hind-guts are broken down at the end of larval life and their lining is
+replaced by fresh tissue derived from two imaginal bands which surround
+the cavity of the digestive tube, one at the hinder end of the fore-gut,
+and the other at the front end of the hind-gut. The larval salivary
+glands in connection with the gullet are also broken down, and fresh
+glands are formed for the imago.
+
+A large part of the substance of an insect larva consists of muscular
+tissue, surrounding the digestive tube, and forming the great muscles
+that move the various parts of the body, and of fat, surrounding the
+organs and serving as a store of food-material. Very many of the
+muscle-fibres and the fat-cells also become disintegrated during the
+late larval and pupal stages, and the corresponding tissues of the adult
+are new formations derived from special groups of imaginal cells, though
+some muscles may persist from the larva to the adult. Similarly the
+complex air-tube or tracheal system of the larva is broken down and a
+fresh set of tubes is developed, adapted to the altered body-form of
+pupa and imago.
+
+The destruction of larval tissue and the development of replacing organs
+from special groups of cells, derived of course from the embryo, and
+carrying on the continuity of cell-lineage to the adult, are among the
+most remarkable facts connected with the life-story of insects. The
+process of tissue-destruction is known as 'histolysis'; the rebuilding
+process is called 'histogenesis.' Considerable difference of opinion has
+existed as to factors causing histolysis, and for a summary of the
+conflicting or complementary theories, the reader is referred to the
+work of L.F. Henneguy (1904, pp. 677-684). In the histolysis of the
+two-winged flies, wandering amoeboid cells--like the white corpuscles or
+leucocytes of vertebrate blood--have been observed destroying the larval
+tissues that need to be broken down, as they destroy invading
+micro-organisms in the body. But students of the internal changes that
+accompany transformation in insects of other orders have often been
+unable to observe such devouring activity of these 'phagocytes,' and
+attribute the dissolution of the larval tissues to internal chemical
+changes. The fact that in all insect transformation a part, and in many
+a large part, of the larval organs pass over to the pupa and imago,
+suggests that only those structures whose work is done are broken down
+through the action of internally formed destructive substances, and one
+function of the phagocytes is to act as scavengers by devouring what has
+become effete and useless.
+
+
+
+
+CHAPTER VI
+
+LARVAE AND THEIR ADAPTATIONS
+
+
+Among the insects that undergo a complete transformation, there is, as
+we have seen in the preceding chapter, an amount of inward change, of
+dissolution and rebuilding of tissues, that varies in its completeness
+in members of different orders. It is now advisable to consider the
+various outward forms assumed by the larvae of these insects, or rather
+by a few examples chosen from a vast array of well-nigh 'infinite
+variety.'
+
+In comparing the transformations of endopterygote insects of different
+orders, it is worthy of notice that in some cases all the members of an
+order have larvae remarkably constant in their main structural features,
+while in others there is great variety of larval form within the order.
+For example, the caterpillars of all Lepidoptera are fundamentally much
+alike, while the grubs of beetles of different families diverge widely
+from one another. A review of a selected series of beetle-larvae will
+therefore serve well to introduce this branch of the subject.
+
+[Illustration: Fig. 12. _a_, Carrion-beetle (_Silpha_) with its larva,
+_b_. Magnified, _a_ 3 times, and _b_ 4 times.]
+
+[Illustration: Fig. 13. Larva of a Ground-beetle (_Aepus_). Magnified
+6 times. After Westwood, _Modern Classification of Insects_.]
+
+Beetles are as a rule remarkable among insects for the firm consistency
+of their chitinous cuticle, the various pieces (_sclerites_) of which
+are fitted together with admirable precision. In some families of
+beetles the larva also is furnished with a complete chitinous armour,
+the sclerites, both dorsal and ventral, of the successive body-segments
+being hard and firm, while the relatively long legs possess well-defined
+segments and are often spiny. Such a larva is evidently far less unlike
+its parent beetle than a caterpillar is unlike a butterfly. Perhaps of
+all beetle larvae, the woodlouse-like grub (fig. 12 _b_) of a
+carrion-beetle (Silpha) or of a semi-aquatic dascillid such as Helodes
+shows the least amount of difference from the typical adult, on account
+of the conspicuous jointed feelers. The larval glow-worm, however, is of
+the same woodlouse-like aspect, and in this case, where the female never
+acquires wings, but becomes mature in a form which does not differ
+markedly from that of the larva, the exceptional resemblance is closer
+still. In all beetle-grubs the legs are simplified, there being only one
+segment (a combined shin and foot) below the knee-joint, whereas in the
+adult there is a shin followed by five, four, or at least three
+distinct tarsal segments. The foot of an adult beetle bears two claws
+at its tip, while the larval foot in the great majority of families has
+only one claw. In one section of the order, however, the Adephaga
+comprising the predaceous terrestrial and aquatic beetles, the larval
+foot has, like that of the adult, two claws. Some adephagous larvae,
+notably those of the large carnivorous water-beetles (Dyticus), often
+destructive to tadpoles and young fish, have completely armoured bodies
+as well as long jointed legs. More commonly, as with most of the
+well-known Ground-beetles (Carabidae), the cuticle is less consistently
+hard, firm sclerites segmentally arranged alternating with considerable
+tracts of cuticle which remain feebly chitinised and flexible. Most of
+the adephagous larvae (fig. 13) have a pair of stiff processes on the
+ninth abdominal segment, and the insect, from its general likeness to a
+bristle-tail of the genus Campodea, is often called a _campodeiform_
+larva (Brauer, 1869). From such as these, a series of forms can be
+traced among larvae of beetles, showing an increasing divergence from
+the imago. The well-known wireworms--grubs of the Click-beetles
+(Elateridae)--that eat the roots of farm crops, have well-armoured
+bodies, but their shape is elongate, cylindrical, worm-like; and their
+legs are relatively short, the build of the insect being adapted for
+rapid motion through the soil. The grubs of the Chafers (Scarabaeidae)
+are also root-eaters, but they are less active in their habits than the
+wireworms, and the cuticle of their somewhat stout bodies is, for the
+most part, pale and flexible; only the head and legs are hard and horny.
+Usually an evident correspondence can be traced between the outward form
+of any larva and its mode of life. For example, in the family of the
+Leaf-beetles (Chrysomelidae) some larvae feed openly on the foliage of
+trees or herbs, while others burrow into the plant tissues. The exposed
+larvae of the Willow-beetles (Phyllodecta, fig. 14) have their somewhat
+abbreviated body segments protected by numerous spine-bearing, firm
+tubercles. But the grub of the 'Turnip Fly' (Phyllotreta) which feeds
+between the upper and lower skins of a leaf, or of _Psylliodes
+chrysocephala_ (fig. 15), which burrows in stalks, has a pale, soft
+cuticle like that of a caterpillar.
+
+[Illustration: Fig. 14. (_a_) Willow-beetle (_Phyllodecta vulgatissima_)
+and its larva (_b_). Magnified 5 times. After Carpenter, _Econ. Proc. R.
+Dublin Soc_. vol. I.]
+
+[Illustration: Fig. 15. (_a_) Cabbage-beetle (_Psylliodes chrysocephala_)
+magnified 5 times, and its larva (_b_) magnified 12 times.]
+
+In the larvae of the little timber-beetles and their allies (Ptinidae),
+including the 'death-watches' whose tapping in old furniture is often
+heard, a marked shortening of the legs and reduction in the size of the
+head accompany the whitening and softening of the cuticle. This
+shortening of the legs is still more marked in the larvae of the
+Longhorn Beetles (Cerambycidae) burrowing in the wood of trees or felled
+trunks; here the legs are reduced to small vestiges.
+
+[Illustration: Fig. 16. _a_, Grain Weevil (_Calandra granaria_); _b_,
+larva; _c_, pupa. Magnified 7 times. After Chittenden, _Yearbook U.S.
+Dept. Agric._ 1894.]
+
+Finally in the large family of the Weevils (Curculionidae, fig. 16) and
+the Bark-beetles (Scolytidae), the grubs, eating underground root or
+stem structures, mining in leaves or seeds, or tunnelling beneath the
+bark of trees, have no legs at all, the place of these limbs being
+indicated only by tiny tubercles on the thoracic segments. Such larvae
+as these latter are examples of the type called _eruciform_ by A.S.
+Packard (1898) who as well as other writers has laid stress on the
+series of transitional steps from the campodeiform to the eruciform type
+afforded by the larvae of the Coleoptera.
+
+A fact of much importance in the transformations of beetles as pointed
+out by Brauer (1869) is that in a few families, the first larval instar
+is campodeiform, while the subsequent instars are eruciform. We may take
+as an example of such 'hypermetamorphosis' the life-story of the Oil or
+Blister-beetles (Meloidae) as first described by J.H. Fabre (1857), and
+later with more elaboration by H. Beaur�gard (1890). From the egg of one
+of these beetles is hatched a minute armoured larva, with long feelers,
+legs, and cerci, whose task is, for example, to seize hold of a bee in
+order that the latter may carry it, an uninvited guest, to her nest.
+Safely within the nest, the little 'triungulin' beetle-grub moults; the
+second instar has a soft cuticle and relatively shorter legs, which, as
+the larva, now living as a cuckoo-parasite, proceeds to gorge itself
+with honey, soon appear still further abbreviated. Later comes a stage
+during which legs are entirely wanting, the larva then resting and
+taking no food. The last larval instar again has short legs like the
+grub of the second period. In connection with this life-history we
+notice that the newly-hatched larva is not in the neighbourhood of its
+appropriate food. Hence the preliminary armoured and active instar is
+necessary in order to reach the feeding place; this journey
+accomplished, the eruciform condition is at once assumed.
+
+In all cases indeed we may say that the particular larval form is
+adapted to the special conditions of life. A few examples from other
+orders of endopterygote insects will illustrate this point. The
+campodeiform type is relatively unusual, but most of the Neuroptera have
+larvae of this kind, active, armoured creatures with long legs, though
+devoid of the tail-processes often associated with similar larvae among
+the Coleoptera. Such are the 'Ant-lions,' larvae of the exotic lacewing
+flies, which hunt small insects, digging a sandy pit for their unwary
+steps in the case of the best-known members of the group, some of which
+are found as far north as Paris. In our own islands the 'Aphis-lions,'
+larvae of Hemerobius and Chrysopa, prowl on plants infested with
+'green-fly' which they impale on their sharp grooved mandibles, sucking
+out the victims' juices, and then, in some cases, using the dried
+cuticle to furnish a clothing for their own bodies. Among these insects,
+while the mouth of the imago is of the normal mandibulate type adapted
+for eating solid food, the larval mouth is constricted and the slender
+mandibles are grooved for the transmission of liquid food.
+
+Turning to eruciform types of larva, we find the _caterpillar_ (fig. 1
+_b_, _c_, _d_) distinguished by its elongate, usually cylindrical body
+with feeble cuticle, short thoracic legs and a variable number of pairs
+of abdominal pro-legs, universal among the moths and butterflies forming
+the great order Lepidoptera, and usual among the saw-flies, which belong
+to the Hymenoptera. The vast majority of caterpillars feed on the leaves
+of plants and their long worm-like bodies with the series of paired
+pro-legs, are excellently adapted for their habit of clinging to twigs,
+and crawling along shoots or the edges of leaves as they go in search of
+food. Of great importance to a caterpillar is its power of spinning
+silk, consisting of fine threads solidified from the secretion of
+specially modified salivary glands whose ducts open in the insect's
+mouth at the tip of the tubular tongue which forms a spinneret.
+
+On the same bush caterpillars of moths and of saw-flies may often be
+seen feeding together. The lepidopterous caterpillar, in our countries
+at least, has never more than five pairs of pro-legs, situated on the
+third, fourth, fifth, sixth, and tenth abdominal segments; each of these
+pro-legs bears a number of minute hooklets, arranged in a circular or
+crescentic pattern, which assist the caterpillar in clinging to its
+food-plant. The saw-fly caterpillar, on the other hand, may have as many
+as eight pairs of pro-legs, the series beginning on the second abdominal
+segment; here, however, the pro-legs have no hooklets. Among the
+Lepidoptera, we notice a reduction in the number of pro-legs in the
+'looper' caterpillars of Geometrid moths. Here only two pairs are
+present, those on the sixth and tenth abdominal segments. Consequently,
+as the caterpillar can cling only by the thorax and by the hinder region
+of the abdomen, the middle region of the body is first straightened out
+and then bent into an arch-like form, as the insect makes its progress
+by alternate movements of stretching and 'looping.'
+
+[Illustration: Fig. 17. _c_, Ruby Tiger Moth (_Phragmatobia
+fuliginosa_); _a_, caterpillar; _b_, cocoon. After Lugger, _Insect
+Life_, vol. II.]
+
+Caterpillars, with their relatively soft bodies, feeding openly on the
+leaves of plants, are exposed to the attacks of many enemies, and the
+various ways in which they obtain protection are well worth studying. A
+clothing of hairs[7] or spines is often present, and it is interesting
+to find that many species of our native Tiger and Eggar Moths (Arctiadae
+and Lasiocampidae) which pass the winter in the larval stage, have
+caterpillars with an especially dense hairy covering (fig. 17).
+Experiments have shown that hairy and spiny insects are distasteful to
+birds and other creatures that prey readily on smooth-skinned species, a
+conclusion that might well have been expected. Certain smooth
+caterpillars however appear to be protected by producing some nauseous
+secretion, which renders them unpalatable. Many of these, as the
+familiar cream yellow and black larva of the Magpie Moth (_Abraxas
+grossulariata_), are very conspicuously adorned, and furnish examples of
+what is known as 'warning coloration,' on the supposition that the gaudy
+aspect of such insects serves as an advertisement that they are not fit
+to eat, and that birds and other possible devourers thus learn to leave
+them alone. On the other hand, smooth caterpillars which are readily
+eaten by birds are usually 'protectively' coloured, so as to resemble
+their surroundings and remain hidden except to careful seekers. Many
+such caterpillars are green, the upper surface, which is naturally
+exposed to the light, being darker than the lower which is in shadow.
+When the caterpillar is large, the green area is often broken up by pale
+lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or
+oblique, as on the great caterpillars of most Hawk Moths (Sphingidae).
+Such an arrangement tends to make the insect less easily seen than were
+it to display a continuous area of the same colour. The 'looper'
+caterpillars mentioned above afford remarkable examples of 'protective'
+resemblance, for many of them show a marvellous likeness to the twigs of
+their food-plant, tubercles on the insect's body resembling closely the
+little outgrowths of the plant's cortex. It has been shown by E.B.
+Poulton (1892) that many caterpillars are, in their early stages,
+directly responsive to their surroundings as regards colour. Usually
+green when hatched, they remain green if kept among leaves or young
+shoots of plants, while they turn red, brown, or blackish if placed
+among twigs of these respective hues. This effect appears to be due to a
+direct response of the subcutaneous tissue to the rays of light
+reflected from the surrounding objects. The sensitiveness dies away as
+the caterpillar grows older, since little or no change of hue in
+response to a change of environment could be induced after the
+penultimate moult.
+
+[7] The 'hairs' of an insect are not in the least comparable to the
+hairs of mammals, being in truth, modified portions of the cuticle,
+secreted by special cells.
+
+Among those families of the Lepidoptera which are usually regarded as
+low in the scale of organisation, caterpillars are very generally
+protected by the habit of feeding in some concealed situation. For
+example, the great larvae of the Goat Moth (Cossus) and the whitish
+caterpillars of the Clearwing Moths (Sesiidae) burrow through the wood
+of trees, eating the timber as they go. The little irritable
+caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the
+edges together with silk, and thus make for themselves a shelter; or
+they bore their way into seeds or fruits, like the larva of the Codling
+Moth that is the cause of 'worm-eaten' apples, too well-known to
+orchard-keepers. Very many small caterpillars mine between the two skins
+of a leaf, eating out the soft green tissue, and giving rise to a
+characteristic blister in form of a spreading patch or a narrow sinuous
+track through the leaf. The caterpillars of the Clothes-moths (Tineidae)
+make for themselves garments out of their own excrement, the particles
+fastened together by silk. In such curious cylindrical cases they wander
+over the wool or fur, feeding and indirectly supplying themselves with
+clothing at the same time.
+
+The case-forming habit of the Clothes-moth caterpillars leads us
+naturally to consider the similar habit adopted by their allies the
+Caddis-larvae which live in the waters of ponds and streams, for the
+Caddis-flies (Trichoptera) have much in common with the more primitive
+Lepidoptera. The caddis-larva is as a rule of the eruciform type, but
+with well-developed thoracic legs, and with hook-like tail-appendages;
+by means of the latter it anchors itself to the extremity of its curious
+'house.' It is of interest to note that in the earlier stages of some
+caddises lately described and figured by A.J. Siltala (1907), the legs
+are relatively very long, and the larva is quite campodeiform in aspect.
+Some of these caddis-grubs retain the campodeiform condition and do not
+shelter permanently in cases, as their relations do. Different genera of
+caddises differ in their mode of building. Some fasten together
+fragments of water-weeds and plant refuse, others take tiny particles of
+stone, of which they make firmly compacted walls, others again lay hold
+of water-snail shells, which may even contain live inhabitants, and bind
+these into a limy rampart behind which their bodies are in safe hiding.
+
+The silk with which the 'caddis-worms' fasten together the materials for
+their houses is produced from spinning-glands which like those of the
+Lepidoptera open into the mouth.
+
+The survey of the various types of beetle-larvae enumerated above (pp.
+50-56) concluded with a short description of the _legless grub_, which
+is the young form of a weevil or a bark-beetle. This is a larva in which
+the head alone has its cuticle firm and hard; the rest of the body is
+covered with a pale, flexible cuticle, so that the grub is often
+described as 'fleshy.' This type of larva is by no means confined to
+certain families of the beetles, it is frequently met with, in more or
+less modified form, in two other important orders of insects, the
+Hymenoptera and the Diptera. Among the Hymenoptera this is indeed the
+predominant larval type. We have just seen that a caterpillar is the
+usual form of larva among the saw-flies, but in all other families of
+the Hymenoptera we find the legless grub. A grub of this order may
+usually be distinguished from the larva of a weevil or other beetle, by
+its relatively smaller head and smoother, less wrinkled cuticle; it
+strikes the observer as a feebler, more helpless creature than a
+beetle-grub. And it is of interest to note that this somewhat degraded
+type of larva is remarkably constant through a great series of
+families--gall-flies, ichneumon-flies, wasps, bees (fig. 18), ants--that
+vary widely in the details of their structure and in their habits and
+mode of life. Almost without exception, however, they make in some way
+abundant provision for their young. The feeble, helpless, larva is in
+every case well sheltered and well fed; it has not to make its own way
+in the world, as the active armoured larva of a ground-beetle or the
+caterpillar of a butterfly is obliged to do.
+
+[Illustration: Fig. 18. Young Larva (_FL_), Full-grown Larva (_SL_) and
+Pupa (_N_) of Hive-bee (_Apis mellifica_). _co_, cocoon; _sp_,
+spiracles; _ce_, eye; _an_, feeler; _m_, mandible; _l_, labium.
+Magnified 4 times. After Cheshire, _Bees_.]
+
+Among those saw-flies whose larvae feed throughout life in a concealed
+situation, we find an interesting transition between the caterpillar
+and the legless grub. For example, the giant saw-flies (so called
+'Wood-wasps') have larvae that burrow in timber, and these larvae
+possess relatively large heads, somewhat flattened bodies with pointed
+tail-end, and very greatly reduced legs. The feeble legless grub,
+characteristic of the remaining families of the Hymenoptera, is provided
+for in a well-nigh endless variety of ways. The female imago among these
+insects is furnished with an elaborate and beautifully formed
+ovipositor, and the act of egg-laying is usually in itself a provision
+for the offspring. Gall-flies pierce plant-tissues within which their
+grubs find shelter and food, the plant responding to the irritation due
+to the presence of the larva by forming a characteristic growth, the
+_gall_, pathological but often regular and shapely, in whose hollow
+chamber the grub lives and eats. Ichneumon-flies and their allies pierce
+the skin of caterpillars and other insect-larvae, laying their eggs
+within the victims' bodies, which their grubs proceed to devour
+internally. Some very small members of these families are content to lay
+their eggs within the eggs of larger insects, thus obtaining rich
+food-supply and effective protection for their tiny larvae. In
+Platygaster and other genera of the family Proctotrypidae, M. Ganin
+(1869) showed the occurrence of hypermetamorphosis somewhat like that
+already described as occurring among the Oil-beetles (Meloidae). The
+larva of Platygaster is at first rather like a small Copepod crustacean,
+with prominent spiny tail-processes; after a moult this form changes
+into the legless grub characteristic of the Hymenoptera, among which
+larvae even approaching the campodeiform type are very exceptional. The
+species of Platygaster pass their larval stages within the larvae of
+gall-midges.
+
+Wasps, bees and ants, have the ovipositor of the female modified into a
+sting, which is often used for the purpose of providing food for the
+helpless grubs. Thus the digging wasps (Sphegidae and Pompilidae) hunt
+for caterpillars, spiders, and other creatures which they can paralyse
+with their stings, and bury them alongside their eggs to furnish a
+food-supply for the newly-hatched young. The social wasps and many ants
+sting and kill flies and other insects, which they break up so as to
+feed their grubs within the nest. It is well known that the labour of
+tending the larvae in these insect societies is performed for the most
+part not by the mother ('Queen') but by the modified infertile females
+or 'workers.' Other ants and the bees feed their grubs (fig. 18), also
+sheltered in well-constructed nests, on honey elaborated from nectar
+within their own digestive canals. In all cases we see that the
+helplessness of the grub is associated with some kind of parental care.
+
+[Illustration: Fig. 19. Larva of Gall-midge (_Contarinia nasturtii_),
+ventral view showing anchor process (_a_), and spiracles projecting at
+sides. Magnified 30 times. From Carpenter, _Journ. Econ. Biol_, vol.
+VI.]
+
+From the Hymenoptera we may pass on to the Diptera or Two-winged Flies,
+an order of which the vast number of species and in many cases the
+myriads of individuals force themselves on the observer's notice. F.
+Brauer (1863) divided the Diptera into two sub-orders[8]; of the first
+of these a Crane-fly or 'Daddy-long-legs' may be taken as typical, of
+the second an ordinary House-fly or Bluebottle. All the larvae of the
+Diptera are legless, those of the Crane-fly group have well-developed
+hard heads, with biting mandibles, but in the House-fly section the
+larva is of the degraded _vermiculiform_ type known as the _maggot_,
+not only legless, but without a definite head, the front end of the
+creature usually tapering to the mouth, where there are a pair of strong
+hooks, used for tearing up the food. A few examples of each of these
+types must suffice in the present brief survey. A few pages back (p. 66)
+reference was made to the production of galls on various plants, through
+the activity of larvae of the hymenopterous family Cynipidae. Many
+plant-galls are due, however, to the presence of grubs of tiny dipterous
+insects, the Cecidomyidae or Gall-midges. A cecid grub (fig. 19) has an
+elongate body with flexible, wrinkled cuticle, tapering somewhat at the
+two ends. The head, if rather narrow, is distinct, and beneath the
+prothorax is a characteristic sclerite known as the 'anchor process' or
+'breast bone.' Along either side of the body is a series of paired
+spiracles, each usually situated at the tip of a little tubular
+outgrowth of the cuticle; the hindmost spiracles are often larger than
+the others. These little grubs live in family communities, their
+presence leading to some deformation of the plant that serves to shelter
+them. A shrivelled fruit or an arrested and swollen shoot, such as may
+be due respectively to the Pear-midge (_Diplosis pyrivora_) or the
+Osier-midge (_Rhabdophaga heterobia_), is a frequent result of the
+irritation set up by these little grubs. In a larva of the crane-fly
+family (Tipulidae, fig. 20) living underground and eating plant-roots,
+like the well-known 'leather-jacket' grubs of the large
+'Daddy-long-legs' (Tipula) or burrowing into a rotting turnip or swollen
+fungus, like the more slender grub of a 'Winter Gnat' (Trichocera), the
+student notices a somewhat tough cuticle, a relatively small but
+distinct head, and frequently prominent finger-like processes on the
+tail-segment. Further examination shows a striking modification in the
+arrangement of the spiracles. Instead of a paired series on most of the
+body-segments, as in caterpillars and the vast majority of insects
+whether larval or adult, there are two large spiracles surrounded by the
+prominent tail-processes, and a pair of very small ones on the
+prothorax, the latter possibly closed up and useless. This restriction
+of the breathing-holes to a front and hind pair (amphipneustic
+condition) or to a hind pair only (metapneustic type) is highly
+characteristic of the larvae of Two-winged flies.
+
+[8] Known as the Orthorrhapha and the Cyclorrhapha; these terms are
+derived from the manner in which the larval or pupal cuticle splits, as
+will be explained in the next chapter (p. 88).
+
+[Illustration: Fig. 20. Crane-fly (_Tipula oleracea_), _a_, female; _b_,
+larva ('leather-jacket' grub). Magnified twice.]
+
+[Illustration: Fig. 21. Maggot of House-fly (_Musca domestica_), _a_,
+side-view, magnified 5 times; _b_, prothoracic spiracle; _c_, feeler;
+_d_, hind-region with posterior spiracles; _e_, _f_, head-region with
+mouth-hooks; _g_, head-region of young maggot; _h_, eggs. All magnified.
+After Howard, _Entom. Bull._ 4, _U.S. Dept. Agric._]
+
+Turning now to the _maggot_, characteristic of the House-fly section
+(fig. 21) of the Diptera, we see the greatest contrast between the larva
+and the imago that can be found throughout the whole class of the
+insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer
+time on exposed meat, not unnaturally arouse feelings of disgust, yet
+they are the prelude to one of the most marvellous of all insect
+life-stories. The fly--with its large globular head, bearing the
+extensive compound eyes, the highly modified feelers with their
+exquisitely feathered slender sensory tips, and the complex suctorial
+jaws; with its compact thorax bearing the glassy fore-wings alone used
+for flight, though the hind-wings modified into tiny drumstick-like
+'halters' are the organs of a fine equilibrating sense--is perhaps the
+most specialised, structurally the 'highest' of all insects. Yet in a
+week or two this swift, alert, winged creature is developed from the
+degraded maggot, white, legless, headless, that buries itself in putrid
+flesh, 'feeding on corruption.'
+
+The broad end of the maggot is the tail, while the narrow extremity
+marks the position of the mouth. Above this are a pair of very short
+feelers (fig. 21 _c_), while from the aperture project the tips of the
+mouth-hooks (fig. 21 _e_, _f_), formidable, black, claw-like structures,
+articulated to the strong pharyngeal sclerites and moved by powerful
+muscles, tearing up the fibres of the flesh. On either side of the
+prothorax is an anterior spiracle, a curious branching or fan-like
+outgrowth (fig. 21 _b_), with a variable number of tiny openings which
+are probably of little use for the admission of air to the tubes. In
+many maggots the mouth-hooks and the front spiracles become more and
+more complex in form in the successive instars. The cuticle, white and
+smooth to the unaided eye, is seen on microscopic study to be set with
+rows of tiny spines which assist the maggot's movements through its
+food-mass. At the tail-end the large hind spiracles are conspicuous on a
+flattened dorsal area of the ninth abdominal segment; each shows a hard
+brown plate, traversed by three slits. And as we watch this curious
+degraded larva thrusting its narrow head-end into the depths of its
+ofttimes loathsome food-supply, we understand the advantage of access to
+the air-tube system being mainly confined to the hinder end of the body.
+
+Maggots, differing from that of the Bluebottle only in minor details,
+are the larval forms of a vast multitude of allied species and display
+great variation in the nature of their food. Most, however, hide their
+soft defenceless bodies in some substance which affords shelter as well
+as food. The Bluebottle maggot burrows into flesh, that of the House-fly
+into horse-dung or vegetable refuse. The maggot of the Cabbage-fly eats
+its way into the roots of cruciferous plants, that of the Mangel-fly
+works out a broad blister between the two skins of a leaf, into which
+the newly-hatched larva crawls directly from the egg. A large number of
+species, forming an entire subfamily (the Tachininae) have larvae that
+feed as parasites within the bodies of other insects.
+
+The habit of parasitism by maggots in back-boned animals has led to some
+remarkable modifications of the larva and to curious adventures in the
+course of the life-story. The Bot-fly of the Horse (_Gastrophilus equi_)
+and the Warble-fly of the Ox (_Hypoderma bovis_, fig. 22) lay eggs
+attached to the hairs of grazing animals, which, at least in the case of
+Gastrophilus, lick the newly-hatched larvae into their mouths. The
+'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach;
+often a whole family attach themselves by their mouth-hooks to a small
+patch of the mucous coat of that organ. The maggot is relatively short
+and stout, with rows of strong spicules surrounding the segments, and
+with spiracles capable of withdrawal through a cup-like inpushing of the
+tail-region of the body, so that the parasite is preserved from drowning
+when the host drinks water. The young maggot of Hypoderma (fig. 22 _e_)
+is elongate and slender, spends its first two stages burrowing in the
+gullet wall and then wandering through the dorsal tissues of its host;
+ultimately it arrives beneath the skin of the back and assumes for its
+third and fourth instars a broad barrel-like form (fig. 22 _b_). The
+supply of free oxygen within the ox's tissues being now insufficient,
+the warble-maggot bores a circular hole through the skin and rests with
+the tail spiracles directed upwards towards the outer air. When fully
+grown the maggot works its way through the hole in the host's skin, and
+falling to the ground pupates in some sheltered spot, the life cycle
+occupying about a year. Similarly the Horse-bot escapes from the host's
+intestine with the excrement, and pupates on the ground.
+
+A curious modification of the maggot is noticeable in the larva of the
+Hover-flies (Syrphus). These, unlike most of their allies, live exposed
+on the foliage of plants, where they feed by preying on aphids.
+
+[Illustration: Fig. 22. Ox Warble-fly (_Hypoderma bovis_), _a_, female;
+_b_, full-grown maggot from back of ox, dorsal view; _c_, egg; _d_,
+empty puparium, ventral view; _e_, young maggot from gullet, ventral
+view. Magnified (lines show natural size). _a-d_, after Theobald, _2nd
+Report Econ. Zool._ (_Brit. Mus._).]
+
+In agreement with this manner of life, the cuticle is roughly
+granulated, often greenish or reddish in hue, and the maggot, despite
+its want of definite head and sense organs, moves actively and
+purposefully about, often rearing up on its broad tail-end with an aphid
+victim impaled on its mouth-hooks.
+
+In a previous chapter reference was made to the exopterygote insects,
+stone-flies, dragon-flies, and may-flies, whose preparatory stages live
+in the water. Among the endopterygote orders many Neuroptera and
+Coleoptera, all Trichoptera, a very few Lepidoptera and many Diptera,
+have aquatic larvae. One or two examples of the adaptations of dipteran
+larvae to life in the water may well bring the present chapter to a
+close. Many members of the hover-fly family (Syrphidae) have maggots
+with the tail-spiracles situated at the end of a prominent tubular
+process. Among the best-known of syrphid flies are the drone-flies
+(Eristalis), often seen hovering over flowers, and presenting a curious
+likeness to hairy bees. The larva of Eristalis is one of the most
+remarkable in the whole order, the 'Rat-tailed maggot' found in the
+stagnant water of ditches and pools. It has a cylindrical body with the
+hinder end drawn out into a long telescopic tube, a more slender
+terminal section being capable of withdrawal into, or protrusion from, a
+thicker basal portion. At the extremity of the slender tube is a crown
+of sharp processes, forming a stellate guard to the spiracles. These
+processes can pierce the surface-film of the water, and place the
+tracheal system of the maggot in touch with the pure upper air; while
+its mouth may be far down, feeding among the foul refuse of the ditch,
+it can still reach out to the medium in which the end of its life-story
+must be wrought out.
+
+Reverting to the first great division of the Diptera, we find varied
+adaptations to aquatic life among many grubs that possess a definite
+head. The larva of a Gnat (Culex[9]) has projecting from the hind region
+of the abdomen a long tubular outgrowth, at the end of which are the
+spiracles, guarded by three pointed flaps forming a valve. When closed
+these pierce the surface-film of the water in which the larva lives;
+when opened a little cup-like depression is formed in the surface-film,
+from which the larva hangs. Or having accumulated a supply of air, it
+can disengage itself from the surface-film and dive through the water,
+its tracheal system safely closed. Another mode of breathing is found in
+the 'Blood-worms' and allied larvae of the Harlequin-midges
+(Chironomidae) whose transformations are described in detail by Miall
+and Hammond (1900). These larvae have two pairs of cylindrical,
+spine-bearing pro-legs--one on the prothorax and the other on the
+hindmost abdominal segment; the latter structures serve to fix the
+larva in the muddy tube which it inhabits at the bottom of its native
+pond. The penultimate abdominal segment has four long hollow outgrowths,
+which contain blood, and have the function of gills, while the hindmost
+segment has four shorter outgrowths of the same nature. Enabled thus to
+breathe dissolved air, the Chironomus larva needs not, like the Culex or
+the Eristalis, to find contact with the atmosphere beyond the
+surface-film.
+
+[9] See _Frontispiece_, A.
+
+Most remarkable, in many respects, of all aquatic larvae are the grubs
+of the Sand-midges (Simulium). These live entirely submerged and, having
+no special gills, carry out an exchange of gases through the general
+surface of the cuticle between the dissolved air in the water and the
+cavities of the air-tube system. The body is shaped like a flask swollen
+slightly at the hinder end and possesses a median pro-leg just behind
+the head, also another at the tail, which serves to attach the larva to
+a stone or to the leaf of an aquatic plant. The head has, in addition to
+feelers and jaws, a pair of processes with wonderful fringes which by
+their motion set up currents in the water, and bring food particles
+within reach of the mouth. A number of the larvae usually live in a
+community. Their power of spinning silken threads by which they can work
+their way back when accidentally dislodged from their resting-place, has
+been vividly described by Miall (1895).
+
+Examples might be multiplied, but enough have been given to enforce the
+conclusion that the forms of insect-larvae are wondrously varied, and
+that frequently, within the limits of the same order or even family,
+modifications of type may be found which are suited to various modes of
+life adopted by different insects. A survey of the multitudes of insect
+larvae--grubs, caterpillars, maggots--living on land, on plants,
+underground, in the water; feeding on leaves, in stems, on roots, on
+carrion, on refuse; by hunting or by lurking after prey; as parasites or
+as scavengers, brings home to us most strongly the conclusion that each
+larva is fitted to some little niche in the vast temple of life, each is
+specially adapted to its part in the great drama of being.
+
+
+
+
+CHAPTER VII
+
+PUPAE AND THEIR MODIFICATIONS
+
+
+The pupal stage is characteristic of the life-story of those insects
+whose larvae have wing-rudiments in the form of inpushed imaginal discs,
+and in all these insects there is, as we have seen, considerable
+divergence in form between larva and imago. In the pupa the wings and
+other characteristically adult structures are, for the first time,
+visible outwardly; it is the instar which marks the great crisis in
+transformation. The pupa rests, as a rule, in a quiescent condition, and
+during the early period of this stage the needful internal changes, the
+breaking down of many larval tissues, and their replacement by imaginal
+organs, go on. Both outwardly and inwardly therefore, the insect
+undergoes, at the pupal stage, a reconstruction necessitated by the
+differences in form and often in habit, between the larva and the winged
+adult; and the greater these differences, the more profound must be the
+changes that mark the pupal stage.
+
+From the prominence of imaginal structures in the pupa, it is at once
+seen that the pupa of any insect must resemble the adult more nearly
+than it resembles the larva. But in different groups of insects we find
+different degrees of likeness between pupa and imago. In a beetle pupa
+(see fig. 16 _c_), the appendages--feelers, jaws, legs, wings--stand out
+from the body as do those of the perfect insect. This type is called a
+_free_ pupa. The pupal cuticle has to be shed for the emergence of the
+imago, but the pupa is already a somewhat reduced model of the final
+instar, with abbreviated wings and doubled-up legs. A free pupa is
+characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera
+and many Diptera. In some cases the pupa requires to be specially
+adapted for a peculiar mode of life; for example, a special arrangement
+of breathing organs may be necessary for life under water, and there
+must needs be temporary pupal structures, not represented in the imago.
+
+On the other hand, in the pupae of most Lepidoptera and of some Diptera,
+there is more or less coalescence between the cuticle of the appendages
+and the cuticle of the body generally, so that the appendages do not
+stand out, but being, as it were, glued down to the body, are somewhat
+masked (see fig. 1 _e_ and fig. 23). Consequently the _obtect_ pupa, as
+this type is called, does not resemble its imago as fully as a free pupa
+does. The outline of the wings for example in a butterfly's pupa can in
+some cases be traced only with difficulty. T.A. Chapman has shown (1893)
+that the completely obtect pupa characterises the more highly developed
+families of Lepidoptera, while in the more primitive families the pupa
+is incompletely obtect. If the pupa of a butterfly or moth be lifted and
+held in the hand, a bending or wriggling motion of the abdomen can be
+observed. In the incompletely obtect pupa, this motion is evident in a
+greater number of segments than in the completely obtect, the number
+concerned varying from five to two in different families. In the
+nymphalid butterflies, the pupa is often called a 'chrysalis' on
+account of the golden hue displayed by the cuticle, and the term
+'chrysalis' is sometimes bestowed indiscriminately on any kind of pupa.
+It has been shown by Poulton (1892) and others, that the colour of a
+butterfly pupa is to some extent affected by the surroundings of the
+caterpillar just before its last moult.
+
+Reference has been made (p. 58) to the power of spinning silk possessed
+by many larvae; often the principal use of this silk is to form some
+protection for the pupa, the larva before its last moult constructing a
+_cocoon_ within which the pupa may rest safely. Many larvae bury
+themselves in the earth, and the pupa lies in an earthen chamber, the
+lining particles of soil fastened together by fine silken threads.
+Larvae that feed in wood, like the caterpillar of the Goat-moth (Cossus)
+make a cocoon of splinters spun together, while hairy caterpillars, such
+as those of the Tiger-moths, work some of their hairs in with the silk
+to make a firm cocoon (fig. 17 _b_). On the other hand, those
+caterpillars known as 'silkworms' make a dense cocoon of pure silk,
+consisting of two layers, the outer of coarse and the inner of fine
+threads. Silken cocoons very similar in appearance are spun by the
+larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed
+of a few interlacing threads, as for example the conspicuous black and
+yellow banded pupa of the Magpie-moth (_Abraxas grossulariata_) and the
+pupae of various leaf-beetles. Others again spin together the edges of
+leaves with connecting silken threads. The grubs of bees and wasps which
+are reared in the comb-chambers of their nests seal up the opening of
+the chamber with a lid, partly silk (fig. 18 _co_) and partly excretion,
+when ready to pass into the pupal state. An additional external
+'capping' may be also supplied by the workers.
+
+The pupae of butterflies are especially interesting, as illustrating the
+extreme reduction of the silken cocoon. The pupa of a 'swallowtail'
+(Papilionid) or a 'white' (Pierid) butterfly (fig. 23) may be found
+attached to a twig of its food-plant or to a wall, in an upright
+position, its tail fastened to a pad of silk and a slender silken girdle
+encircling its thorax. The pupa of a 'Tortoiseshell' or 'Admiral'
+(Nymphalid) butterfly hangs head downwards from a twig, supported only
+by the tail-pad of silk, which, useless as a shelter, serves only for
+attachment. The pupa is fastened to this pad by a spiny hook or process,
+the _cremaster_ (fig. 23 _cr_), on the last abdominal segment. The
+cremaster is a characteristic structure in the pupa of a moth or
+butterfly. C.V. Riley (1880) and W. Hatchett-Jackson (1890) have shown
+that it corresponds with a spiny area, the suranal plate, which lies
+above the opening of the caterpillar's intestine. The means by which the
+suspended pupa of a nymphalid butterfly attaches its cremaster to the
+silken pad which the larva has spun in preparation for pupation, is
+worthy of brief attention. The caterpillar, hanging head downwards, is
+attached to the silken pad by its hindmost pair of pro-legs or claspers
+and by the suranal plate, and the cuticle is slowly worked off from
+before backwards, so as to expose the pupa. Were the process of moulting
+to be simply completed while the insect hangs by the claspers, the pupa
+would of course fall to the ground. But there is enough adhesion between
+the pupal and larval cuticles at the hinder end of the body, especially
+by means of the everted lining of the hind-gut, for the pupa to be
+supported while it jerks its cremaster out of the larval cuticle and
+works it into the meshes of the silken pad. The moult is thus completed
+and the pupa hangs securely all the time. In the numerous cases where
+the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa
+to the surrounding silk. Chapman (1893) has drawn attention to the fact
+that among the more highly organised moths the pupa remains in the
+cocoon, the emergence being entirely left to the imago, while the pupae
+of the more primitive moths work their way partly out of the cocoon
+before the final moult begins. In the latter case, the cremaster is
+anchored by a strand of silk which allows a certain degree of emergence,
+and the pupa has rows of spines on its abdominal segments, of which a
+greater number retain the power of mutual motion than in those pupae
+which do not come out of their cocoons.
+
+[Illustration: Fig. 23. Pupa of White Butterfly (_Pieris_), side view;
+_f_, feeler; _w_, wing; _sp_, spiracle; _p_, anal pro-leg; _cr_,
+cremaster. Magnified 8 times. In part after Hatchett-Jackson, _Trans.
+Linn. Soc._ 1900, and Tutt's _British Butterflies_.]
+
+While the pupa on the whole resembles the imago that is to emerge from
+it, there are not a few cases in which a special structure necessary for
+some contingency in pupal life is retained or adopted in this stage. A
+butterfly pupa, like the imago, has no mandibles, but in the case of the
+Caddis-flies (Trichoptera) and two families of small moths, the most
+primitive of all Lepidoptera, the pupa, like the larva, has
+well-developed mandibles. These enable the caddis pupa to bite its way
+out of the shortened larval case in which it has pupated, and then to
+swim upwards through the water ready for the caddis-fly's emergence into
+the air. Pupae that are submerged require special breathing-organs. In
+the previous chapter (p. 77) mention was made of the gnat's aquatic
+larva with its tail-spiracles adapted for procuring atmospheric air
+through the surface-film. The pupa of the gnat[10] also has 'respiratory
+trumpets' serving the same purpose, but these are a pair of processes on
+the prothorax, so that the pupa, which is fairly active, hangs from the
+surface-film with its abdomen pointing downwards through the water. This
+change of position is correlated with the necessity for the imago to
+emerge into the air; were the pupa to hang head downwards as the larva
+does, the gnat would perforce have to dive into the water. With the
+beautifully adapted transfer of the functional spiracles, their position
+is appropriately arranged for the gnat's emergence at the surface, and
+the empty pupal cuticle floats serving the insect as a raft. On this it
+rests securely and the crumpled wings have opportunity to expand and
+harden before the insect takes to flight.
+
+[10] See _Frontispiece_, B.
+
+The aquatic pupae of other Diptera, many species of the midges
+Chironomus and Simulium for example, breathe dissolved air by means of
+tufts of thread-like gills, which arise on either side of the prothorax.
+The pupae of Simulium rest in their curious little cup-like dwellings,
+attached to submerged stones or plants. The Chironomus pupa is usually
+found in an elongate gelatinous case adhering to a stone. From this case
+the pupa rises to the surface of the water, that the midge may emerge
+into the air. Miall and Hammond (1900) describe the arrangement by
+which, when the pupal stage ends, and these gills are no longer
+required, their connection with the air-tube system is severed 'without
+undue violence.' The walls of the fine air-tubes that pass into the
+gills are specially strengthened, but just below the pupal cuticle these
+walls are exceedingly thin and delicate. Thus when the pupal cuticle is
+cast, they are readily broken there, and the cuticle of the midge
+forming beneath has a spiracular opening into the main air-trunk, ready
+for use during the insect's aerial life.
+
+Among those Diptera whose larva is the headless maggot a most
+remarkable arrangement for protecting the pupa is to be found. The last
+larval cuticle, instead of being as usual worked off and cast, after
+separation from the underlying structures, becomes hard and firm,
+forming a protective case (_puparium_) within which by the processes of
+histolysis and histogenesis already described the organs of the pupa and
+imago are built up. This puparium (fig. 22 _d_) is usually dark in
+colour, often brown and barrel-shaped, and a subcircular lid splits off
+from it at the head-end to allow the emergence of the fly[11]. While the
+maggot breathes by its tail-spiracles, the functional spiracles of the
+puparium (connected with the tracheal system of the enclosed pupa) are
+far forward, and these may be situated at the tips of long sometimes
+branching processes, which recall the thoracic gills of the aquatic
+pupae mentioned a few pages above. Adaptations, various and beautiful,
+to special modes of life, are thus seen to characterise pupae as well as
+larvae.
+
+[11] The presence of this sub-circular lid characterises Brauer's
+suborder Cyclorrhapha. Those Diptera in which the pupal cuticle splits
+in the normal, longitudinal manner are included in the Orthorrhapha (see
+p. 67).
+
+
+
+
+CHAPTER VIII
+
+THE LIFE-STORY AND THE SEASONS
+
+
+A number of interesting questions are associated with the seasonal cycle
+of an insect's life-history. In a previous chapter (IV. pp. 30, 34)
+reference has been made to the contrast between the long aquatic life of
+the larval dragon-fly or may-fly, extending over several years, and the
+short aerial existence of the winged adult restricted in the case of the
+may-flies to a few hours. Here we see that the feeding activities of the
+insect are carried on during the larval stage only; the may-fly in its
+winged condition takes no food, pairing and egg-laying form the whole of
+its appointed task. A similar though less extreme shortening of the
+imaginal life may be noticed in many endopterygote insects. For example,
+the bot- and warble-flies have the jaws so far reduced that they are
+unable to feed, and the parasitic life of the maggot (see p. 74)
+extending over eight or nine months in the body of the horse or ox,
+prepares for a winged existence of probably but a few days. Again in
+many moths the jaws are reduced or vestigial so that no food can be
+taken in the winged state, as for example in the 'Eggars'
+(Lasiocampidae) and the 'Tussocks' (Lymantriidae). It is noteworthy
+that in these short-lived insects the male is often provided with
+elaborate sense-organs which, we may believe, assist him to find a mate
+with as little delay as possible; the male may-fly has especially
+complex eyes, while the feelers of the male silk-moth or eggar are
+comb-like or feathery, the branches bearing thousands of sensory hairs.
+A box with a captive living female of one of these moths, if taken into
+a wood haunted by the species becomes rapidly surrounded by a swarm of
+would-be suitors, attracted by the odour emitted from the prisoner's
+scent-glands.
+
+Very exceptionally the imaginal stage may be omitted from the life-story
+altogether. Nearly fifty years ago N. Wagner (1865) made the remarkable
+discovery that in the larvae of certain gall-midges (Cecidomyidae) the
+ovaries might become precociously mature and unfertilised eggs might be
+developed into small larvae observable within the body of the
+mother-larva; ultimately these abnormally reared young break their way
+out. In this case therefore there may be a series of larval generations,
+neither pupa nor imago being formed. Extended observations on the
+precocious reproductive processes of these midges have lately been
+published by W. Kahle (1908). A less extreme instance of an abbreviated
+life-story was made known by O. Grimm (1870) who saw pupae of
+Harlequin-midges (Chironomus) lay unfertilised eggs, which developed
+into larvae. Here the imaginal stage only is omitted from the
+life-history. Not always however is it the imaginal stage of the
+life-history which is shortened. Reference (p. 18) has already been made
+to the case of the virgin female aphids, whose eggs develop within the
+mother's body, so that active, formed young are brought forth. Among the
+Diptera it is not unusual to find similar cases, the female fly giving
+birth to young maggots instead of laying eggs. Such is the habit of the
+great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.)
+whose larvae live as parasites on other insects, and occasionally of the
+Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a
+shortened larval period, and Brace's investigations in 1895, summarised
+by E.E. Austen (1911), have shown that females of the dreaded African
+Tsetse flies (Glossinia) bring forth nearly mature larvae, which pupate
+soon after birth. In another group of Diptera, the blood-sucking
+parasites of the Hippoboscidae and allied families, the whole larval
+development is passed through within the mother's body, and a full-grown
+larva is born the cuticle of which hardens and darkens immediately to
+form a puparium; hence these flies are often called, though incorrectly,
+Pupipara. Still more astonishing is the mode of reproduction in the
+allied family of the Termitoxeniidae, curious, degraded, wingless
+'guests' of the termites, or 'white ants,' lately made known through the
+researches of E. Wasmann (1901). Here the individual is hermaphrodite--a
+most exceptional condition among insects--and lays a large egg, whence
+is usually hatched a fully-developed adult! Here then we find that all
+the early stages, usual in the higher insects, are omitted from the
+life-story.
+
+Interesting comparison may be made between the total duration of various
+insect life-stories. To some extent at least, the length of an insect's
+life is correlated with its size, its food, the season of the year when
+it breeds. Small insects have, as a rule, shorter lives than large ones;
+those whose larvae devour highly nutritive food generally develop more
+quickly than those which have to live on dry, poor, substances;
+life-cycles follow one another most rapidly in summer weather when
+temperature is high and food plentiful.
+
+In early chapters we have already noticed the long aquatic life of the
+larva and nymph of a dragon-fly, relatively a large insect, and the
+rapid multiplication of the repeated summer broods of virgin aphids (p.
+18). Within the one order of the Coleoptera it is instructive to compare
+the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose
+larvae mine in the green tissues, and complete their transformations so
+rapidly that several successive broods appear in the spring and early
+summer, with the larger click-beetles whose larvae, the equally
+notorious 'wireworms,' feed on roots for three or four years before they
+become fully grown. Among the Diptera, the 'leather-jacket' grub of the
+crane-fly, feeding like the wireworm on roots, has a larval life
+extending through the greater part of a year, while the maggot of the
+bluebottle, feeding on a rich meat diet, becomes mature in a few days.
+As examples of excessively long life-cycles the 'thirteen-year' and
+'seventeen-year' cicads of North America, described by C.L. Marlatt
+(1895), are noteworthy. Certain specially populous 'broods' of these
+insects are known and localised, so that the appearance of the imagos in
+future years can be accurately predicted. Here again we have to do with
+bulky insects whose subterranean larvae and nymphs feed on comparatively
+innutritious roots.
+
+In our own climate, it is of interest to notice the variation among
+insects as to the stage which carries the race over the winter. The
+click-beetles, mentioned just above, emerge from their buried pupae in
+summer, hibernate under stones or clods, and lay eggs among the herbage
+next spring. At the same time of course, owing to the extended term of
+the larval life, many more individuals of the species are wintering
+underground as 'wireworms' of various ages, and these, except in very
+severe frosts, can continue their occupation of feeding on roots. But in
+the case of the 'turnip-flies' the food-supply is cut off in winter, and
+all those beetles of the latest summer brood that survive hibernate in
+some sheltered spot, waiting for the return of spring, that they may lay
+their eggs, and start the life-cycle once again. Among the Diptera, most
+species pass the winter as pupae, the sheltering puparium being a good
+protection against most adverse conditions, or as flies. But where there
+is a prolonged parasitic larval life, as with the bot- and warble-flies,
+the maggot, warm and well-fed within the body of its mammalian host,
+affords an appropriate wintering stage.
+
+Among the Hymenoptera an especially interesting seasonal life-cycle is
+afforded by the alternation of summer and winter generations in many
+Gall-flies (Cynipidae) as H. Adler (1881, 1896) demonstrated for most of
+our common species. The well-known 'oak-apples' are tenanted in summer
+by grubs, which after pupation develop into winged males and wingless
+females. The latter, after pairing, burrow underground and lay their
+eggs in the roots, the larvae causing the presence there of globular
+swellings or root-galls within which they live, pass through their
+transformations and develop into wingless virgin females. These shelter
+until February or March in their underground chambers, then climb up
+the tree and lay on the shoots eggs, from which will be hatched the
+grubs destined to grow within the oak-apples into the summer sexual
+brood of flies.
+
+The Lepidoptera afford examples of hibernation in all stages of the
+life-history. In this order a few large moths with wood-boring
+caterpillars, the 'Goat' (Cossus) for example, undergo a development
+extending over several years, while at the other extreme a few small
+species may have three or more complete cycles within the twelve months.
+But in the vast majority of Lepidoptera we find either one or two
+generations, definitely seasonal, within the year; the insect is either
+'single-brooded' or 'double-brooded.'
+
+Almost every winter one or more letters may be read in some newspaper
+recording the writer's surprise at seeing on a sunny day during the cold
+season, one of our common gaily-coloured butterflies of the Vanessa
+group, a 'Tortoiseshell' or 'Red Admiral,' flitting about. Surprise
+might be greater did the observers realise that the imaginal is the
+normal hibernating stage for these species. Emerging from the pupa in
+late summer or autumn, they shelter during winter in hollow trees, under
+thatched eaves, in outbuildings or in similar situations, coming out in
+spring to lay their eggs on the leaves of their caterpillars'
+food-plants. The larvae feed and grow through the early summer months,
+in the case of the Small Tortoiseshell (_Vanessa urticae_) pupating
+before midsummer and developing into a July brood of butterflies whose
+offspring after a late summer life-cycle, hibernate; while for the
+larger species of the group there is, in our islands, only one complete
+life-cycle in the year, though the same insects in warmer countries may
+be double-brooded. C.G. Barrett records (1893, vol. I. pp. 153-4) how in
+the August of 1879 hundreds and thousands of 'Painted Ladies' (_Pyrameis
+cardui_) migrated into the south of England from the European continent
+where in many places great swarms had been observed early in the summer.
+'These August butterflies, the progeny of the June swarms, coming from a
+warmer climate, had no intention of hibernating, but paired and laid
+eggs. Some of the larvae were collected and reared indoors [butterflies]
+emerging in November and December, but out of doors all must have been
+destroyed by damp or frost, in either the larva or pupa state, for no
+freshly emerged specimens were noticed in the spring, and no trace of
+the great migration remained.'
+
+In September and October the pedestrian, even in a suburban square, may
+see moths with pretty brown, white-spotted wings flying around trees.
+These are males of the common 'Vapourer' (_Orgyia antiqua_), in search
+of the females which, wingless and helpless, rest on the cocoons
+surrounding the pupae whence they have just emerged, the cocoons being
+attached to the branches of the trees where the caterpillars have fed.
+After pairing, the female lays her eggs among the silk of the cocoon,
+partly covering them with hairs shed from her body, and then dies. The
+eggs thus protected remain through the winter, the larvae not being
+hatched till springtide, when the young leaves begin to sprout forth.
+The caterpillars, adorned and probably protected by their 'tussocks' of
+black or coloured bristles, feed vigorously. Their activity and habit of
+occasional migration from one tree to another, compensates, to some
+extent, as Miall (1908) has pointed out, for the females' enforced
+passivity; only in the larval state can moths with such wingless females
+extend their range. The caterpillars spin their cocoons towards the end
+of summer, and then pupate, the moths emerging in the autumn and the
+eggs, as we have seen, furnishing the winter stage.
+
+After midsummer, the conspicuous cream, black and yellow-spotted
+'Magpie' moth (_Abraxas grossulariata_) is common in gardens. The female
+lays her eggs on a variety of shrubby plants; gooseberry and currant
+bushes are often chosen. From the eggs caterpillars are hatched in
+autumn, but these, instead of beginning to feed, seek almost at once for
+rolled-up leaves, cracks in walls, crannies of bark, or similar places,
+which may afford winter shelters. Here they remain until the spring,
+when they come out to feed on the young foliage and grow rapidly into
+the conspicuous cream, yellow and black 'looper' caterpillars mentioned
+in a previous chapter (p. 60). These, when fully-grown, spin among the
+twigs of the food-plant a light cocoon, in which the black and
+yellow-banded wasp-like pupa spends its short summer term before the
+emergence of the moth.
+
+An equally familiar garden insect, the common 'Tiger' moth (_Arctia
+caia_) with its 'woolly bear' caterpillar, affords a life-cycle slightly
+differing from that of the 'Magpie.' The gaudy winged insects are seen
+in July and August, and lay their eggs on a great variety of plants. The
+larvae hatched from these eggs begin to feed at once, and having moulted
+once or twice and attained about half their full size, they rest through
+the winter, the dense hairy covering wherewith they are provided forming
+an effective protection against the cold. At the approach of spring they
+begin to feed again, and the fully-grown 'woolly bear' is a common
+object on garden paths in May and June. Before midsummer it has usually
+spun its yellow cocoon under some shelter on the ground and changed into
+a pupa.
+
+Another modification with respect to seasonal change is shown by the
+Turnip moth (_Agrotis segetum_) and other allied Noctuidae (Owl-moths).
+These are insects with brown-coloured wings, flying after dark in June.
+The dull greyish larvae feed on many kinds of low-growing plants,
+usually hiding in the earth by day and wandering along the surface of
+the ground by night, biting off the farmer's ripening corn, or burrowing
+into his turnips or potatoes. On account of the burrowing habits of this
+insect it can feed throughout the winter, except when a hard frost puts
+a temporary stop to its activity. By April it has become fully grown and
+pupates in an earthen chamber a few inches below the surface. The Turnip
+moth in our countries is partially double-brooded, a minority of the
+autumn caterpillars growing more rapidly than their comrades so that
+they pupate, and a second brood of moths appear in September. These pair
+and lay eggs, the resulting caterpillars going as Barrett suggests
+(1896, vol. III. p. 291) 'to reinforce the great army of wintering
+larvae.'
+
+Such underground caterpillars, to a great extent protected from cold,
+can continue to feed through the winter. With other species we find that
+the larva becomes fully grown in autumn, yet lives through the winter
+without further change. This is the case with the Codling moth
+(_Carpocapsa pomonella_), a well-known orchard pest, which in our
+countries is usually single-brooded. The moth is flying in May and lays
+her eggs on the shoots or leaves of apple-trees, more rarely on the
+fruitlets, into which however the caterpillar always bores by the upper
+(calyx) end. Here it feeds, growing with the growth of the fruit,
+feeding on the tissue around the cores, ultimately eating its way out
+through a lateral hole, and crawling upwards if its apple-habitation has
+fallen, downwards if it still remains on the bough, to shelter under a
+loose piece of bark where it spins its cocoon about midsummer and
+hibernates still in the larval condition. Not until spring is the pupal
+form assumed, and then it quickly passes into the imaginal state. In the
+south of England, as F.V. Theobald (1909) has lately shown, and also in
+southwestern Ireland, this species may be double-brooded, the usual
+condition on the European continent and in the United States of America.
+There the midsummer larvae pupate at once and the moths of an August
+brood lay eggs on the hanging or stored fruit; in this case, again,
+however, the full-grown larva, quickly fed-up within the developed
+apples, is the wintering stage.
+
+Several of the insects mentioned in this survey, like the last-named
+codling moth, are occasionally double-brooded. As an example of the many
+Lepidoptera, which in our islands have normally two complete life-cycles
+in the year, we may take the very familiar White butterflies (Pieris) of
+which three species are common everywhere. The appearance of the first
+brood of these butterflies on the wing in late April or May is hailed as
+a sign of advanced spring-time. They pair and lay their eggs on
+cabbages and other plants, and the green hairy caterpillars feed in June
+and July, after which the spotted pupae may be found on fences and
+walls, attached by the silken tail-pad and supported by the
+waist-girdle. In August and September butterflies of the second brood
+have emerged from these and are on the wing; their offspring are the
+autumn caterpillars which feed in some seasons as late as November,
+doing often serious damage to the late cruciferous crops before they
+pupate. The pupae may be seen during the winter months, waiting for the
+spring sunshine to call out the butterflies whose structures are being
+formed beneath the hard cuticle.
+
+Reviewing the small selection of life-stories of various Lepidoptera
+just sketched, we notice an interesting and suggestive variety in the
+wintering stage. The vanessid butterflies hibernate as imagos; the
+'vapourer' winters in the egg, the magpie as a young ungrown larva, the
+'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
+winter, growing all the time; the codling caterpillar completes its
+growth in the autumn, and winters as a full-size resting larva; lastly,
+the 'whites' hibernate in the pupal state. And in every case it is
+noteworthy that the form or habit of the wintering stage is well adapted
+for enduring cold.
+
+Our native 'whites' afford illustration of another interesting feature
+often to be noticed in the life-story of double-brooded Lepidoptera. The
+butterflies of the spring brood differ slightly but constantly from
+their summer offspring, affording examples of what is called _seasonal
+dimorphism_. All three species have whitish wings marked with black
+spots, larger and more numerous in the female than in the male. In the
+spring butterflies these spots tend towards reduction or replacement by
+grey, while in the summer insects they are more strongly defined, and
+the ground colour of the wings varies towards yellowish. In the
+'Green-veined' white (_Pieris napi_) the characteristic greenish-grey
+lines of scaling beneath the wings along the nervures, are much broader
+and more strongly marked in the spring than in the summer generation,
+whose members are distinguished by systematic entomologists under the
+varietal name _napaeae_. The two forms of this insect were discussed by
+A. Weismann in his classical work on the Seasonal Dimorphism of
+butterflies (1876). He tried the effect of artificially induced cold
+conditions on the summer pupae of _Pieris napi_, and by keeping a batch
+for three months at the temperature of freezing water, he succeeded in
+completely changing every individual of the summer generation into the
+winter form. The reverse of this experiment also was attempted by
+Weismann. He took a female of _bryoniae_, an alpine and arctic variety
+of _Pieris napi_, showing in an intensive degree the characters of the
+spring brood. This female laid eggs the caterpillars from which fed and
+pupated. The pupae although kept through the summer in a hothouse all
+produced typical _bryoniae_, and none of these with one exception
+appeared until the next year, for in the alpine and arctic regions this
+species is only single-brooded. Weismann experimented also with a small
+vanessid butterfly, _Araschnia levana_, common on the European
+continent, though unknown in our islands, which is double (or at times
+treble) brooded, its spring form (_levana_) alternating with a larger
+and more brightly coloured summer form (_prorsa_). Here again by
+refrigerating the summer pupae, butterflies were reared most of which
+approached the winter pattern, but it was impossible by heating the
+winter pupae to change _levana_ into _prorsa_. Experiments with North
+American dimorphic species have given similar results. Weismann argued
+from these experiments that the winter form of these seasonally
+dimorphic species is in all cases the older, and that the butterflies
+developing within the summer pupae can be made to revert to the
+ancestral condition by repeating the low-temperature stimulus which
+always prevailed during the geologically recent Ice Age. On the other
+hand, a high temperature stimulus applied to one generation of the
+winter pupae cannot induce the change into the summer pattern, which has
+been evolved still more recently by slow stages, as the continental
+climate has become more genial. In tropical countries where instead of
+an alternation of winter and summer, alternate dry and rainy seasons
+prevail, somewhat similar seasonal dimorphism has been observed among
+many butterflies. Not a few forms of Precis, an African and Indian genus
+allied to our Vanessa, that had long been considered distinct species
+are now known, thanks to the researches of G.A.K. Marshall (1898), to be
+alternating seasonal forms of the same insect. The offspring when adult
+does not closely resemble the parent; its appearance is modified by the
+climatic environment of the pupa. The experiments of Weismann just
+sketched in outline show at least that the same principle holds for our
+northern butterflies.
+
+We are thus led to see from the life-story of such insects, that the
+course of the story is not rigidly fixed; the creature in its various
+stages is plastic, open to influence from its surroundings, capable of
+marked change in the course of generations. And so the seasonal changes
+in the history of the individual from egg to imago point us to changes
+in the age-long history of the race.
+
+
+
+
+CHAPTER IX
+
+PAST AND PRESENT; THE MEANING OF THE STORY
+
+
+In the previous chapter we recognised how the seasonal changes in
+various species of butterflies as observable in two or three
+generations, indicate changes in the history of the race as it might be
+traced through innumerable generations. The endless variety in the form
+and habits of insect-larvae and their adaptations to various modes of
+life, which have been briefly sketched in this little book, suggest
+vaster changes in the class of insects, as a whole, through the long
+periods of geological time. Every student of life, influenced by the
+teaching of Charles Darwin (1859) and his successors, now regards all
+groups of animals from the evolutionary standpoint, and believes that
+comparisons of facts of structure and life-history of orders and classes
+evidently akin to each other, furnish at least some indications of the
+course of development in the greater systematic divisions, even as the
+facts of seasonal dimorphism, mentioned in the last chapter, give hints
+as to the course of development in those restricted groups that we call
+species or varieties. A brief discussion of the main outlines of the
+life-story of insects in the wide, evolutionary sense may thus fitly
+conclude this book.
+
+In the first place we turn to the 'records' of those rocks, in whose
+stratified layers[12] are entombed remains, often fragmentary and
+obscure, of the insects of past ages of the earth's history. Compared
+with the thousands of extinct types of hard-shelled marine animals, such
+as the Mollusca, fossil insects are few, as could only be expected,
+seeing that insects are terrestrial and aerial creatures with slight
+chance of preservation in sediments formed under water. Yet a number of
+insect remains are now known to naturalists, who are, in this
+connection, more particularly indebted to the researches of S.H. Scudder
+(1885), C. Brongniart (1894), and A. Handlirsch (1906).
+
+[12] See Table of Geological Systems, p. 123.
+
+We are now considering insects from the standpoint of their
+life-histories, and the individual life-story of an insect of which we
+possess but a few fragments of wings or body, entombed in a rock formed
+possibly before the period of the Coal Measures, can only be a matter of
+inference. Still it may safely be inferred that when the structure of
+these remains clearly indicates affinity to some existing order or
+family, the life-history of the extinct creature must have resembled, on
+the whole, that of its nearest living allies. And all the fossil
+insects known can be either referred to existing orders, or shown to
+indicate definite relationship to some existing group.
+
+Passing over some doubtful remains of Silurian age, we find in rocks
+usually regarded as Devonian[13] the most ancient fossils that can be
+certainly referred to the insects, while from beds of the succeeding
+Carboniferous period, a number of insect remains have been disinterred.
+These Palaeozoic insects were frequently of large size, and they show
+distinct affinities with our recent may-flies, dragon-flies,
+stone-flies, and cockroaches. In the Permian period, the latest of the
+divisions of the Palaeozoic, lived Eugereon, an insect with hemipteroid
+jaws and orthopteroid wings. All these insects must have been
+exopterygote in their life-history, if we may trust the indications of
+affinity furnished by their structure. In the Mesozoic period, however,
+insects with complete transformations must have been fairly abundant.
+Rocks of Triassic age have yielded beetles and lacewing-flies, while
+from among Jurassic fossils specimens have been described as
+representing most of our existing orders, including Lepidoptera,
+Hymenoptera and Diptera. In Cainozoic rocks fossil insects of nearly six
+thousand species have been found, which are easily referable to
+existing families and often to existing genera. We may conclude then,
+imperfect though our knowledge of extinct insects is, that some of the
+most complex of insect life-stories were being worked out before the
+dawn of the Cainozoic era. Some instructive hints as to differences in
+the rate of change among different insect groups may be drawn from the
+study of parasites. For example, V.L. Kellogg (1913) points out that an
+identical species of the Mallophaga (Bird-lice) infests an Australian
+Cassowary and two of the South American Rheas; while two species of the
+same genus (Lipeurus) are common to the African Ostrich and a third kind
+of South American Rhea. These parasites must have been inherited
+unchanged by the various members of these three families of flightless
+birds from their common ancestors, that is from early Cainozoic times at
+latest. On the other hand, the various kinds of such highly specialised
+parasites as the warble-flies of the oxen and deer, must have become
+differentiated during those later stages of the Cainozoic period which
+witnessed the evolution of their respective mammalian hosts.
+
+[13] The 'Little River' beds of St John, New Brunswick, Canada, by some
+modern geologists however considered as Carboniferous.
+
+The foregoing brief outline of our knowledge of the geological
+succession of insects shows that the exopterygote preceded, in time, the
+endopterygote type of life-history. We have already seen that those
+insects undergoing little change in the life-cycle, and with visible,
+external wing-rudiments, are on the whole less specialised in structure
+than those which pass through a complete transformation. These two
+considerations, taken together, suggest strongly that in the evolution
+of the insect class, the simpler life-history preceded the more complex.
+Such a conclusion seems reasonable and what might have been expected,
+but we are confronted with the difficulty that if the most highly
+organised insects pass through the most profound transformations, then
+insects present a remarkable and puzzling exception to the general rules
+of development among animals, as has already been pointed out in the
+first chapter of this volume (p. 7). A few students of insect
+transformation have indeed supposed that the crawling caterpillar or
+maggot must be regarded as a larval stage which recalls the worm-like
+nature of the supposed far-off ancestors of insects generally. Even in
+Poulton's classical memoir (1891, p. 190), this view finds some support,
+and it may be hard to give up the seductive idea that the worm-like
+insect-larva has some phylogenetic meaning. But the weight of evidence,
+when we take a comprehensive survey of the life-story of insects, must
+be pronounced to be strongly in favour of the view put forward by Brauer
+(1869), and since supported by the great majority of naturalists who
+have discussed the subject, that the caterpillar or the maggot is itself
+a specialised product of the evolutionary process, adapted to its own
+particular mode of larval life.
+
+The explanation of insect transformation is, in brief, to be found in an
+increasing amount of divergence between larva and imago. The most
+profound metamorphosis is but a special type of growth, accompanied by
+successive castings and renewings of the chitinous cuticle, which
+envelopes all arthropods. In the simplest type of insect life-story,
+there is no marked difference in form between the newly-hatched young
+and the adult, and in such cases we find that the young insect lives in
+the same way as the adult, has the same surroundings, eats the same
+food. This is the rule (see Chapters II and III) with the Apterygota,
+the Orthoptera, and most of the Hemiptera. In the last-named order,
+however, we find in certain families marked divergence between larva and
+imago, for example in the cicads, whose larvae live underground, while
+in the coccids, whose males are highly specialised and females degraded,
+there succeeds to the larva--very like the young stage in allied
+families--a resting instar, which in the case of the male, suggests
+comparison with the pupa of a moth or beetle.
+
+Turning to the stone-flies, dragon-flies and may-flies, whose
+life-stories have been sketched in Chapter IV, we find that the early
+stages are passed in water, whence before the final moult, the insects
+emerge to the upper air. Except for the possession of tufted gills,
+adapting them to an aquatic life, the stone-fly nymphs differ but
+slightly from the adults; the grubs of the dragon-flies and may-flies,
+however, are markedly different from their parents. In connection with
+these comparisons, it is to be noted that the dragon-flies and may-flies
+are more highly specialised insects than stone-flies, divergent
+specialisation of the adult and larva is therefore well illustrated in
+these groups, which nevertheless have, like the Hemiptera and
+Orthoptera, visible external wing-rudiments.
+
+From the vast array of insects that show internal wing-growth and a true
+pupal stage, a few larval types were chosen for description in Chapter
+VI, and a review of these suggests again the thought of increasing
+divergence between larva and imago. Reference has been made previously
+to the many instances in which the former has become pre-eminently the
+feeding, and the latter the breeding stage in the life-cycle. It seems
+impossible to avoid the conclusion that the active, armoured
+campodeiform grub differing less from its parent than an eruciform larva
+differs from its parent, is as a larval type more primitive than the
+caterpillar or maggot. A. Lameere has indeed, while admitting the
+adaptive character of insect larvae generally, argued (1899) with much
+ingenuity that the eruciform or vermiform type must have been primitive
+among the Endopterygota, believing that the original environment of the
+larvae of the ancestral stock of all these insects must have been the
+interior of plant tissues. He is thus forced to the necessity of
+suggesting that the campodeiform larvae of ground-beetles or lacewings
+must be regarded as due to secondarily acquired adaptations; 'they
+resemble Thysanura and the larvae of Heterometabola only as whales
+resemble fishes.' There are two considerations which render these
+theories untenable. The Neuroptera and Coleoptera among which
+campodeiform larvae are common, are less specialised than Lepidoptera,
+Hymenoptera, and Diptera, in which they are unknown. And among the
+Coleoptera which as we have seen (pp. 50 _f._) display a most
+interesting variety of larval structure, the legless, eruciform larva
+characterises families in which the imago shows the greatest
+specialisation, while in the same life-story, as in the case of the
+oil-beetles (pp. 56-7), the newly-hatched grub may be campodeiform,
+changing to the eruciform type as soon as it finds itself within reach
+of its host's rich store of food.
+
+A certain amount of difficulty may be felt with regard to the theory of
+divergent evolution between imago and larva, in the case of those
+insects with complete transformation whose grubs and adults live in much
+the same conditions. By turning over stones the naturalist may find
+ground-beetles in company with the larvae of their own species. On the
+leaves of a willow tree he may observe leaf-beetles (Phyllodecta and
+Galerucella) together with their grubs, all greedily eating the foliage;
+or lady-bird beetles (Coccinella) and their larvae hunting and devouring
+the 'greenfly.' All of these insects are, however, Coleoptera, and the
+adult insects of this order are much more disposed to walk and crawl and
+less disposed to fly than other endopterygote insects. Their heavily
+armoured bodies and their firm shield-like forewings render them less
+aerial than other insects; in many genera the power of flight has been
+altogether lost. It is not surprising, therefore, that many beetles,
+even when adult, should live as their larvae do; since the acquirement
+of complete metamorphosis they have become modified towards the larval
+condition, and an extreme case of such modification is afforded by the
+wingless grub-like female Glow-worm (Lampyris).
+
+With most insects, however, the larva must be regarded as the more
+specially modified, even if degraded, stage. Miall (1895) has pointed
+out that the insect grub is not a precociously hatched embryo, like the
+larvae of multitudes of marine animals, but that it exhibits in a
+modified form the essential characters of the adult. Comparison for
+example can be readily made between the parts of the caterpillar and the
+butterfly, whose story was sketched in the first chapter of this book,
+widely different though caterpillar and butterfly may appear at a
+superficial glance. And the survey of variety in form, food, and habit
+of insect larvae given in Chapter VI enforces surely the conclusion that
+the larva is eminently plastic, adaptable, capable of changing so as to
+suit the most diverse surroundings. In a most suggestive recent
+discussion on the transformation of insects P. Deegener (1909) has
+claimed that the larva must be regarded as the more modified stage,
+because while all the adult's structures are represented in the larva,
+even if only as imaginal buds, there are commonly present in the larva
+special adaptive organs not found in the imago, for example the pro-legs
+of caterpillars or the skin-gills of midge-grubs. The correspondence of
+parts in butterfly and caterpillar just referred to, may still be
+traced, though less easily, in bluebottle and maggot. The latter is an
+extreme example of degenerative evolution, and its contrast with the
+elaborately organised two-winged fly marks the greatest divergence
+observable between the larva and imago. With this divergence the resting
+pupal stage, during which more or less dissolution and reconstruction of
+organs goes on, becomes a necessity, and it has already been pointed out
+how the amount of this reconstruction is greatest where the divergence
+between the larval and perfect stages is most marked. Whatever
+differences of opinion may prevail on points of detail, the general
+explanation of insect metamorphosis as the result of divergent evolution
+in the two active stages of the life-story must assuredly be accepted.
+No other explanation accords with the increasing degree of divergence to
+be observed as we pass from the lower to the higher insect orders.
+
+The successive incidents of the life-story of most insects are largely
+connected with the acquisition of wings. Wings, and the power of flight
+wherewith they endow their possessors, are evidently beneficial to the
+race in giving power of extending the range during the breeding period
+and thus ensuring a wide distribution of the eggs. In no case are wings
+fully developed until the closing stage of the insect's life, they are
+always acquired after hatching or birth. We have already noticed (p. 40)
+how Sharp (1899) has laid stress on the essential difference between the
+exopterygote and endopterygote insects, the wing-rudiments of the former
+growing outwards throughout life while those of the latter remain hidden
+until the pupal instar. Sharp considers that there is some difficulty in
+bridging, in thought, the gap between these two methods of wing-growth,
+and has put forward an ingenious suggestion to meet it (1902). Reference
+has already been made to insects of various orders in which one sex is
+wingless, the Vapourer Moth (p. 96) for example, or all the individuals
+of both sexes are wingless, as the aberrant cockroaches mentioned in
+Chapter II (p. 15), or certain generations of virgin females are
+wingless, for example aphids (pp. 18-19) and gall-flies (pp. 94-5).
+Insects may thus become secondarily wingless, that is to say be
+manifestly the offspring of winged parents, and such wingless forms may
+on the other hand give rise to offspring or descendants with
+well-developed wings. Frequently, as in the case of the aphids, many
+wingless generations intervene between two winged generations. A
+striking illustration of this fact is afforded by an aquatic bug, _Velia
+currens_, commonly to be seen skating over the surface of running water.
+The adults of Velia are nearly always wingless, but now and then the
+naturalist meets with a specimen provided with functional wings, the
+possession of which enables the insect to make its way to a fresh
+stream. Moreover there are whole orders of parasitic insects, such as
+the lice and fleas, which, showing clear affinity to orders of winged
+insects, are believed to be secondarily wingless. These orders are
+designated by Sharp 'Anapterygota.' And from the analogy of the periodic
+loss and recovery of wings in various generations of the same species,
+he has concluded that the gap between the exopterygote and the
+endopterygote method of development may have been bridged by an
+anapterygote condition; that the ancestors of those insects with
+complete transformations were the wingless descendants of primitive
+insects which grew their wings from visible external rudiments, and
+that in later times re-acquiring wings, they developed these organs in a
+new way, from inwardly directed rudiments or imaginal buds.
+
+This theory of Sharp's is original, daring, and ingenious, but the loss
+and re-acquisition of wings which it presupposes is difficult to imagine
+in large groups during a prolonged evolutionary history, while the
+sudden appearance of a totally new mode of wing-growth in the offspring
+of wingless insects would be an extreme example of discontinuity in
+development.
+
+On the whole the most probable suggestion which can be made as to the
+origin of 'complete' transformation in insects is that the instar in
+which wings were first visible externally became later and later in the
+course of the evolution of the more highly organised groups. In this way
+a gradual transition from the exopterygote to the endopterygote type of
+life-story is at least conceivable. It will be remembered that a may-fly
+(p. 33) undergoes a moult after acquiring functional wings, emerging
+into the air as a 'sub-imago.' In not a few endopterygote insects, the
+pupa shows more or less activity, swimming through water intermittently
+(gnats) or just before the imago has to emerge (caddis-flies); working
+its way out of the ground (crane-flies) or coming half-way out of its
+cocoon (many moths). The pupa of the higher insects almost certainly
+corresponds with the may-fly's sub-imago, and the facts just recalled as
+to remnants of pupal activity suggest that in the ancestors of
+endopterygote insects what is now the pupal instar was represented by an
+active nymphal or sub-imaginal stage, possibly indeed by more than one
+stage, as Packard and other writers have stated that pupae of bees and
+wasps undergo two or three moults before the final exposure of the
+imago. Such an early pupal instar has been defined as a 'pro-nymph' or a
+'semi-pupa.' Examples have been given of the exceptional passive
+condition of the penultimate instar in Exopterygota. The instars
+preceding this presumably had originally outward wing-rudiments in all
+insect life-histories, and the endopterygote condition was attained by
+the postponement of the outward appearance of these to successively
+later stages. The leg and wing rudiments of the male coccid (pp. 20-1)
+beneath the cuticle of the second instar are strictly comparable to
+imaginal buds, and these are present in one instar of what is generally
+regarded as an exopterygote life-history. The first instar in all
+insects has no visible wing-rudiments, but when they grow outwardly from
+the body, they necessarily become covered with cuticle, so that they
+must be visible after the first moult. There is no supreme difficulty in
+supposing that the important change was for these early rudiments to
+become sunk into the body, so that the cuticle of the second, and,
+later, of the third and succeeding instars, showed no outward sign of
+their presence. This suggestion is confirmed by Heymons' (1896, 1907)
+observation of the occasional appearance of outward wing-rudiments on
+the thoracic segments of a mealworm, the larva of the beetle _Tenebrio
+molitor_, and by F. Silvestri's discovery (1905) of a 'pro-nymph' stage
+with short external wing-rudiments between the second larval and the
+pupal instars of the small ground-beetle _Lebia scapularis_. Whatever
+may be the exact explanation of these abnormalities, they show that in
+the life-story of the higher insects outward wing-rudiments may even yet
+appear before the pupal stage, confirming our belief that such
+appearance is an ancestral character. The inward growth of these
+wing-rudiments may well have been correlated with a difference in form
+between the newly-hatched insect and its parent. As this difference
+persisted until a constantly later stage, and the pre-imaginal instar
+became necessarily a stage for reconstruction, the present condition of
+complete metamorphosis in the more highly organised orders was finally
+attained.
+
+To explain satisfactorily these complex life-stories is however
+admittedly a difficult task. The acquisition of wings is, as we have
+seen, a dominating feature in them all, but if we try to go yet a step
+farther back and speculate on the origin of wings in the most primitive
+exopterygote insects, the task becomes still more difficult. Many years
+ago Gegenbaur (1878) was struck by the correspondence of insect wings to
+the tracheal gills of may-fly larvae, which are carried on the abdominal
+segments somewhat as wings are on the thoracic segments. But B�rner has
+recently (1909) brought forward evidence that these abdominal gills
+really correspond serially with legs. Moreover Gegenbaur's theory
+suggests that the ancestral insects were aquatic, whereas the presence
+of tubes for breathing atmospheric air in well-nigh all members of the
+class, and the fact that aquatic adaptations, respiratory and otherwise,
+in insect-larvae are secondary force the student to regard the ancestral
+insects as terrestrial. It is indeed highly probable that insects had a
+common origin with aquatic Crustacea, but all the evidence points to the
+ancestors of insects having become breathers of atmospheric air before
+they acquired wings. How the wings arose, what function their precursors
+performed before they became capable of supporting flight, we can hardly
+even guess.
+
+Our study of the life-story of insects, therefore, while it has taught
+us something of what is going on around us to-day, and has given us
+hints of the course of a few threads of that long life-story which runs
+through the ages, brings us face to face with the most instructive, if
+humbling fact that 'there are many more things of which we are
+ignorant.' The passage from creeping to flight, as the caterpillar
+becomes transformed into the butterfly, was a mystery to those who first
+observed it, and many of its aspects remain mysterious still. Perhaps
+the most striking result of the study of insect transformation is the
+appreciation of the divergent specialisation of larva and imago, and it
+is a suggestive thought that of the two the larva has in many cases
+diverged the more from the typical condition. The caterpillar crawling
+over the leaf, or the fly-grub swimming through the water, may thus be
+regarded as a creature preparing for a change to the true conditions of
+its life. It is a strange irony that the preparation is often far longer
+than the brief hours of achievement. But the light which research has
+thrown on the nature of these wonderful life-stories, the demonstration
+of the unseen presence and growth within the insect, during its time of
+preparation among strange surroundings, of the organs required for
+service in the coming life amid its native air, confirm surely the
+intuition of the old-time students, who saw in these changes, so
+familiar and yet so wonderful, a parable and a prophecy of the higher
+nature of man.
+
+
+
+
+OUTLINE CLASSIFICATION OF INSECTS
+
+
+Class INSECTA or HEXAPODA.
+
+Sub-class A, APTERYGOTA.
+
+Order 1. _Thysanura_ (Bristle-tails).
+      2. _Collembola_ (Spring-tails).
+
+Sub-class B, EXOPTERYGOTA.
+
+Order 1. _Dermaptera_ (Earwigs).
+      2. _Orthoptera_ (Cockroaches, Grasshoppers, Crickets).
+      3. _Plecoptera_ (Stone-flies).
+      4. _Isoptera_ (Termites or 'White Ants').
+      5. _Corrodentia_
+         (_a_) _Copeognatha_ (Book-lice).
+         (_b_) _Mallophaga_ (Biting-lice).
+      6. _Ephemeroptera_ (May-flies).
+      7. _Odonata_ (Dragon-flies).
+      8. _Thysanoptera_ (Thrips).
+      9. _Hemiptera_
+         (_a_) _Heteroptera_ (Bugs, Pond-skaters)
+         (_b_) _Homoptera_ (Cicads, 'Greenfly,' Scales).
+     10. _Anoplura_ (Lice).
+
+Sub-class C, ENDOPTERYGOTA.
+
+Order 1. _Neuroptera_ (Alder-flies, Ant-lions, Lacewings).
+      2. _Coleoptera_ (Beetles).
+      3. _Mecaptera_ (Scorpion-flies).
+      4. _Trichoptera_ (Caddis-flies).
+      5. _Lepidoptera_ (Moths and Butterflies).
+      6. _Diptera_ (Two-winged flies)
+         (_a_) _Orthorrhapha_ (Crane-flies, Midges, Gnats)
+         (_b_) _Cyclorrhapha_ (Hover-flies, House-flies, Bot-flies, &c).
+      7. _Siphonaptera_ (Fleas).
+      8. _Hymenoptera_
+         (_a_) _Symphyta_ (Saw-flies)
+         (_b_) _Apocrita_ (Gall-flies, Ichneumon-flies, Wasps, Bees, Ants).
+
+
+
+
+TABLE OF GEOLOGICAL SYSTEMS
+
+
+These names, given by geologists to the various divisions of rocks, as
+indicated by the fossils entombed in them, are arranged in 'descending'
+order, the more recent formations above, the more ancient below, as
+newer deposits necessarily lie over older beds.
+
+CALNOZOIC OR TERTIARY GROUP.
+
+Pleistocene.
+Pliocene.
+Miocene.
+Eocene.
+
+
+MESOZOIC OR SECONDARY GROUP.
+
+Cretaceous.
+Jurassic.
+Triassic.
+
+
+PALAEOZOIC OR PRIMARY GROUP.
+
+Permian.
+Carboniferous.
+Devonian.
+Silurian.
+Cambrian.
+
+
+
+
+BIBLIOGRAPHY
+
+
+The following list of some books and papers, referred to in this little
+volume or of especial service to the author in its preparation, is
+needless to say very far from exhaustive. To save space, titles are
+often abbreviated. Most of the works in the general list (A) contain
+extensive lists of literature on insects and their transformations,
+these should be consulted by the serious student.
+
+
+A. GENERAL WORKS.
+
+1909.     C. B�rner. Die Verwandlungen der Insekten. _Sitzb. d. Gesellsch.
+          naturforsch. Freunde, Berlin._
+
+1869.     F. Brauer. Betrachtung �ber die Verwandlung der Insekten.
+          _Verhandl. der K.K. zool.-bot. Gesellschaft in Wien._ XIX.
+
+1899.     G.H. Carpenter. Insects, their Structure and Life. London.
+
+1859.     C. Darwin. The Origin of Species. London.
+
+1909.     P. Deegener. Die Metamorphose der Insekten. Leipzig.
+
+1906.     J.W. Folsom. Entomology. London.
+
+1878.     C. Gegenbaur. Grundriss der Vergleichende Anatomie. Leipzig.
+
+1906.     A. Handlirsch. Die fossilen Insekten. Leipzig.
+
+1904.     L.F. Henneguy. Les Insectes. Paris.
+
+1907.     R. Heymons. Die verschiedenen Formen der Insectenmetamorphose.
+          _Ergebnisse der Zoologie._ I.
+
+1899.     A. Lameere. La raison d'�tre des Metamorphoses chez les Insectes.
+          _Ann. Soc. Entom. Bruxelles._ XLIII.
+
+1874.     J. Lubbock. The Origin and Metamorphoses of Insects. London.
+
+1895.     L.C. Miall. (_a_) The Transformations of Insects. _Nature._ LIII.
+
+1895.     ---- (_b_) The Natural History of Aquatic Insects. London.
+
+1908.     ---- Injurious and Useful Insects. 2nd edition. London.
+
+1839.     G. Newport. Insects. _Todd Cyclopaedia._ II. London.
+
+1898.     A.S. Packard. Text book of Entomology. New York.
+
+1734-42.  R.A.F. de R�aumur. M�moires pour servir � l'Histoire naturelle
+          et � l'anatomie des Insectes. Paris.
+
+1895-8.   D. Sharp. The Cambridge Natural History, V, VI. London.
+
+1899.     ---- Some points in the Classification of Insects. IV. _Internat.
+          Zoolog. Congress._
+
+1902.     ---- Insects in _Encycl. Brit._ 10th Edition, XXIX. London.
+
+1910.     ---- and G.H. Carpenter. Hexapoda in _Encycl. Brit._ 11th
+          Edition. Cambridge.
+
+1737.     J. Swammerdam. Biblia Naturae. Leyden (incorporates works on
+          Insects published during the author's lifetime 1669-75).
+
+1909.     F.V. Theobald. Insect Pests of Fruit. Wye.
+
+
+B. SPECIAL WORKS.
+
+1881.     H. Adler. Ueber den Generationswechsel den Eichen-Gallwespen.
+          _Zeitsch. f. wissensch. Zoologie._ XXXV.
+
+1896.     ---- and C.R. Straton. Alternating Generations. Oxford.
+
+1902.     J. Anglas. Nouvelles Observations sur les M�tamorphoses Internes.
+          _Arch. d'Anat. Microscop._ IV.
+
+1911.     E.E. Austen. Handbook of the Tsetse-Flies. London (Brit. Museum).
+
+1909.     F. Balfour-Browne. Life-History of Agrionid Dragonfly. _Proc.
+          Zool. Soc. Lond._
+
+1893, &c. C.G. Barrett. Lepidoptera of the British Islands. London.
+
+1890.     H. Beaur�gard. Les Insectes V�sicants. Paris.
+
+1909.     C. B�rner. Die Tracheenkiemen der Ephemeriden. _Zoolog. Anz._
+          xxxiii.
+
+1863.     F. Brauer. Monographie der Oestriden. Wien.
+
+1894.     C. Brongniart. R�cherches pour servir � l'histoire des Insectes
+          fossiles des Temps Primaires. St Etienne.
+
+1893.     T.A. Chapman. Structure of Pupae of Heterocerous Lepidoptera.
+          _Trans. Entom. Soc. Lond._
+
+1891.     H. Dewitz. Das geschlossene Tracheensystem bei Insektenlarven.
+          _Zoolog. Anz._ xiii.
+
+1857-8.   J.H. Fabre. L'Hyperm�tamorphose et les Moeurs des Meloides.
+          _Ann. Sci. Nat._ (_Zool._), (4). VII. IX.
+
+1869.     M. Ganin. Die Entwicklungsgeschichte bei den Insekten. _Zeitsch.
+          f. wissensch. Zoolog._ xix.
+
+1894.     J. Gonin. La M�tamorphose des Lepidopt�res. _Bull. Soc. Vaud.
+          Sci. Nat._ xxx.
+
+1870.     O. Grimm. Die ungeschechtliche Fortpflanzung einer Chironomus.
+          _Mem. Acad. Imp�r. St P�tersbourg_ (7). xv.
+
+1890.     W. Hatchett-Jackson. Morphology of the Lepidoptera. _Trans. Linn.
+          Soc. (Zool.) Lond._ (2). v.
+
+1896.     R. Heymons. Fl�gelbildung bei der Larve von Tenebrio molitor.
+          _Sitzb. d, Gesellsch. Naturforsch. Freunde, Berlin._
+
+1906.     ---- Ueber die ersten Jugendformen von Machilis alternata. _Ib._
+
+1908.     W. Kahle. Die Paedogenesis der Cecidomyiden. _Zoologica._ IV.
+
+1913.     V.L. Kellogg. Distribution and Species-forming of Ectoparasites.
+          _Amer. Naturalist._ XLVII.
+
+1887.     A. Kowalevsky. Die nachembryonale Entwicklung der Musciden.
+          _Zeitsch. f. wissensch. Zool._ XLV.
+
+1904.     O.H. Latter. Natural History of Common Animals (chaps. III, IV,
+          V). Cambridge.
+
+1890-95.  B.T. Lowne. The Blowfly, 2 vols. London.
+
+1863.     J. Lubbock. Development of Chloeon. _Trans. Linn. Soc. Lond._
+          XXIII.
+
+1762.     P. Lyonet. Trait� anatomique de la Chenille. Haag.
+
+1669.     M. Malpighi. De Bombyce. London.
+
+1898.     C.L. Marlatt. The periodical Cicada. _Entom. Bull._ 14, _U.S.
+          Dept. Agric._
+
+1898.     G.A.K. Marshall. Seasonal Dimorphism in Butterflies. _Ann. Mag.
+          Nat. Hist._ (7). II.
+
+1900.     L.C. Miall and A.B. Hammond. The Harlequin Fly. Oxford.
+
+1901-3.   R. Newstead. Coccidae of the British Isles. London.
+
+1877.     J.A. Palm�n. Zur Morphologie des Tracheensystems. Leipzig.
+
+1891.     E.B. Poulton. External Morphology of the Lepidopterous Pupa.
+          _Trans. Linn. Soc. Zool._ (2). V.
+
+1892.     ---- Colour-relation between Lepidopterous Larvae &c. and their
+          surroundings. _Trans. Entom. Soc. Lond._
+
+1880.     C.V. Riley. Pupation of Butterflies. _Proc. Amer. Assoc._ XXVIII.
+
+1902.     E.D. Sanderson. Report of Entomologist. Delaware. U.S.A.
+
+1885.     E.O. Schmidt. Metamorphose und Anatomie des m�nnlichen
+          Aspidiotus. _Archiv f. Naturgeschichte._ LI.
+
+1885.     S.H. Scudder. Insekten in Zittel's Paleontologie. II.
+
+1907.     A.J. Siltala. Die postembryonale Entwicklung der
+          Trichopteren-Larven. _Zoolog. Jahrb. Suppl._ IX.
+
+1905.     F. Silvestri. Metamorfosi e Costumi della Lebia scapularis.
+          _Redia._ II.
+
+1900.     J.B. Smith. The Apple Plant-louse. _New Jersey Agric. Exp.
+          Station Bull._ 143.
+
+1888.     J. Van Rees. Die innere Metamorphose von Musca. _Zoolog. Jahrb.
+          Anat._ III.
+
+1911.     K.W. Verhoeff. Ueber Felsenspringer, Machiloidea. _Zoolog. Anz._
+          XXXVIII.
+
+1865.     N. Wagner. Die viviparen Gallm�ckenlarven. _Zeitsch. f.
+          wissensch. Zoolog._ XV.
+
+1901.     E. Wasmann. Termitoxenia. _Zeitsch. f. wissensch. Zoolog._ LXX.
+
+1864.     A. Weismann. Die nachembryonale Entwicklung der Musciden.
+          _Zeitsch. f. wissensch. Zoolog._ XIV.
+
+1865.     ---- Die Metamorphose von Corethra. _Ib._ XVI.
+
+1876.     ---- Studien zur Descendenz-Theorie. Leipzig. (English
+          Translation by R. Meldola, London, 1882.)
+
+
+
+
+INDEX
+
+
+_Abraxas grossulariata_, 60, 83, 97-8
+
+Adaptation of larvae, 57, 79, 114
+
+Adephaga, 51
+
+Adler, H., 94
+
+Aeschnidae, 27, 29, 31
+
+Agrionidae, 27, 28
+
+_Agrotis segetum_, 98
+
+Air-tubes, 2, 11, 23, 47, 70, 77, 87, 120
+
+Alternation of generations, 17, 94
+
+Ametabola, 11, 35
+
+Anapterygota, 116
+
+Anglas, J., 46
+
+Ant-lions, 57
+
+Ants, 64, 66
+
+Aphidae, 17-20, 116
+
+_Aphis pomi_, 18-19
+
+Aphis-lion, 57
+
+Apterygota, 41, 110
+
+Aquatic insects, 23-34, 76-9, 120
+
+_Araschnia levana_ and var. _prorsa_, 103
+
+_Arctia caia_, 98
+
+Arctiadae, 59
+
+Arthropoda, 9
+
+Austen, E.E., 91
+
+Avebury, Lord, _see_ Lubbock, J.
+
+
+Balfour-Browne, F., 28
+
+Bark-beetles, 55
+
+Barrett, C.G., 96, 99
+
+Beaur�gard, H., 56
+
+Bees, 40, 46, 64, 83
+
+Beetles, 40, 50-7, 80, 107, 112-3, 119
+
+Bell Moths, 62
+
+Bird-lice, 108
+
+Birth, 18, 91
+
+_Blatta orientalis_, 15
+
+Blister-beetles, 56
+
+Blowfly or Bluebottle, 43, 44, 46, 67, 71-3, 93, 114
+
+B�rner, C., 32, 120
+
+Bot-flies, 73-4, 89, 91
+
+Brain, 44
+
+Brauer, F., 6, 52, 56, 67, 109
+
+Bristle-tails, 11
+
+Brongniart, C., 106
+
+Butterflies, 1, 83, 95-6, 114
+
+
+Cabbage-butterflies, 39, 41, 85, 100-1
+
+Cabbage-fly, 73
+
+Caddis-flies, 62-3, 86, 117
+
+Cainozoic insects, 107
+
+Calliphora, 43.
+  _See also_ Blowfly
+
+Campodeiform larvae, 52, 56, 111
+
+Carabidae, 52
+
+Carboniferous insects, 107
+
+_Carpocapsa pomonella_, 99-100
+
+Carrion-beetles, 50
+
+Caterpillar, 4, 36, 49, 58-62, 95-101, 109, 114
+
+Cecidomyidae, 68-70, 90
+
+Cerambycidae, 55
+
+Cercopods, 12, 15
+
+Chafers, 52
+
+Chapman, T.A., 81, 84
+
+Chironomus, 43, 77, 87, 91
+
+Chloeon, 33
+
+Chrysalis, 82.
+  _See also_ Pupa
+
+Chrysomelidae, 53.
+  _See also_ Leaf-beetles
+
+Chrysopa, 57
+
+Cicads, 22, 93, 110
+
+Classification, 122
+
+Clearwing Moths, 62
+
+Click-beetles, 52, 93
+
+Clothes-moths, 62
+
+Coccidae, 20, 110, 118
+
+Coccinella, 113
+
+Cockroaches, 11, 14, 15, 107, 115
+
+Cocoons, 82
+
+Codling Moth, 62, 99
+
+Coleoptera, 50-6, 80, 112, 119
+
+Collembola, 11
+
+Complete transformation, 35, 107, 119.
+  _See also_ Endopterygota
+
+Corethra, 43
+
+Cossus, 38, 62, 82, 95
+
+Crane-flies, 67, 70, 93, 117
+
+Cremaster, 83
+
+Crustacea, 7, 120
+
+Culex, 43, 77, 86
+
+Curculionidae, 55
+
+Cuticle, 2, 9, 29, 37, 40, 50, 81, 87, 110
+
+Cynipidae, 94.
+  _See also_ Gall-flies
+
+
+Daddy-long-legs, 69-70
+
+Darwin, C., 105
+
+Deegener, P., 6, 114
+
+Devonian insects, 107
+
+Dewitz, H., 28
+
+Digestive system, 10, 45-7
+
+_Diplosis pyrivora_, 70
+
+Diptera, 42, 64, 67-79, 81, 86-8, 91, 94, 107
+
+Divergence between larva and imago, 110, 114, 121
+
+Double-brooded Lepidoptera, 95, 100-4
+
+Dragon-flies, 26-31, 107, 110
+
+Drone-flies, 76
+
+Duration of life, 34, 89, 92-3, 95
+
+Dyticus, 51
+
+
+Ecdysis, 10.
+  _See also_ Moult
+
+Ectoderm, 9, 11, 47
+
+Eggar Moths, 59, 89
+
+Eggs, 6, 17-18, 26, 34, 65-7, 71, 90, 94-5, 97
+
+Elateridae, 52
+
+Endopterygota, 41, 49, 108, 112, 115-6
+
+Ephemeroptera, 24.
+  _See also_ May-flies
+
+Epidermis, 9, 40
+
+Eristalis, 76
+
+Eruciform larvae, 56, 58-70, 111
+
+Evolution, 16, 103, 105-21
+
+Exopterygota, 41, 108, 115-6, 118
+
+Exoskeleton, 9
+
+
+Fabre, J.H., 56
+
+Fat-body, 47
+
+Feeding-period, 27, 32, 36, 89, 111
+
+Feelers, 1, 4, 42, 71
+
+Fleas, 116
+
+Fore-gut, 47
+
+Free pupa, 80
+
+
+Gall-flies, 64-6, 94, 115
+
+Gall-midges, 68-70, 90
+
+Ganin, M., 66
+
+_Gastrophilus equi_, 73-4
+
+Gegenbaur, C., 120
+
+Geological history, 106-8, 123
+
+Geometridae, 59
+
+Gills, 24, 27, 32, 78, 87, 114, 120
+
+Glossinia, 91
+
+Glow-worm, 50, 113
+
+Gnats, 43, 77, 86
+
+Goat Moth, 38, 62, 82, 95
+
+Gonin, J., 38, 41
+
+Grasshoppers, 11, 14, 15
+
+Grimm, O., 90
+
+Ground-beetles, 52, 112
+
+Growth, 9
+
+Grub, 63-70.
+  _See also_ Caterpillar, Larva
+
+
+Hairs, 59, 82, 98
+
+Hammond, A.R., 43, 77, 87
+
+Handlirsch, A., 106
+
+Harvey, William, 7
+
+Hatchett-Jackson, W., 83
+
+Hawk Moths, 60
+
+Heart, 45
+
+Helodes, 50
+
+Hemerobius, 57
+
+Hemimetabola, 35
+
+Hemiptera, 17, 110
+
+Henneguy, L.F., 45, 48
+
+Heymons, R., 6, 11, 119
+
+Hibernation. _See_ Wintering stages
+
+Hind-gut, 47
+
+Hippoboscidae, 91
+
+Histogenesis and Histolysis, 48
+
+Holometabola, 35
+
+House-fly, 67, 71, 73
+
+Hover-flies, 74-6
+
+Hymenoptera, 58, 64, 94, 107
+
+Hypermetamorphosis, 56
+
+_Hypoderma bovis_, 73-5
+
+Hypodermis, 9
+
+
+Ichneumon-flies, 64, 66, 82
+
+Imaginal buds or discs, 34-48, 114, 117-8
+
+Imago, 24, 34, 114
+
+Instar, 13, 33, 56, 117-9
+
+
+Jaws of imago and larva, 2, 4, 5, 32, 42, 89
+
+Jurassic insects, 107
+
+
+Kahle, W., 90
+
+Kellogg, V.L., 108
+
+Kowalevsky, A., 46
+
+
+Labium, 2, 27
+
+Lacewing-flies, 57, 107
+
+Ladybirds, 113
+
+Lameere, A., 111
+
+Lampyris, 113
+
+Larva, 4, 22, 26-7, 32, 49-79, 110-15
+
+Larval reproduction, 90
+
+Lasiocampidae, 59, 89
+
+Latter, O.H., 28
+
+Leaf-beetles, 53, 83, 92-3, 113
+
+_Lebia scapularis_, 119
+
+Lepidoptera, 1, 36, 38, 49, 58, 81, 95-104, 107
+
+Libellulidae, 27
+
+Lice, 116
+
+Lipeurus, 108
+
+Longhorn Beetles, 55
+
+Looper caterpillars, 59, 61
+
+Lowne, B.T., 42
+
+Lubbock, J., 6, 32
+
+Lymantriidae, 90
+
+Lyonet, P., 38
+
+
+Machilis, 11
+
+Maggot, 44, 67, 71-6, 109, 114
+
+Magpie Moth, 60, 82, 97-8
+
+Mallophaga, 108
+
+Mandibles, 4, 17, 26, 58, 67, 86
+
+Mangel-fly, 73
+
+Marlatt, C.L., 93
+
+Marshall, G.A.K., 104
+
+Maxillae, 2, 17, 37, 42
+
+May-flies, 31-4, 107, 110, 117, 120
+
+Meloidae, 56
+
+Mesozoic insects, 107
+
+Metabola, 35
+
+Metamorphosis (in general), 6, 109;
+  (degrees of in insects) 8, 35, 109, 117-19
+
+Miall, L.C., 6, 28, 33, 43, 77, 78, 87, 97, 113
+
+Mosquito. _See_ Culex, Gnats
+
+Moths, 1, 58-62, 84, 95-100, 117
+
+Moult, 10, 32, 36, 41
+
+_Musca domestica_, 71
+
+Muscidae, 44
+
+Muscles, 47
+
+
+Nervous system, 44-5
+
+Neuroptera, 57, 80, 112
+
+Newport, G., 41, 44
+
+Noctuidae, 60, 98
+
+Nymph, 15, 28, 33
+
+
+Oak-apples, 94
+
+Obtect pupa, 81
+
+Odonata, 24.
+  _See also_ Dragon-flies
+
+_Oestrus ovis_, 91
+
+Oil-beetles, 56, 112
+
+_Orgyia antiqua_, 96-7
+
+Orthoptera, 17, 35, 110
+
+Owl Moths, 60, 98
+
+
+Packard, A.S., 56, 118
+
+Paedogenesis. _See_ Larval reproduction
+
+Painted Lady Butterfly, 96
+
+Palaeozoic insects, 107
+
+Palm�n, J.A., 25
+
+Parasitic insects, 73-4, 108, 116
+
+Parental care, 64-6
+
+Parthenogenesis, 18
+
+Partial transformation, 35, 37
+
+Perla, 24
+
+Permian insects, 107
+
+Phagocytes, 48
+
+Phyllodecta, 53, 113
+
+Phyllotreta, 53
+
+_Pieris brassicae_, 39, 41, 85, 100
+
+_Pieris napi_ and var. _bryoniae_, 102-3
+
+Platygaster, 66
+
+Plecoptera, 24.
+  _See also_ Stone-flies
+
+Pompilidae, 66-7
+
+Poulton, E.B., 61, 82, 109
+
+Precis, 104
+
+Proctotrypidae, 66
+
+Pro-legs, 4, 58-9, 84, 114
+
+Pro-nymph, 118, 119
+
+Protective coloration, 60-1
+
+_Psylliodes chrysocephala_, 54
+
+Ptinidae, 54
+
+Pupa, 4, 37, 40, 79-88, 114, 117
+
+Puparium, 88
+
+Pupipara, 91
+
+_Pyrameis cardui_, 96
+
+
+Rat-tailed maggot, 76
+
+R�aumur, R.A.F. de, 8, 28, 33, 41
+
+Reproductive larvae, 90;
+  pupae, 91
+
+Reproductive organs, 45
+
+_Rhabdophaga heterobia_, 70
+
+Riley, C.V., 83
+
+
+Sanderson, E.D., 17
+
+Sand-midges, 78
+
+Sarcophaga, 91
+
+Saw-flies, 58-9
+
+Scale-insects, 20.
+  _See also_ Coccidae
+
+Scarabaeidae, 52
+
+Schmidt, E.O., 21
+
+Scolytidae, 55
+
+Scudder, S.H., 106
+
+Seasonal changes, 89-104
+
+Seasonal dimorphism, 102
+
+Semi-pupa, 118
+
+Sesiidae, 62
+
+Sexual differences, 15, 20-1, 90
+
+Sharp, D., 13, 36, 40, 115
+
+Silk-spinning, 58, 62-3, 82
+
+Silkworms, 82
+
+Silpha, 50
+
+Siltala, A.J., 63
+
+Silvestri, F., 119
+
+Simulium, 78, 87
+
+Smith, J.B., 17
+
+Sphegidae, 66-7
+
+Sphingidae, 60
+
+Spinneret, 58
+
+Spiracles, 2, 23, 70, 72, 77, 86, 87
+
+Spring-tails, 11
+
+Stone-flies, 24, 107, 110
+
+Sub-imago, 33, 117
+
+Sucking insects, 17
+
+Swammerdam, J., 33
+
+Syrphus, 74-6
+
+
+Tachininae, 73, 91
+
+_Tenebrio molitor_, 119
+
+Termitoxeniidae, 92
+
+Theobald, F.V., 100
+
+Thysanura, 11
+
+Tiger Moths, 59, 82, 98
+
+Timber-beetles, 54
+
+Tineidae, 62
+
+Tipulidae, 70
+
+Tortoiseshell Butterfly, 45, 95
+
+Tortricidae, 62
+
+Tracheal system. _See_ Air-tubes, Spiracles
+
+Transformation. _See_ Metamorphosis
+
+Triassic insects, 107
+
+Trichocera, 70
+
+Trichoptera, 62-3, 76, 80, 86
+
+Tsetse Flies, 91
+
+Turnip-fly, 53, 92, 94
+
+Turnip Moth, 98-9
+
+Tussock Moths, 90, 97
+
+
+_Vanessa urticae_, 45, 95
+
+Van Rees, J., 42
+
+Vapourer Moth, 96-7, 115
+
+_Velia currens_, 116
+
+Verhoeff, K.W., 11
+
+Vermiculiform larvae, 67, 71-6, 111
+
+Virgin stem-mothers, 18
+
+Viviparous reproduction. _See_ Birth
+
+
+Wagner, N., 90
+
+Warble-fly, 73-4, 89, 108
+
+Warning coloration, 60
+
+Wasmann, E., 92
+
+Wasps, 46, 64, 66-7, 83
+
+Water-insects. _See_ Aquatic insects
+
+Weevils, 55
+
+Weismann, A., 38, 42, 102
+
+White Butterflies, 41, 83, 85, 100-3
+
+Willow-beetles, 53
+
+Wingless insects, 15, 18, 20, 96, 115
+
+Wing-rudiments, 13, 18, 20, 22, 24, 28, 33, 36-8, 40, 111, 115, 117-19
+
+Wings, 1, 14, 115, 119-20
+
+Winter broods, 102-3
+
+Wintering stages, 93-101
+
+Wireworms, 52, 93
+
+Wood-wasps, 65
+
+
+
+
+CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS
+
+
+
+
+                                 THE
+                          CAMBRIDGE MANUALS
+                      OF SCIENCE AND LITERATURE
+
+             Published by the Cambridge University Press
+
+                           GENERAL EDITORS
+                          P. GILES, Litt.D.
+                      Master of Emmanuel College
+                                 and
+                      A.C. SEWARD, M.A., F.R.S.
+          Professor of Botany in the University of Cambridge
+
+                         70 VOLUMES NOW READY
+
+
+HISTORY AND ARCHAEOLOGY
+
+     Ancient Assyria. By Rev. C.H.W. Johns, Litt.D. Ancient Babylonia.
+     By Rev. C.H.W. Johns, Litt.D.
+
+     A History of Civilization in Palestine. By Prof. R.A.S. Macalister,
+     M.A., F.S.A.
+
+     China and the Manchus. By Prof. H.A. Giles, LL.D.
+
+     The Civilization of Ancient Mexico. By Lewis Spence.
+
+     The Vikings. By Prof. Allen Mawer, M.A.
+
+     New Zealand. By the Hon. Sir Robert Stout, K.C.M.G., LL.D., and J.
+     Logan Stout, LL.B. (N.Z.).
+
+     The Ground Plan of the English Parish Church. By A. Hamilton
+     Thompson, M.A., F.S.A.
+
+     The Historical Growth of the English Parish Church. By A. Hamilton
+     Thompson, M.A., F.S.A.
+
+     English Monasteries. By A.H. Thompson, M.A., F.S.A.
+
+     Brasses. By J.S.M. Ward, B.A., F.R.Hist.S.
+
+     Ancient Stained and Painted Glass. By F.S. Eden.
+
+
+ECONOMICS
+
+     Co-partnership in Industry. By C.R. Fay, M.A.
+
+     Cash and Credit. By D.A. Barker.
+
+     The Theory of Money. By D.A. Barker.
+
+
+LITERARY HISTORY
+
+     The Early Religious Poetry of the Hebrews. By the Rev. E.G. King,
+     D.D.
+
+     The Early Religious Poetry of Persia. By the Rev. Prof. J. Hope
+     Moulton, D.D., D.Theol. (Berlin).
+
+     The History of the English Bible. By John Brown, D.D.
+
+     English Dialects from the Eighth Century to the Present Day. By
+     W.W. Skeat, Litt.D., D.C.L., F.B.A.
+
+     King Arthur in History and Legend. By Prof. W. Lewis Jones, M.A.
+
+     The Icelandic Sagas. By W.A. Craigie, LL.D.
+
+     Greek Tragedy. By J.T. Sheppard, M.A.
+
+     The Ballad in Literature. By T.F. Henderson.
+
+     Goethe and the Twentieth Century. By Prof. J.G. Robertson, M.A.,
+     Ph.D.
+
+     The Troubadours. By the Rev. H.J. Chaytor, M.A.
+
+     Mysticism in English Literature. By Miss C.F.E. Spurgeon.
+
+
+PHILOSOPHY AND RELIGION
+
+     The Idea of God in Early Religions. By Dr F.B. Jevons.
+
+     Comparative Religion. By Dr F.B. Jevons.
+
+     Plato: Moral and Political Ideals. By Mrs A.M. Adam.
+
+     The Moral Life and Moral Worth. By Prof. Sorley, Litt.D.
+
+     The English Puritans. By John Brown, D.D.
+
+     An Historical Account of the Rise and Development of
+     Presbyterianism in Scotland. By the Rt Hon. the Lord Balfour of
+     Burleigh, K.T., G.C.M.G.
+
+     Methodism. By Rev. H.B. Workman, D.Lit.
+
+
+EDUCATION
+
+     Life in the Medieval University. By R.S. Rait, M.A.
+
+
+LAW
+
+     The Administration of Justice in Criminal Matters (in England and
+     Wales). By G. Glover Alexander, M.A., LL.M.
+
+
+BIOLOGY
+
+     The Coming of Evolution. By Prof. J.W. Judd, C.B., F.R.S.
+
+     Heredity in the Light of Recent Research. By L. Doncaster, M.A.
+
+     Primitive Animals. By Geoffrey Smith, M.A.
+
+     The Individual in the Animal Kingdom. By J.S. Huxley, B.A.
+
+     Life in the Sea. By James Johnstone, B.Sc.
+
+     The Migration of Birds. By T.A. Coward.
+
+     Spiders. By C. Warburton, M.A.
+
+     Bees and Wasps. By O.H. Latter, M.A.
+
+     House Flies. By C.G. Hewitt, D.Sc.
+
+     Earthworms and their Allies. By F.E. Beddard, F.R.S.
+
+     The Wanderings of Animals. By H.F. Gadow, F.R.S.
+
+
+ANTHROPOLOGY
+
+     The Wanderings of Peoples. By Dr A.C. Haddon, F.R.S.
+
+     Prehistoric Man. By Dr W.L.H. Duckworth.
+
+
+GEOLOGY
+
+     Rocks and their Origins. By Prof. Grenville A.J. Cole.
+
+     The Work of Rain and Rivers. By T.G. Bonney, Sc.D.
+
+     The Natural History of Coal. By Dr E.A. Newell Arber.
+
+     The Natural History of Clay. By Alfred B. Searle.
+
+     The Origin of Earthquakes. By C. Davison, Sc.D., F.G.S.
+
+     Submerged Forests. By Clement Reid, F.R.S.
+
+
+BOTANY
+
+     Plant-Animals: a Study in Symbiosis. By Prof. F.W. Keeble.
+
+     Plant-Life on Land. By Prof. F.O. Bower, Sc.D., F.R.S.
+
+     Links with the Past in the Plant-World. By Prof. A.C. Seward.
+
+
+PHYSICS
+
+     The Earth. By Prof. J.H. Poynting, F.R.S.
+
+     The Atmosphere. By A.J. Berry, M.A.
+
+     Beyond the Atom. By John Cox, M.A.
+
+     The Physical Basis of Music. By A. Wood, M.A.
+
+
+PSYCHOLOGY
+
+     An Introduction to Experimental Psychology. By Dr C.S. Myers.
+
+     The Psychology of Insanity. By Bernard Hart, M.D.
+
+
+INDUSTRIAL AND MECHANICAL SCIENCE
+
+     The Modern Locomotive. By C. Edgar Allen, A.M.I.Mech.E.
+
+     The Modern Warship. By E.L. Attwood.
+
+     Aerial Locomotion. By E.H. Harper, M.A., and Allan E. Ferguson,
+     B.Sc.
+
+     Electricity in Locomotion. By A.G. Whyte, B.Sc.
+
+     Wireless Telegraphy. By Prof. C.L. Fortescue, M.A.
+
+     The Story of a Loaf of Bread. By Prof. T.B. Wood, M.A.
+
+     Brewing. By A. Chaston Chapman, F.I.C.
+
+
+
+
+SOME VOLUMES IN PREPARATION
+
+
+HISTORY AND ARCHAEOLOGY
+
+     The Aryans. By Prof. M. Winternitz.
+
+     Ancient India. By Prof. E.J. Rapson, M.A.
+
+     The Peoples of India. By J.D. Anderson, M.A.
+
+     The Balkan Peoples. By J.D. Bourchier.
+
+     Canada of the present day. By C.G. Hewitt, D.Sc.
+
+     The Evolution of Japan. By Prof. J.H. Longford.
+
+     The West Indies. By Sir Daniel Morris, K.C.M.G.
+
+     The Royal Navy. By John Leyland.
+
+     Gypsies. By John Sampson.
+
+     A Grammar of Heraldry. By W.H. St John Hope, Litt.D.
+
+     Celtic Art. By Joseph Anderson, LL.D.
+
+
+ECONOMICS
+
+     Women's Work. By Miss Constance Smith.
+
+
+LITERARY HISTORY
+
+     Early Indian Poetry. By A.A. Macdonell.
+
+     The Book. By H.G. Aldis, M.A.
+
+     Pantomime. By D.L. Murray.
+
+     Folk Song and Dance. By Miss Neal and F. Kidson.
+
+
+PHYSICS
+
+     The Natural Sources of Energy. By Prof. A.H. Gibson, D.Sc.
+
+     The Sun. By Prof. R.A. Sampson.
+
+     R�ntgen Rays. By Prof. W.H. Bragg, F.R.S.
+
+
+BIOLOGY
+
+     The Life-story of Insects. By Prof. G.H. Carpenter.
+
+     The Flea. By H. Russell.
+
+     Pearls. By Prof. W.J. Dakin.
+
+
+GEOLOGY
+
+     Soil Fertility. By E.J. Russell, D.Sc.
+
+     Coast Erosion. By Prof. T.J. Jehu.
+
+
+INDUSTRIAL AND MECHANICAL SCIENCE
+
+     Coal Mining. By T.C. Cantrill.
+
+     Leather. By Prof. H.R. Procter.
+
+
+
+
+                      Cambridge University Press
+                          C.F. Clay, Manager
+                      London: Fetter Lane, E.C.
+                    Edinburgh: 100, Princes Street
+
+
+
+
+
+End of Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE LIFE-STORY OF INSECTS ***
+
+***** This file should be named 16410-8.txt or 16410-8.zip *****
+This and all associated files of various formats will be found in:
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+<title>The Project Gutenberg eBook of The Life-Story of Insects, by Geo. H. Carpenter.</title>
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+<pre>
+
+Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Life-Story of Insects
+
+Author: Geo. H. Carpenter
+
+Release Date: August 1, 2005 [EBook #16410]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE LIFE-STORY OF INSECTS ***
+
+
+
+
+Produced by Justin Kerk, Laura Wisewell and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<p class="center">The Cambridge Manuals of Science and<br /> Literature</p>
+
+<h1>THE LIFE-STORY OF INSECTS</h1>
+
+
+<p class="center">CAMBRIDGE UNIVERSITY PRESS<br />
+London: FETTER LANE, E.C.<br />
+C.&nbsp;F. CLAY, <span class="smcap">Manager</span></p>
+
+<div class="center">
+   <a id="logo" name="logo"></a>
+   <img class="plain" src="images/logo.png" height="100"
+      alt="Publisher logo."
+      title="Publisher logo." />
+</div>
+
+<p class="center">Edinburgh: 100, PRINCES STREET<br />
+London: H.&nbsp;K. LEWIS, 136, GOWER STREET, W.C.<br />
+WILLIAM WESLEY &amp; SON, 28, ESSEX STREET, STRAND<br />
+Berlin: A. ASHER AND CO.<br />
+Leipzig: F.&nbsp;A. BROCKHAUS<br />
+New York: G.&nbsp;P. PUTNAM'S SONS<br />
+Bombay and Calcutta: MACMILLAN AND CO., <span class="smcap">Ltd</span>.</p>
+
+
+<div class="center">
+   <a id="frontis" name="frontis"></a>
+   <img style="padding-top:20px; padding-bottom:20px; margin-top:3em;" src="images/frontispiece.jpg" height="700"
+      alt="Frontispiece. Stages in the Transformations of a Gnat."
+      title="Frontispiece. Stages in the Transformations of a Gnat." />
+   <div class="caption"><p class="center"><i>Frontispiece</i>. Transformation of a Gnat (<i>Culex</i>).
+Magnified 5 times.</p>
+<ul class="caption" style="text-indent:0;">
+<li>Larva. (The head is directed downwards and the tail-siphon with
+spiracle points upwards to the surface of the water.)</li>
+<li>Pupal Cuticle from which the Imago is emerging. (The pair of
+'respiratory trumpets' on the thorax of the pupa are conspicuous. The
+wings of the Imago are crumpled, and the hind feet are not yet
+withdrawn.)</li>
+<li>Adult Gnat. Female.</li>
+</ul>
+</div>
+</div><p><a name="Page_-4" id="Page_-4"></a></p>
+
+<div class="center">
+   <a id="title" name="title"></a>
+   <img class="plain" style="margin:2em;" src="images/title.png" height="700"
+      alt="THE LIFE-STORY OF INSECTS by GEO. H. CARPENTER
+      Professor of Zoology in the Royal College of Science, Dublin
+      Cambridge: at the University Press
+      New York: G.P. Putnam's Sons
+      1913."
+      title="THE LIFE-STORY OF INSECTS by GEO. H. CARPENTER
+      Professor of Zoology in the Royal College of Science, Dublin
+      Cambridge: at the University Press
+      New York: G.P. Putnam's Sons
+      1913." />
+</div>
+
+<p><a name="Page_-3" id="Page_-3"></a></p>
+
+<p class="center">Cambridge:<br />
+PRINTED BY JOHN CLAY, M.A.<br />
+AT THE UNIVERSITY PRESS</p>
+
+<p><i>With the exception of the coat of arms at the foot, the design on
+the title page is a reproduction of one used by the earliest known
+Cambridge printer John Siberch 1521</i></p>
+
+
+
+<p><a name="Page_-2" id="Page_-2"></a></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE</h2>
+
+<p>The object of this little book is to afford an outline sketch of the
+facts and meaning of insect-transformations. Considerations of space
+forbid anything like an exhaustive treatment of so vast a subject, and
+some aspects of the question, the physiological for example, are almost
+neglected. Other books already published in this series, such as Dr
+Gordon Hewitt's <i>House-flies</i> and Mr O H. Latter's <i>Bees and Wasps</i>, may
+be consulted with advantage for details of special insect life-stories.
+Recent researches have emphasised the practical importance to human
+society of entomological study, and insects will always be a source of
+delight to the lover of nature. This humble volume will best serve its
+object if its reading should lead fresh observers to the brookside and
+the woodland.</p>
+
+<p style="text-indent:80%; margin-top:0;">G.&nbsp;H.&nbsp;C.</p>
+
+<p class="quotsig"><span class="smcap">Dublin,</span><br />
+<i>July</i>, 1913.</p>
+
+
+
+
+<h2>CONTENTS</h2>
+<ul class="preol"><li><span  class="smcap lalign">Chap.</span>&nbsp; <span class="smcap ralign">Page.</span></li>
+</ul>
+<ol class="TOC">
+
+<li>Introduction    <span class="ralign"><a href="#CHAPTER_I">1</a></span></li>
+
+<li>Growth and Change    <span class="ralign"><a href="#CHAPTER_II">8</a></span></li>
+
+<li>The Life-stories of some Sucking Insects    <span class="ralign"><a href="#CHAPTER_III">16</a></span></li>
+
+<li>From Water to Air    <span class="ralign"><a href="#CHAPTER_IV">23</a></span></li>
+
+<li>Transformations, Outward and Inward    <span class="ralign"><a href="#CHAPTER_V">35</a></span></li>
+
+<li>Larvae and their Adaptations    <span class="ralign"><a href="#CHAPTER_VI">49</a></span></li>
+
+<li>Pupae and their Modifications    <span class="ralign"><a href="#CHAPTER_VII">79</a></span></li>
+
+<li>The Life-story and the Seasons    <span class="ralign"><a href="#CHAPTER_VIII">89</a></span></li>
+
+<li>Past and Present&mdash;the Meaning of the Story    <span class="ralign"><a href="#CHAPTER_IX">105</a></span></li>
+
+<li class="off">Outline Classification of Insects    <span class="ralign"><a href="#OUTLINE_CLASSIFICATION_OF_INSECTS">122</a></span></li>
+
+<li class="off">Table of Geological Systems    <span class="ralign"><a href="#TABLE_OF_GEOLOGICAL_SYSTEMS">123</a></span></li>
+
+<li class="off">Bibliography    <span class="ralign"><a href="#BIBLIOGRAPHY">124</a></span></li>
+
+<li class="off">Index    <span class="ralign"><a href="#INDEX">129</a></span></li>
+</ol>
+
+
+
+
+<h2><a name="LIST_OF_ILLUSTRATIONS" id="LIST_OF_ILLUSTRATIONS"></a>LIST OF ILLUSTRATIONS</h2>
+
+<ul class="preol">
+<li>Stages in the Transformations of a Gnat        <span class="ralign"><a href="#frontis"><i>Frontispiece</i></a></span><br /><span class="lalign smcap">Fig.</span>&nbsp;<span class="ralign smcap">Page.</span></li>
+</ul>
+<ol class="postul">
+<li>Stages of the Diamond-back Moth (<i>Plutella
+cruciferarum</i>)                                             <span class="ralign"><a href="#fig1">3</a></span></li>
+
+<li>Head of typical Moth                                       <span class="ralign"><a href="#fig2">5</a></span></li>
+
+<li>Head of Caterpillar                                        <span class="ralign"><a href="#fig3">5</a></span></li>
+
+<li>Common Cockroach (<i>Blatta orientalis</i>)               <span class="ralign"><a href="#fig4">12</a></span></li>
+
+<li>Nymph of Locust (<i>Schistocera americana</i>)            <span class="ralign"><a href="#fig5">13</a></span></li>
+
+<li><i>Aphis pomi</i>, winged and wingless females            <span class="ralign"><a href="#fig6">19</a></span></li>
+
+<li>Mussel Scale-Insect (<i>Mytilaspis pomorum</i>)           <span class="ralign"><a href="#fig7">21</a></span></li>
+
+<li>Emergence of Dragon-fly (<i>Aeschna cyanea</i>)           <span class="ralign"><a href="#fig8ab">29-31</a></span></li>
+
+<li>Nymph of May-fly (<i>Chloeon dipterum</i>)                <span class="ralign"><a href="#fig9">33</a></span></li>
+
+<li>Imaginal buds of Butterfly                               <span class="ralign"><a href="#fig10">39</a></span></li>
+
+<li>Imaginal buds of Blow-fly                                <span class="ralign"><a href="#fig11">43</a></span></li>
+
+<li>Carrion Beetle (<i>Silpha</i>) and larva                 <span class="ralign"><a href="#fig12">51</a></span></li>
+
+<li>Larva of Ground-beetle (<i>Aepus</i>)                    <span class="ralign"><a href="#fig13">52</a></span></li>
+
+<li>Willow-beetle (<i>Phyllodecta</i>) and larva             <span class="ralign"><a href="#fig14">53</a></span></li>
+
+<li>Cabbage-beetle (<i>Psylliodes</i>) and larva             <span class="ralign"><a href="#fig15">54</a></span></li>
+
+<li>Corn Weevil (<i>Calandra</i>) and larva                  <span class="ralign"><a href="#fig16">55</a></span></li>
+
+<li>Ruby Tiger Moth (<i>Phragmatobia fuliginosa</i>)         <span class="ralign"><a href="#fig17">61</a></span></li>
+
+<li>Larvae and Pupa of Hive-bee (<i>Apis mellifica</i>)      <span class="ralign"><a href="#fig18">65</a></span></li>
+
+<li>Larva of Gall-midge (<i>Contarinia nasturtii</i>)        <span class="ralign"><a href="#fig19">68</a></span></li>
+
+<li>Crane-fly (<i>Tipula oleracea</i>) and larva             <span class="ralign"><a href="#fig20">69</a></span></li>
+
+<li>Maggot of House-fly (<i>Musca domestica</i>)             <span class="ralign"><a href="#fig21">71</a></span></li>
+
+<li>Ox Warble-fly (<i>Hypoderma bovis</i>) with egg, larva,
+and puparium                                                 <span class="ralign"><a href="#fig22">75</a></span></li>
+
+<li>Pupa of White Butterfly (<i>Pieris</i>)                  <span class="ralign"><a href="#fig23">85</a></span></li>
+</ol>
+
+
+
+<p><a name="Page_1" id="Page_1"></a></p>
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I<br />
+INTRODUCTION</h2>
+
+
+<p>Among the manifold operations of living creatures few have more strongly
+impressed the casual observer or more deeply interested the thoughtful
+student than the transformations of insects. The schoolboy watches the
+tiny green caterpillars hatched from eggs laid on a cabbage leaf by the
+common white butterfly, or maybe rears successfully a batch of silkworms
+through the changes and chances of their lives, while the naturalist
+questions yet again the 'how' and 'why' of these common though wondrous
+life-stories, as he seeks to trace their course more fully than his
+predecessors knew.</p>
+
+<div class="center">
+   <a id="fig1" name="fig1"></a>
+   <img src="images/01fig.png" height="700"
+      alt="Stages of the Diamond-back Moth (Plutella cruciferarum)."
+      title="Stages of the Diamond-back Moth (Plutella cruciferarum)." />
+   <div class="caption"><p>Fig. 1. <i>a</i>, Diamond-back Moth (<i>Plutella
+cruciferarum</i>); <i>b</i>, young caterpillar, dorsal view; <i>c</i>, full-grown
+caterpillar, dorsal view; <i>d</i>, side view; <i>e</i>, pupa, ventral view.
+Magnified 6 times. From <i>Journ. Dept. Agric. Ireland</i>, vol. I.</p></div>
+</div>
+
+<p>Everyone is familiar with the main facts of such a life-story as that of
+a moth or butterfly. The form of the adult insect (<a href="#fig1">fig.&nbsp;1</a> <i>a</i>) is
+dominated by the wings&mdash;two pairs of scaly wings, carried respectively
+on the middle and hindmost of the three segments that make up the
+<i>thorax</i> or central region of the insect's body. Each of these three
+segments carries a pair of legs. In front of the thorax is the head on
+which the pair of long jointed feelers and the pair <a name="Page_2" id="Page_2"></a>of large,
+sub-globular, compound eyes are the most prominent features. Below the
+head, however, may be seen, now coiled up like a watch-spring, now
+stretched out to draw the nectar from some scented blossom, the
+butterfly's sucking trunk or proboscis, situated between a pair of short
+hairy limbs or palps (<a href="#fig2">fig.&nbsp;2</a>). These palps belong to the appendages of
+the hindmost segment of the head, appendages which in insects are
+modified to form a hind-lip or <i>labium</i>, bounding the mouth cavity below
+or behind. The proboscis is made up of the pair of jaw-appendages in
+front of the labium, the <i>maxillae</i>, as they are called. Behind the
+thorax is situated the <i>abdomen,</i> made up of nine or ten recognisable
+segments, none of which carry limbs comparable to the walking legs, or
+to the jaws which are the modified limbs of the head-segments. The whole
+cuticle or outer covering of the body, formed (as is usual in the group
+of animals to which insects belong) of a horny (chitinous) secretion of
+the skin, is firm and hard, and densely covered with hairy or scaly
+outgrowths. Along the sides of the insect are a series of paired
+openings or spiracles, leading to a set of air-tubes which ramify
+throughout the body and carry oxygen directly to the tissues.</p>
+
+<div class="center">
+   <a id="fig2" name="fig2"></a>
+   <img src="images/02fig.png" height="400"
+      alt="Head of typical Moth."
+      title="Head of typical Moth." />
+   <div class="caption"><p>Fig. 2. A. Head of a typical Moth, showing proboscis
+formed by flexible maxillae (<i>g</i>) between the labial palps (<i>p</i>); <i>c</i>,
+face; <i>e</i>, eye; the structure <i>m</i> has been regarded as the vestige of a
+mandible. B. Basal part (<i>b</i>) of maxilla removed from head, with
+vestigial palp (<i>p</i>). Magnified.</p></div>
+</div>
+
+<p>Such a butterfly as we have briefly sketched lays an egg on the leaf of
+some suitable food-plant, and there is hatched from it the well-known
+crawling<a name="Page_3" id="Page_3"></a><a name="Page_4" id="Page_4"></a> larva<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> (<a href="#fig1">fig.&nbsp;1</a> <i>b, c, d</i>) called a caterpillar, offering in
+many superficial features a marked contrast to its parent. Except on the
+head, whose surface is hard and firm, the caterpillar's cuticle is as a
+rule thin and flexible, though it may carry a protective armature of
+closely set hairs, or strong sharp spines. The feelers (<a href="#fig3">fig.&nbsp;3</a> <i>At</i>) are
+very short and the eyes are small and simple. In connection with the
+mouth, there are present in front of the maxillae a pair of <i>mandibles</i>
+(<a href="#fig3">fig.&nbsp;3</a> <i>Mn</i>), strong jaws, adapted for biting solid food, which are
+absent from the adult butterfly, though well developed in cockroaches,
+dragon-flies, beetles, and many other insects. The three pairs of legs
+on the segments of the thorax are relatively short, and as many as five
+segments of the abdomen may carry short cylindrical limbs or pro-legs,
+which assist the clinging habits and worm-like locomotion of the
+caterpillar. No trace of wings is visible externally. The caterpillar,
+therefore, differs markedly from its parent in its outward structure, in
+its mode of progression, and in its manner of feeding; for while the
+butterfly sucks nectar or other liquid food, the caterpillar bites up
+and devours solid vegetable substances, such as the leaves of herbs or
+trees. It is well-known that between the close of its larval life and
+its attainment of perfection as a butterfly,<a name="Page_5" id="Page_5"></a><a name="Page_6" id="Page_6"></a> the insect spends a
+period as a <i>pupa</i> (<a href="#fig1">fig.&nbsp;1</a> <i>e</i>) unable to move from place to place, and
+taking no food.</p>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span> </a> The term <i>larva</i> is applied to any young animal which
+differs markedly from its parent.</p></div>
+
+<div class="center">
+   <a id="fig3" name="fig3"></a>
+   <img src="images/03fig.png" height="250"
+      alt="Head of Caterpillar."
+      title="Head of Caterpillar." />
+   <div class="caption"><p>Fig. 3. Head of Caterpillar of Goat-moth (<i>Cossus</i>) seen
+from behind. <i>At</i>, feeler; <i>Mn</i>, mandible; <i>Mx</i>, maxilla; <i>Lm</i>, labium,
+spinneret projecting beyond it. Magnified. After Lyonet from Miall and
+Denny's <i>Cockroach</i>.</p></div>
+</div>
+
+<p>Such, in brief, is the course of the most familiar of insect
+life-stories. For the student of the animal world as a whole, this
+familiar transformation raises some startling problems, which have been
+suggestively treated by <a href="#Brauer1869">F. Brauer (1869)</a>, <a href="#Miall1895">L.&nbsp;C. Miall (1895)</a>, <a href="#Lubbock1874">J. Lubbock
+(1874)</a>, <a href="#Heymons1907">R. Heymons (1907)</a>, <a href="#Deegener1909">P. Deegener (1909)</a> and other writers<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a>. To
+appreciate these problems is the first step towards learning the true
+meaning of the transformation.</p>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span> </a> The dates in brackets after authors' names will facilitate
+reference to the Bibliography (pp. <a href="#BIBLIOGRAPHY">124-8</a>).</p></div>
+
+<p>The butterfly's egg is absolutely and relatively of large size, and
+contains a considerable amount of yolk. As a rule we find that young
+animals hatched from such eggs resemble their parents rather closely and
+pass through no marked changes during their lives. A chicken, a
+crocodile, a dogfish, a cuttlefish, and a spider afford well-known
+examples of this rule. Land-animals, generally, produce young which are
+miniature copies of themselves, for example horses, dogs, and other
+mammals, snails and slugs, scorpions and earthworms. On the other hand,
+metamorphosis among animals is associated with eggs of small size, with
+aquatic habit, and with relatively low zoological rank. The young of a
+starfish, for example, has hardly a character in common with its parent,
+while a marine <a name="Page_7" id="Page_7"></a>segmented worm and an oyster, unlike enough when adult,
+develop from closely similar larval forms. If we take a class of
+animals, the Crustacea, nearly allied to insects, we find that its more
+lowly members, such as 'water-fleas' and barnacles, pass through far
+more striking changes than its higher groups, such as lobsters and
+woodlice. But among the Insects, a class of predominantly terrestrial
+and aerial creatures producing large eggs, the highest groups undergo,
+as we shall see, the most profound changes. The life-story of the
+butterfly, then, well-known as it may be, furnishes a puzzling exception
+to some wide-reaching generalisations concerning animal development. And
+the student of science often finds that an exception to some rule is the
+key to a problem of the highest interest.</p>
+
+<p>During many centuries naturalists have bent their energies to explain
+the difficulties presented by insect transformations. Aristotle, the
+first serious student of organised beings whose writings have been
+preserved for us, and William Harvey, the famous demonstrator of the
+mammalian blood circulation two thousand years later, agreed in
+regarding the pupa as a second egg. The egg laid by a butterfly had not,
+according to Harvey, enough store of food to provide for the building-up
+of a complex organism like the parent; only the imperfect larva could be
+produced from it. The larva was regarded as <a name="Page_8" id="Page_8"></a>feeding voraciously for the
+purpose of acquiring a large store of nutritive material, after which it
+was believed to revert to the state of a second but far larger egg, the
+pupa, from which the winged insect could take origin. Others again,
+following <a href="#Reaumur1734">de R&eacute;aumur (1734)</a>, have speculated whether the development of
+pupa within larva, and of winged insect within pupa might not be
+explained as abnormal births. But a comparison of the transformation of
+butterflies with simpler insect life-stories will convince the enquirer
+that no such heroic theories as these are necessary. It will be realised
+that even the most profound transformation among insects can be
+explained as a special case of growth.</p>
+
+
+
+
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II<br />
+GROWTH AND CHANGE</h2>
+
+
+<p>The caterpillar differs markedly from the butterfly. As we pursue our
+studies of insect growth and transformation we shall find that in some
+cases the difference between young and adult is much greater&mdash;as for
+example between the maggot and the house-fly, in others far less&mdash;as
+between the young and full-grown grasshopper or plant-bug. It is
+evidently wise to begin a general survey of the subject with some of
+<a name="Page_9" id="Page_9"></a>those simpler cases in which the differences between the young and
+adult insect are comparatively slight. We shall then be in a position to
+understand better the meaning of the more puzzling and complex cases in
+which the differences between the stages are profound.</p>
+
+<p>In the first place it is necessary to realise that the changes which any
+insect passes through during its life-story are essentially
+accompaniments of its growth. The limits of this little book allow only
+slight reference to features of internal structure; we must be content,
+in the main, to deal with the outward form. But there is an important
+relation between this outward form and the underlying living tissues
+which must be clearly understood. Throughout the great race of
+animals&mdash;the Arthropoda&mdash;of which insects form a class, the body is
+covered outwardly by a <i>cuticle</i> or secretion of the underlying layer of
+living cells which form the outer skin or <i>epidermis</i><a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> (see <a href="#fig10">fig.&nbsp;10</a>
+<i>ep</i>, <i>cu</i>, <a href="#Page_39">p. 39</a>). This cuticle has regions which are hard and firm,
+forming an <i>exoskeleton</i>, and, between these, areas which are relatively
+soft and flexible. The firm regions are commonly segmental in their
+arrangement, and the intervening flexible connections render possible
+accurate motions of the exoskeletal parts in relation <a name="Page_10" id="Page_10"></a>to each other,
+the motions being due to the contraction of muscles which are attached
+within the exoskeleton.</p>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span> </a> The term 'hypodermis' frequently applied to this layer is
+misleading. The layer is the true outer skin&mdash;ectoderm or epidermis.</p></div>
+
+<p>Now this jointed exoskeleton&mdash;an admirably formed suit of armour though
+it often is&mdash;has one drawback: it is not part of the insect's living
+tissues. It is a cuticle formed by the solidifying of a fluid secreted
+by the epidermal cells, therefore without life, without the power of
+growth, and with only a limited capacity for stretching. It follows,
+therefore, that at least during the period through which the insect
+continues to grow, the cuticle must be periodically shed. Thus in the
+life-story of an insect or other arthropod, such as a lobster, a spider,
+or a centipede, there must be a succession of cuticle-castings&mdash;'moults'
+or <i>ecdyses</i> as they are often called.</p>
+
+<p>When such a moult is about to take place the cuticle separates from the
+underlying epidermis, and a fluid collects beneath. A delicate new
+cuticle (see <a href="#fig10">fig.&nbsp;10</a> <i>cu'</i>) is then formed in contact with the
+epidermis, and the old cuticle opens, usually with a slit lengthwise
+along the back, to allow the insect in its new coat to emerge. At first
+this new coat is thin and flabby, but after a period of exposure to the
+air it hardens and darkens, becoming a worthy and larger successor to
+that which has been cast. The cuticle moreover is by no means wholly
+external. The greater part of the digestive canal and the whole
+<a name="Page_11" id="Page_11"></a>air-tube system are formed by inpushings of the outer skin (ectoderm)
+and are consequently lined with an extension of the chitinous cuticle
+which is shed and renewed at every moult.</p>
+
+<p>In all insects these successive moults tend to be associated with change
+of form, sometimes slight, sometimes very great. The new cuticle is
+rarely an exact reproduction of the old one, it exhibits some new
+features, which are often indications of the insect's approach towards
+maturity. Even in some of those interesting and primitive insects the
+Bristle-tails (Thysanura) and Spring-tails (Collembola), in which wings
+are never developed, perceptible differences in the form and arrangement
+of the abdominal limbs can be traced through the successive stages, as
+<a href="#Heymons1906">R. Heymons (1906)</a> and <a href="#Verhoeff1911">K.&nbsp;W. Verhoeff (1911)</a> have shown for Machilis. But
+the changes undergone by such insects are comparatively so slight, that
+the creatures are often known as 'Ametabola' or insects without
+transformation in the life-history. Now there are a considerable number
+of winged insects&mdash;cockroaches and grasshoppers for example&mdash;in which
+the observable changes are also comparatively slight. We will sketch
+briefly the main features of the life-story of such an insect.</p>
+
+<div class="center">
+   <a id="fig4" name="fig4"></a>
+   <img src="images/04fig.png" height="400"
+      alt="Common Cockroach (Blatta orientalis)."
+      title="Common Cockroach (Blatta orientalis)." />
+   <div class="caption"><p>Fig. 4. Common Cockroach (<i>Blatta orientalis</i>). <i>a</i>,
+female; <i>b</i>, male; <i>c</i>, side view of female; <i>d</i>, young. After Marlatt,
+<i>Entom. Bull.</i> 4, <i>U.S. Dept. Agric.</i></p></div>
+</div>
+
+<p>The young creature is hatched from the egg in a form closely resembling,
+on the whole, that of its parent, so that the term 'miniature adult'
+sometimes <a name="Page_12" id="Page_12"></a>applied to it, is not inappropriate. The baby cockroach (<a href="#fig4">fig.&nbsp;4</a> <i>d</i>) is known by its flattened body, rounded prothorax, and stiff,
+jointed tail-feelers or cercopods; the baby grasshopper by its strong,
+elongate hind-legs, adapted, like those of the adult, for vigorous
+leaping. During the growth of the insect to the adult state there may be
+four or five moults, each preceded and succeeded by a <a name="Page_13" id="Page_13"></a>characteristic
+instar<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>. The first instar differs, however, from the adult in one
+conspicuous and noteworthy feature, it possesses no trace of wings. But
+after the first or the second moult, definite wing-rudiments are visible
+in the form of outgrowths on the corners of the second and third
+thoracic segments. In each succeeding instar these rudiments become more
+prominent, and in the fourth or the fifth stage, they show a branching
+arrangement of air-tubes, prefiguring the nervures of the adult's <a name="Page_14" id="Page_14"></a>wing
+(<a href="#fig5">fig.&nbsp;5</a>). After the last moult the wings are exposed, articulated to the
+segments that bear them, and capable of motion. Having been formed
+beneath the cuticle of the wing-rudiments of the penultimate instar, the
+wings are necessarily abbreviated and crumpled. But during the process
+of hardening of the cuticle, they rapidly increase in size, blood and
+air being forced through the nervures, so that the wings attaining their
+full expanse and firmness, become suited for the function of flight.</p>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span> </a> The convenient term 'instar' has been proposed by Fischer
+and advocated by <a href="#Sharp1895">Sharp (1895)</a> for the form assumed by an insect during a
+stage of its life-story. Thus the creature as hatched from the egg is
+the <i>first instar</i>, after the first moult it has become the <i>second
+instar</i>, and so on, the number of moults being always one less than the
+number of instars.</p></div>
+
+<div class="center">
+   <a id="fig5" name="fig5"></a>
+   <img src="images/05fig.png" height="300"
+      alt="Nymph of Locust (Schistocera americana)."
+      title="Nymph of Locust (Schistocera americana)." />
+   <div class="caption"><p>Fig. 5. Nymph of Locust (<i>Schistocera americana</i>) with
+distinct wing-rudiments. After Howard, <i>Insect Life</i>, vol. VII.</p></div>
+</div>
+
+<p>The changes through which these insects pass are therefore largely
+connected with the development of the wings. It is noteworthy that in an
+immature cockroach the entire dorsal cuticle is hard and firm. In the
+adult, however, while the cuticle of the prothorax remains firm, that of
+the two hinder thoracic and of all the abdominal segments is somewhat
+thin and delicate on the dorsal aspect. It needs not now to be
+resistant, because it is covered by the two firm forewings, which shield
+and protect it, except when the insect is flying. There are, indeed,
+slight changes in other structures not directly connected with the
+wings. In a young grasshopper, for example, the feelers are relatively
+stouter than in the adult, and the prothorax does not show the
+specifically distinctive shape with its definite keels and furrows.
+Changes in the secondary sexual characters may also be noticed. For
+instance, in an immature cockroach <a name="Page_15" id="Page_15"></a>both male and female carry a pair of
+jointed tail-feelers or cercopods on the tenth abdominal segment, and a
+pair of unjointed limbs or stylets on the ninth. In the adult stage,
+both sexes possess cercopods, but the males only have stylets, those of
+the female disappearing at the final moult.</p>
+
+<p>Reviewing the main features of the life-story of a grasshopper or
+cockroach, we notice that there is no marked or sudden change of form.
+The newly-hatched insect resembles generally its parent, except that it
+has no wings. Wing-rudiments appear, however, in an early instar as
+visible outgrowths on the thoracic segments, and become larger after
+each moult. All through its various stages the immature insect&mdash;<i>nymph</i>
+as it is called&mdash;lives in the same kind of situations and on the same
+kind of food as its parent, and it is all along active and lively,
+undergoing no resting period like the pupal stage in the transformation
+of the butterfly.</p>
+
+<p>One interesting and suggestive fact remains to be mentioned. There are
+grasshoppers and cockroaches in which the changes are even less than
+those just sketched, because the wings remain, even in the adult, in a
+rudimentary state (as for example in the female of the common kitchen
+cockroach, <i>Blatta orientalis</i>, see <a href="#fig4">fig.&nbsp;4</a> <i>a</i>), or are never developed
+at all. Such exceptional winglessness in members of a winged family can
+only be explained by the <a name="Page_16" id="Page_16"></a>recognition of a life-story, not merely in the
+individual but in the race. We cannot doubt that the ancestors of these
+wingless insects possessed wings, which in the course of time have been
+lost by the whole species or by the members of the female sex. It is
+generally assumed that this loss has been gradual, and so in many cases
+it probably may have been. But there are species of insects in which
+some generations are winged and others wingless; a winged mother gives
+birth to wingless offspring, and a wingless parent to young with
+well-developed wings. Such discontinuity in the life-story of a single
+generation forces us to recognise the possibility of similar sudden
+mutations in the course of that age-long process of evolution to which
+the facts of insect growth, and indeed of all animal development, bear
+striking testimony.</p>
+
+
+
+
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III<br />
+THE LIFE-STORIES OF SOME SUCKING INSECTS</h2>
+
+
+<p>We may now turn our attention to some examples of the remarkable
+alternation of winged and wingless generations in the yearly life-cycle
+of the same species, mentioned at the end of the last chapter.
+<a name="Page_17" id="Page_17"></a>Cockroaches and grasshoppers belong to an order of insects, the
+Orthoptera<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a>, characterised by firm forewings and biting jaws; in all
+of them the change of form during the life-history is comparatively
+slight. A great contrast to those insects in the structure of the
+mouth-parts is presented by the Hemiptera, an order including the bugs,
+pond-skaters, cicads, plant-lice, and scale-insects. These all have an
+elongated, grooved labium projecting from the head in form of a beak,
+within which work, to and fro, the slender needle-like mandibles and
+maxillae by means of which the insect pierces holes through the skin of
+a leaf or an animal, and is thus enabled to suck a meal of sap or blood,
+according to its mode of life. In many Hemiptera&mdash;the various families
+of bugs both aquatic and terrestrial, for example&mdash;the life-history is
+nearly as simple as that of a cockroach. It is the family of the
+plant-lice (Aphidae) that affords typical illustrations of that
+alternation of generations to which reference has been made.</p>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span> </a> See outline classification of insects, <a href="#Page_122">p. 122</a>.</p></div>
+
+<p>The yearly cycle of the common Aphids of the apple tree has been lately
+worked out in detail by <a href="#Smith1900">J.&nbsp;B. Smith (1900)</a> and <a href="#Sanderson1902">E.&nbsp;D. Sanderson (1902)</a>. In
+late autumn tiny wingless males and females are found in large numbers
+on the withered leaves. The sexes pair together, and the females lay
+their relatively large, smooth, hard-coated black eggs on the <a name="Page_18" id="Page_18"></a>twigs;
+these resistant eggs carry the species safely over the winter. At
+springtide, when the leaves begin to sprout from the opening buds the
+aphid eggs are hatched, and the young insects after a series of moults,
+through which hardly any change of form is apparent, all grow into
+wingless 'stem-mothers' much larger than the egg-laying females of the
+autumn. The stem-mothers have the power, unusual among animals as a
+whole, but not very infrequent in the insects and their allies, of
+reproducing their kind without having paired<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> with a male. Eggs
+capable of parthenogenetic development, produced in large numbers in the
+ovaries of these females, give rise to young which, developing within
+the body of the mother, are born in an active state. Successive broods
+of these wingless virgin females (<a href="#fig6">fig.&nbsp;6</a> <i>a</i>) appear through the spring
+and summer months, and as the rate of their development is rapid, often
+the whole life-story is completed within a week. The aphid population
+increases very fast. Later a generation appears in which the thoracic
+segments of the nymphs are seen to bear wing-rudiments like those of the
+young cockroach, and a host of winged females (<a href="#fig6">fig.&nbsp;6</a> <i>b</i>) are produced;
+these have the power of migrating to other plants. We understand that
+wings are not necessary to the earlier broods whose members have plenty
+of room and food on their native <a name="Page_19" id="Page_19"></a>shoots, but that when the population
+becomes crowded, a winged brood capable of emigration is advantageous to
+the race.</p>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span> </a> Such virgin reproduction is termed 'parthenogenesis.'</p></div>
+
+<p>Many generations of virgin female aphids, some wingless, others winged
+when adult, succeed each other through the summer months. At the close
+of the year the latest brood of these bring forth young, which develop
+into males and egg-laying females; thus the yearly cycle is completed.
+Variations in points of detail may be noticed in different species of
+aphids. The autumn males and egg-laying females are, for example,
+frequently winged, and the same species may have constantly recurring
+generations of different forms adapted for different food-plants, or for
+different regions of the same food-plant. But taking a general view of
+the life-story of aphids for comparison with the life-story of other
+insects, three <a name="Page_20" id="Page_20"></a>points are especially noteworthy. Virgin reproduction
+recurs regularly, parthenogenetic broods being succeeded by a single
+sexual brood. A winged parent brings forth young which remain always
+wingless, and wingless adults produce young which acquire wings. The
+wings are developed, as in the cockroach, from outward and visible
+wing-rudiments.</p>
+
+<div class="center">
+   <a id="fig6" name="fig6"></a>
+   <img src="images/06fig.png" height="250"
+      alt="Apple Aphid (Aphis pomi), winged and wingless females."
+      title="Apple Aphid (Aphis pomi), winged and wingless females." />
+   <div class="caption"><p>Fig. 6. Apple Aphid (<i>Aphis pomi</i>), virgin females, <i>a</i>,
+wingless; <i>b</i>, winged. Magnified 20 times.</p></div>
+</div>
+
+<p>A family of Hemiptera, related to the Aphidae and equally obnoxious to
+the gardener, is that of the Coccidae or scale-insects. These furnish an
+excellent illustration of features noticeable in certain insect
+life-histories. In the first place, the newly-hatched young differs
+markedly from the parent in the details of its structure. A young coccid
+(<a href="#fig7">fig.&nbsp;7</a> <i>c</i>) is flattened oval in shape, has well-developed feelers
+(<a href="#fig7">fig.&nbsp;7</a> <i>d</i>) and legs, and runs actively about, usually on the leaves or
+bark of trees and shrubs, through which it pierces with its long jaws,
+so that it may suck sap from the soft tissues beneath. After a time it
+fixes itself by means of these jaws and the characteristic scale or
+protective covering, composed partly of a waxy secretion and partly of
+dried excrement, begins to grow over its body. The female loses legs and
+feelers, and never acquires wings, becoming little more than a sluggish
+egg-bag (<a href="#fig7">fig.&nbsp;7</a> <i>e</i>). The male on the other hand passes into a second
+larval stage in which there are no functional legs, but rudiments of
+legs and of wings are present on <a name="Page_21" id="Page_21"></a>the epidermis beneath the cuticle, as
+shown by B.&nbsp;O. Schmidt for Aspidiotus <a href="#Schmidt1885">(1885)</a>. The penultimate instar of
+this sex in which the wing-rudiments are visible externally lies
+passively beneath the scale, <a name="Page_22" id="Page_22"></a>its behaviour resembling that of a
+butterfly pupa. The adult winged male (<a href="#fig7">fig.&nbsp;7</a> <i>a</i>) leads a short, but
+active life.</p>
+
+<div class="center">
+   <a id="fig7" name="fig7"></a>
+   <img src="images/07fig.png" height="500"
+      alt="Mussel Scale-Insect (Mytilaspis pomorum."
+      title="Mussel Scale-Insect (Mytilaspis pomorum)." />
+   <div class="caption"><p>Fig. 7. Mussel Scale-insect (<i>Mytilaspis pomorum</i>). <i>a</i>,
+male; <i>b</i>, foot of male; <i>c</i>, larva, ventral view; <i>d</i>, feeler of larva;
+<i>e</i>, female, ventral view. After Howard, <i>Yearbook U.S. Dept. Agric.</i>
+1904. Magnified, <i>a, c, e</i> x 20; <i>b, d</i> x 120.</p></div>
+</div>
+
+<p>Another family allied to the Aphidae is that of the Cicads, hardly
+represented in our fauna but abundant in many of the warmer regions of
+the earth. Here also the young insect differs widely from its parent in
+form, living underground and being provided with strong fore-legs for
+digging in the soil. After a long subterranean existence, usually
+extending over several years, the insect attains the penultimate stage
+of its life-story, during which it rests passively within an earthen
+cell, awaiting the final moult, which will usher in its winged and
+perfect state.</p>
+
+<p>In the life-histories of cicads and coccids, then, there are some
+features which recall those of the caterpillar's transformation into the
+butterfly. The newly-hatched insect is externally so unlike its parent
+that it may be styled a larva. The penultimate instar is quiescent and
+does not feed. But while the caterpillar shows throughout its life no
+outward trace of wings, external wing-rudiments are evident in the young
+stages of the cicad. In the male coccid we find a late larval stage with
+hidden wing-rudiments, the importance of which, for comparison with the
+caterpillar, will be appreciated later.</p>
+
+
+
+<p><a name="Page_23" id="Page_23"></a></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV<br />
+FROM WATER TO AIR</h2>
+
+
+<p>Insects as a whole are preeminently creatures of the land and the air.
+This is shown not only by the possession of wings by a vast majority of
+the class, but by the mode of breathing to which reference has already
+been made (<a href="#Page_2">p. 2</a>), a system of branching air-tubes carrying atmospheric
+air with its combustion-supporting oxygen to all the insect's tissues.
+The air gains access to these tubes through a number of paired air-holes
+or spiracles, arranged segmentally in series.</p>
+
+<p>It is of great interest to find that, nevertheless, a number of insects
+spend much of their time under water. This is true of not a few in the
+perfect winged state, as for example aquatic beetles and water-bugs
+('boatmen' and 'scorpions') which have some way of protecting their
+spiracles when submerged, and, possessing usually the power of flight,
+can pass on occasion from pond or stream to upper air. But it is
+advisable in connection with our present subject to dwell especially on
+some insects that remain continually under water till they are ready to
+undergo their final moult and attain the winged state, which <a name="Page_24" id="Page_24"></a>they pass
+entirely in the air. The preparatory instars of such insects are
+aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
+caddis-flies, many beetles and two-winged flies, and a few moths thus
+divide their life-story between the water and the air. For the present
+we confine attention to the Stone-flies, the May-flies, and the
+Dragon-flies, three well-known orders of insects respectively called by
+systematists the Plecoptera, the Ephemeroptera and the Odonata.</p>
+
+<p>In the case of many insects that have aquatic larvae, the latter are
+provided with some arrangement for enabling them to reach atmospheric
+air through the surface-film of the water. But the larva of a stone-fly,
+a dragon-fly, or a may-fly is adapted more completely than these for
+aquatic life; it can, by means of gills of some kind, breathe the air
+dissolved in water.</p>
+
+<p>The aquatic young of a stone-fly does not differ sufficiently in form
+from its parent to warrant us in calling it a larva; the life-history is
+like that of a cockroach, all the instars however except the final
+one&mdash;the winged adult or <i>imago</i>&mdash;live in the water. The young of one of
+our large species, a Perla for example, has well-chitinised cuticle,
+broad head, powerful legs, long feelers and cerci like those of the
+imago; its wings arise from external rudiments, which are conspicuous in
+the later aquatic stages. <a name="Page_25" id="Page_25"></a>But it lives completely submerged, usually
+clinging or walking beneath the stones that lie in the bed of a clear
+stream, and examination of the ventral aspect of the thorax reveals six
+pairs of tufted gills, by means of which it is able to breathe the air
+dissolved in the water wherein it lives. At the base of the tail-feelers
+or cerci also, there are little tufts of thread-like gills as <a href="#Palmen1877">J.&nbsp;A.
+Palm&eacute;n (1877)</a> has shown. An insect that is continually submerged and has
+no contact with the upper air cannot breathe through a series of paired
+spiracles, and during the aquatic life-period of the stone-fly these
+remain closed. Nevertheless, breathing is carried on by means of the
+ordinary system of branching air-tubes, the trunks of which are in
+connection with the tufted hollow gill-filaments, through whose delicate
+cuticle gaseous exchange can take place, though the method of this
+exchange is as yet very imperfectly understood. When the stone-fly nymph
+is fully grown, it comes out of the water and climbs to some convenient
+eminence. The cuticle splits open along the back, and the imago, clothed
+in its new cuticle, as yet soft and flexible, creeps out. The spiracles
+are now open, and the stone-fly breathes atmospheric air like other
+flying insects. But throughout its winged life, the stone-fly bears
+memorials of its aquatic past in the little withered vestiges of gills
+that can still be distinguished beneath the thorax.</p>
+
+<p><a name="Page_26" id="Page_26"></a>The adult dragon-fly (<a href="#fig8d">fig.&nbsp;8 <i>d</i></a>) is specialised in such a way that it
+captures its prey&mdash;flies and other small insects&mdash;on the wing, swooping
+through the air like a hawk and feeding voraciously. The head is
+remarkable for its large globular compound eyes, its short bristle-like
+feelers, and its very strong mandibles which bite up the bodies of the
+victims. The thorax bears the two pairs of ample wings, firm and almost
+glassy in texture, and its segments are projected forward ventrally, so
+that all six legs, which are armed with rows of sharp, slender spines,
+can be held in front of the mouth, where they form an effective
+fly-trap. The abdomen is very long and usually narrow.</p>
+
+<p>A female dragon-fly after a remarkable mode of pairing, the details of
+which are beside our present subject, drops her eggs in the water, or
+lays them on water-weeds, perhaps cutting an incision where they can be
+the more safely lodged, or even goes down below the surface and deposits
+them in the mud at the bottom of a pond. From the eggs are hatched the
+aquatic larvae which differ in many respects from the imago. The
+dragon-fly larva has the same predaceous mode of life as its parent, but
+it is sluggish in habit, lurking for its prey at the bottom of the pond,
+among the mud or vegetation, which it resembles in colour. The thoracic
+segments have not the specialisation that they show in the imago; the
+<a name="Page_27" id="Page_27"></a>abdomen is relatively shorter and broader. The larval head has, like
+that of the imago, short feelers, and the eyes are somewhat large,
+though far from attaining the size of the great globular eyes of the
+dragon-fly. But the third pair of jaws, forming the labium, are most
+remarkably modified into a 'mask,' the distal central portion (mentum)
+being hinged to the basal piece (sub-mentum) which is itself jointed
+below the head. The mentum carries at its extremity a pair of lobes with
+sharp fangs. Thus the mask can be folded under the head when the larva
+lurks in its hiding place, or be suddenly darted out so as to secure any
+unwary small insect that may pass close enough for capture. Dragon-fly
+larvae walk, and also swim by movements of the abdomen or by expelling a
+jet of water from the hind-gut. The walls of this terminal region of the
+intestine have areas lined with delicate cuticle and traversed by
+numerous air-tubes, so that gaseous exchange can take place between the
+air in the tubes and that dissolved in the water. The larvae of the
+larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
+mostly in this way. Those of the slender and delicate 'Demoiselles'
+(Agrionidae) are provided with three leaf-like gill-plates at the tail,
+between whose delicate surfaces numerous air-tubes ramify. These
+gill-plates are at times used for propulsion. Thus air supply is ensured
+during aquatic life. But occasionally, when <a name="Page_28" id="Page_28"></a>the water in which the
+larva lives is foul and poor in oxygen, the tail is thrust out of the
+water so that air can be admitted directly into the intestinal chamber.
+The aquatic life of these insects lasts for more than a year, and <a href="#Balfour1909">F.
+Balfour-Browne (1909)</a> has observed from ten to fourteen moults in
+Agrion. Outward wing-rudiments are early visible on the thoracic
+segments; when these have become conspicuous the insect, beginning in
+some respects to approach the adult condition, is often called a nymph.
+In an advanced dragon-fly nymph, <a href="#Dewitz1891">H. Dewitz (1891)</a> has shown that the
+thoracic spiracles are open, and, as the time for its final moult draws
+near, the insect may thrust the front part of its body out of the water,
+and breathe atmospheric air through these. Thus before the great change
+takes place the nymph has foretastes of the aerial mode of breathing
+which it will practise when the perfect stage shall have been attained.
+The emergence of the dragon-fly from its nymph-cuticle has been
+described by many naturalists from <a href="#Reaumur1734">de R&eacute;aumur (1740)</a> to <a href="#Miall1895">L.&nbsp;C. Miall
+(1895)</a> and <a href="#Latter1904">O.&nbsp;H. Latter (1904)</a>. The nymph climbs out of the water by
+ascending some aquatic plant, and awaits the change so graphically
+sketched by Tennyson:</p>
+
+<blockquote class="poem">
+<p>A hidden impulse rent the veil,<br />
+Of his old husk, from head to tail,<br />
+Came out clear plates of sapphire mail.</p>
+</blockquote>
+
+<p>'From head to tail,' for the nymph-cuticle splits <a name="Page_29" id="Page_29"></a>lengthwise down the
+back, and the head and thorax of the imago are freed from it (<a href="#fig8ab">fig.&nbsp;8&nbsp;<i>a</i></a>), then the legs clasp the empty cuticle, and the abdomen is drawn
+out (<a href="#fig8ab">fig.&nbsp;8&nbsp;<i>b</i></a>, <a href="#fig8c"><i>c</i></a>). After a short rest, the newly-emerged fly climbs
+yet higher up the water-weed, and remains for some hours with the
+abdomen bent concave dorsalwards (<a href="#fig8d">fig.&nbsp;8&nbsp;<i>d</i></a>), to allow space for the
+expansion and hardening of the wings. For some <a name="Page_30" id="Page_30"></a>days after emergence the
+cuticle of the dragon-fly has a dull pale hue, as compared with the dark
+or brightly metallic aspect that characterises it when fully mature. The
+life of the imago endures but a short time compared with the long
+aquatic larval and nymphal stages. After some weeks, or at most <a name="Page_31" id="Page_31"></a>a few
+months, the dragon-flies, having paired and laid their eggs, die before
+the approach of winter.</p>
+
+<div class="center">
+   <a id="fig8ab" name="fig8ab"></a>
+   <img src="images/08abfig.png" height="500"
+      alt="Dragon-fly (Aeschna cyanea). Two stages in emergence of fly from nymph-cuticle."
+      title="Dragon-fly (Aeschna cyanea). Two stages in emergence of fly from nymph-cuticle." />
+   <div class="caption"><p>Fig. 8. <i>a, b</i>. Dragon-fly (<i>Aeschna cyanea</i>). Two stages
+in emergence of fly from nymph-cuticle. From Latter's <i>Natural
+History</i>.</p></div>
+</div>
+<div class="center">
+   <a id="fig8c" name="fig8c"></a>
+   <img src="images/08cfig.png" height="600"
+      alt="Dragon-fly emerged, wings expanding."
+      title="Dragon-fly emerged, wings expanding." />
+   <div class="caption"><p>Fig. 8. <i>c</i>. Dragon-fly emerged, wings
+expanding. From Latter's <i>Natural History</i>.</p></div>
+</div>
+<div class="center">
+   <a id="fig8d" name="fig8d"></a>
+   <img src="images/08dfig.png" height="500"
+      alt="Dragon-fly with expanded wings."
+      title="Dragon-fly with expanded wings." />
+   <div class="caption"><p>Fig. 8. <i>d</i>. Dragon-fly (<i>Aeschna cyanea</i>) with
+expanded wings.</p></div>
+</div>
+
+<p>The life-story of a may-fly follows the same general course as that just
+described for the dragon-flies, but there are some suggestive
+differences. In the first place, we notice a wider divergence between
+the <a name="Page_32" id="Page_32"></a>imago and the larva. An adult may-fly is one of the most delicate
+of insects; the head has elaborate compound eyes, but the feelers are
+very short, and the jaws are reduced to such tiny vestiges that the
+insect is unable to feed. Its aquatic larva is fairly robust, with a
+large head which is provided with well-developed jaws, as the larval and
+nymphal stages extend over one or two years, and the insects browse on
+water-weeds or devour creatures smaller and weaker than themselves. They
+breathe dissolved air by means of thread-like or plate-like gills
+traversed by branching air-tubes, somewhat resembling those of the
+demoiselle dragon-fly larva. But in the may-fly larva, there is a series
+of these gills (<a href="#fig9">fig.&nbsp;9</a><i>b</i>) arranged laterally in pairs on the abdominal
+segments, and <a href="#Boerner1909a"><ins class="correction" title="Transcriber's note: It is not clear which of the two B&ouml;rner 1909 entries in the Bibliography is meant here.">C. B&ouml;rner (1909)</ins></a> has recently given reasons, from the
+position and muscular attachments of these organs, for believing that
+they show a true correspondence to (in technical phraseology are
+homologous with) the thoracic legs. One feature in which the larva often
+agrees with the imago is the possession on the terminal abdominal
+segment of a pair of long jointed cerci, and in many genera a median
+jointed tail-process (see <a href="#fig9">fig.&nbsp;9</a>) is also present, in some cases both in
+the larva and the imago, in others in the larva during its later stages
+only. The prolonged larval life in may-flies often involves a large
+series of moults; <a href="#Lubbock1863">Lubbock (1863)</a> has enumerated twenty-one in the
+<a name="Page_33" id="Page_33"></a>life-history of Chloeon. In the second year of aquatic life
+wing-rudiments (<a href="#fig9">fig.&nbsp;9</a> <i>a</i>) are visible, and the larva becomes a nymph.
+When the time for the winged condition approaches the nymphs leave the
+water in large swarms. The vivid accounts of these swarms given by
+<a href="#Swammerdam1737">Swammerdam (1675)</a>, <a href="#Reaumur1734">de R&eacute;aumur (1742)</a> and other old-time observers are
+available in summarised form for English readers in Miall's admirable
+book <a href="#Miall1895">(1895)</a>. May-flies are eagerly sought as food by trout, and the rise
+of the fly on many lakes ushers in a welcome season to the angler.</p>
+
+<p>The nymph-cuticle opens and the winged insect emerges. But this is not
+the final instar; may-flies are exceptional among insects in undergoing
+yet another moult after they have acquired wings which they can use for
+flight. The instar that emerges from the nymph-cuticle is a sub-imago,
+dull in hue, with a curious immature aspect about it. A few <a name="Page_34" id="Page_34"></a>hours later
+the final moult takes place, a very delicate cuticle being shed and
+revealing the true imago. Then follow the dancing flight over the calm
+waters, the mating and egg-laying, the rapid death. The whole winged
+existence prepared for by the long aquatic life may be over in a single
+evening; at most it lasts but for a few days.</p>
+
+<div class="center">
+   <a id="fig9" name="fig9"></a>
+   <img src="images/09fig.png" height="500"
+      alt="Nymph of May-fly (Chloeon dipterum)."
+      title="Nymph of May-fly (Chloeon dipterum)." />
+   <div class="caption"><p>Fig. 9. Nymph of May-fly (<i>Chloeon dipterum</i>) showing on
+right side wing-rudiment (<i>a</i>), on left tracheal gills (<i>b</i>). Magnified
+4 times. [Feelers and legs are cut short.] From Miall and Denny after
+Vayssi&egrave;re.</p></div>
+</div>
+
+<p>In the development of the may-flies, then, we notice not only a
+considerable divergence between larva and imago, both in habitat and
+structure; we see also what is to be observed often in more highly
+organised insects&mdash;a feeding stage prolonged through the years of larval
+and nymphal life, while the winged imago takes no food and devotes its
+energies through its short existence to the task of reproduction. Such
+division of the life-history into a long feeding, and a short breeding
+period has, as will be seen later, an important bearing on the question
+of insect transformation generally, and the dragon-flies and may-flies
+afford examples of two stages in its specialisation. The sub-imaginal
+instar of the may-fly furnishes also a noteworthy fact for comparison
+with other insect histories. In two points, however, the life-story of
+these flies with their aquatic larvae recalls that of the cockroach. All
+the larval and nymphal instars are active, and the wing-rudiments are
+outwardly visible long before the final moult.</p>
+
+
+
+<p><a name="Page_35" id="Page_35"></a></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V<br />
+TRANSFORMATIONS,&mdash;OUTWARD AND INWARD</h2>
+
+
+<p>We are now in a position to study in some detail the transformation of
+those insects whose life-story corresponds more or less closely with
+that of the butterfly, sketched in the opening pages of this little
+book. In the case of some of the insects reviewed in the last three
+chapters, the may-flies and cicads for example, a marked difference
+between the larva and the imago has been noticed; in others, as the
+coccids, we find a resting instar before the winged condition is
+assumed, suggesting the pupal stage in the butterfly's life-story.</p>
+
+<p>The various insect orders whose members exhibit no marked divergence
+between larva and imago (the Orthoptera for example) are often said to
+undergo no transformation, to be 'Ametabola.' Those with life-stories
+such as the dragon-flies' are said to undergo partial transformation,
+and are termed 'Hemimetabola.' Moths, caddis-flies, beetles, two-winged
+flies, saw-flies, ants, wasps, bees, and the great majority of insects,
+having the same type of life-story as the butterfly, are said to undergo
+complete transformation and are classed as 'Metabola' or 'Holometabola.'
+<a name="Page_36" id="Page_36"></a>Wherein lies the fundamental difference between these Holometabola on
+the one hand and the Hemimetabola and Ametabola on the other? It is not
+that the larva differs from the imago or that there is a passive stage
+in the life-history; these conditions are observable among insects with
+a 'partial' transformation as we have seen, though the resting instar
+that simulates the butterfly pupa is certainly exceptional. It has been
+pointed out by <a href="#Sharp1899">Sharp (1899)</a> that the most important indication of the
+difference between the two modes of development is furnished by the
+position of the wing-rudiments. In all Ametabola and Hemimetabola these
+are visible externally long before the penultimate instar has been
+reached; in the Holometabola they are not seen until the pupal stage.</p>
+
+<p>Attention has already been drawn to the contrast in outward form between
+a butterfly and its caterpillar. As in the case of dragon-fly or
+may-fly, the larval period is essentially a time for feeding and growth,
+and during this period the larval cuticle is cast four or five, in some
+species even seven or eight times. After each moult some changes in
+detail may be observable, for example in the proportions of the
+body-segments or their outgrowths, in the colour or the closeness of the
+hairy or spiny armature. But in all main features the caterpillar
+retains throughout its life the characteristic form in which <a name="Page_37" id="Page_37"></a>it left
+the egg. From the tiny, newly-hatched larva to the full-fed caterpillar,
+possibly several inches in length, there is all along the same crawling,
+somewhat worm-like body, destitute of any outward trace of wings. When
+however the last larval cuticle has split open lengthwise along the
+back, and has been worked off by vigorous wriggling motions of the
+insect, the pupa thus revealed shows the wing-rudiments conspicuous at
+the sides of the body, and lying neatly alongside these are to be seen
+the forms of feelers, legs, and maxillae of the imago prefigured in the
+cuticle of the pupa (<a href="#fig1">fig.&nbsp;1</a> <i>e</i>). The pupa thus resembles the imago much
+more closely than it resembles the larva; even in the proportions of the
+body a relative shortening is to be noticed, and the imago of any insect
+with complete transformation is reduced in length as compared with the
+full-fed larva. Now these wings and other structures characteristic of
+the imago, appear in the pupa which is revealed by the shedding of the
+last larval cuticle. From these facts we infer that the wing-rudiments
+must be present in the larva, hidden beneath the cuticle; and until the
+last larval instar, not beneath the cuticle only, but growing in
+such-wise that they are hidden by the epidermis. For if they were
+growing outwardly the new cuticle would be formed over them, so that
+they would be apparent after the next moult. But it is clear that only
+in the pupa, <a name="Page_38" id="Page_38"></a>forming beneath the cuticle of the last larval instar, can
+they grow outwards.</p>
+
+<p>Anatomical study of the caterpillar at various stages verifies the
+conclusions just drawn from superficial observation. A hundred and fifty
+years ago P. Lyonet in his monumental work <a href="#Lyonet1762">(1762)</a> on the caterpillar of
+the Goat Moth (Cossus) detected, in the second and third thoracic
+segments, four little white masses buried in the fat-body, and, while
+doubtful as to their real meaning, he suggested that their number and
+position might well give rise to the suspicion that they were rudiments
+of the wings of the moth. But it was a century later that A. Weismann in
+his classical studies <a href="#Weismann1864">(1864)</a> on the development of common flies, showed
+the presence in the maggot of definite rudiments of wings, and other
+organs of the adult&mdash;rudiments to which he gave the name of <i>imaginal
+discs</i>. We will recur later to these transformations of the Diptera. For
+the present, we pursue our survey of changes in the life-history of the
+Lepidoptera and can take to guide us the excellent researches of <a href="#Gonin1894">J.
+Gonin (1894)</a>.</p>
+
+<p>Careful study of the imaginal discs of the wings in a caterpillar (<a href="#fig10">fig.&nbsp;10</a>) made by examining microscopically sections cut through them, shows
+that the epidermis is pushed in to form a little pouch (<i>C, p</i>) and that
+into this grows the actual wing-rudiment. Consequently the whitish disk
+which seems to lie <a name="Page_39" id="Page_39"></a><a name="Page_40" id="Page_40"></a>within the body-wall of the larva, is really a
+double fold of the epidermis, the outer fold forming the pouch, the
+inner the actual wing-bud. Into the cavity of the latter pass branches
+from the air-tube system. In its earliest stage, the wing-bud is simply
+an ingrowing mass of cells (<a href="#fig10">fig.&nbsp;10</a> <i>A</i>) which subsequently becomes an
+inpushed pouch (<i>B</i>). Until the last stage of larval life the wing-bud
+remains hidden in its pouch, and no cuticle is formed over it. When the
+pupal stage draws near the bud grows out of its sheath, and projecting
+from the general surface of the epidermis becomes covered with cuticle
+to be revealed, as we have seen, after the last larval moult, as the
+pupal wing. Thus all through the life of the humble, crawling
+caterpillar, 'it doth not yet appear what it shall be,' but there are
+being prepared, hidden and unseen, the wondrous organs of flight, which
+in due time will equip the insect for the glorious aerial existence that
+awaits it.</p>
+
+<div class="center">
+   <a id="fig10" name="fig10"></a>
+   <img src="images/10fig.png" height="600"
+      alt="Imaginal Buds of Butterfly."
+      title="Imaginal Buds of Butterfly." />
+   <div class="caption"><p>Fig. 10. A, B, C, Sections through epidermis and cuticle,
+showing three stages in growth of the imaginal disc (<i>w</i>) of a wing in
+the caterpillar of a White Butterfly (<i>Pieris</i>). <i>ep</i>, epidermis; <i>cu</i>,
+cuticle; <i>t</i>, air-tube, whence branches pass into the developing wing.
+In C, <i>cu'</i> represents the new cuticle forming beneath the old one, and
+(<i>p</i>) the pouch within which the wing-disc (<i>w</i>) lies. Highly magnified.
+After Gonin, <i>Bull. Soc. Vaud.</i> <span class="smcap">xxx</span>.</p></div>
+</div>
+
+<p>As mentioned above, this hidden growth of the wing-rudiments, in
+butterflies, beetles, flies, bees, and the great majority of the winged
+insects, has been emphasised by <a href="#Sharp1899">Sharp (1899)</a> as a character contrasting
+markedly with the outward and visible growth of the wing-rudiments in
+such insects as cockroaches, bugs, and dragon-flies. The divergence
+between the two modes of development is certainly very striking, and a
+conceivable method of transition <a name="Page_41" id="Page_41"></a>from the one to the other is not easy
+to explain. Sharp has expressed the divergence by the terms
+<i>Endopterygota</i>, applied to all the orders of insects with hidden
+wing-rudiments (the 'Metabola' or 'Holometabola' of most
+classifications) and <i>Exopterygota</i>, including all those insects whose
+wing-rudiments are visible throughout growth ('Hemimetabola' and
+'Ametabola'). Those curious lowly insects, belonging to the two orders
+of the Collembola and Thysanura, none of whose members ever develop
+wings at all, form a third sub-class, the <i>Apterygota</i> (see
+Classificatory Table, <a href="#Page_122">p. 122</a>).</p>
+
+<p>Not the wings only, but other structures of the imago, varying in extent
+in different orders, are formed from the imaginal discs. For example, de
+R&eacute;aumur and <a href="#Newport1839">G. Newport (1839)</a> found that if the thoracic leg of a
+late-stage caterpillar were cut off, the corresponding leg of the
+resulting butterfly would still be developed, although in a truncated
+condition. Gonin has shown that in the Cabbage White butterfly (<i>Pieris
+brassicae</i>) the legs of the imago are represented, through the greater
+part of larval life, only by small groups of cells situated within the
+bases of the larval legs. After the third moult these imaginal discs
+grow rapidly and the proximal portion of each, destined to develop into
+the thigh and shin of the butterfly's leg, sinks into a depression at
+the side of the thorax, while the tip of the shin and the
+<a name="Page_42" id="Page_42"></a>five-segmented foot project into the cavity of the larval leg. Hence we
+understand that the amputation of the latter by the old naturalists
+truncated only and did not destroy the imaginal limb. In the blow-fly
+maggot, Weismann, <a href="#Lowne1890">B.&nbsp;T. Lowne (1890)</a> and <a href="#Van1888">J. Van Rees (1888)</a> have shown
+that the imaginal discs of the legs (<a href="#fig11">fig.&nbsp;11</a>&mdash;1, 2, 3) grow out from
+deep dermal inpushings. Simple at first, these outgrowths by partial
+splitting, become differentiated into thigh and shin.</p>
+
+<div class="center">
+   <a id="fig11" name="fig11"></a>
+   <img src="images/11fig.png" height="200"
+      alt="Imaginal Buds of Blow-fly."
+      title="Imaginal Buds of Blow-fly." />
+   <div class="caption"><p>Fig. 11. Front region of Maggot of Blow-fly
+(<i>Calliphora</i>) showing diagrammatically the imaginal discs, which are
+shaded. <i>e</i>, eye; <i>f</i>, feeler; <i>W</i>, fore-wing; <i>w</i>, hind-wing; 1, 2, 3,
+legs. <i>H</i> is the 'cephalic vesicle,' which becomes everted at the close
+of the metamorphosis, so as to bring the feelers and eyes to the front,
+the brain (<i>B</i>) moving forwards at the same time. After Van Rees, <i>Zool.
+Jahrb.</i> 1894, and Lowne's <i>Blow-fly</i>.</p></div>
+</div>
+
+<p>Similarly the feelers and jaws of the butterfly are developed from
+imaginal discs, and this fact explains how it comes to pass that they
+differ so widely from the corresponding structures in the caterpillar.
+The larval feelers (<a href="#fig3">fig.&nbsp;3</a> <i>At</i>) are short and stumpy, those of the
+butterfly long and many-jointed. The maxilla of the larva (<a href="#fig3">fig.&nbsp;3</a> <i>Mx</i>)
+consists of a base carrying two short jointed processes; in the
+butterfly a certain portion of the maxilla, the hood or galea, is
+modified into a long, flexible grooved process, capable of forming with
+its fellow the trunk through which the insect sucks its liquid food
+(<a href="#fig2">fig.&nbsp;2</a>). Nothing but some such provision as that of the imaginal discs
+could render possible the wonderful replacement of the caterpillar's
+jaws, biting solid food, into those of the butterfly sipping nectar from
+flowers.</p>
+
+<p>A curious segmental displacement of the imaginal discs with regard to
+the larva is noticeable in some Diptera. In the larva of the
+harlequin-midge <a name="Page_43" id="Page_43"></a>(Chironomus) as described by <a href="#Miall1900">Miall and Hammond (1900)</a>
+the brain is situated in the thorax, and the imaginal discs for the
+head, eyes, and feelers of the adult lie in close association with it,
+though they arise from inpushings of the larval head. These rudiments do
+not appear until the last larval stage has been reached. In the gnats
+Culex and Corethra, on the other hand, the imaginal discs for the
+head-appendages retain their normal position within the larval head, and
+appear in an early stage of larval <a name="Page_44" id="Page_44"></a>life. Among the flies of the
+bluebottle group (Muscidae) the brain (<a href="#fig11">fig.&nbsp;11</a> <i>B</i>) is situated, as in
+Chironomus, in the thoracic region of the legless maggot, which is the
+larva of an insect of this family, and the imaginal discs for eyes and
+feelers (<a href="#fig11">fig.&nbsp;11</a> <i>e</i>, <i>f</i>) lie just in front of it. Here, the imaginal
+buds of the legs (<a href="#fig11">fig.&nbsp;11</a>&mdash;1, 2, 3) and wings (<a href="#fig11">fig.&nbsp;11</a> <i>W</i>, <i>w</i>) are
+deeply inpushed, retaining their connection with the skin only by means
+of a thread of cells. As the larva is legless and headless its outer
+form is not affected by the discs and it is not surprising to learn that
+they appear early. It has indeed been suggested that the pharyngeal
+region of the larva, in connection with which the imaginal head-discs
+are developed, should be regarded, though it lies in the thorax, as an
+inpushed anterior section of the larval head. In any case this region is
+pushed out during the formation of the pupa within the final larval
+cuticle, so that the imaginal head with its contained brain, its
+compound eyes, and its complex feelers, takes its rightful place at the
+front end of the insect.</p>
+
+<p>The mention of the brain suggests a few brief remarks on the changes in
+the internal organs during insect transformation. There are no imaginal
+discs for the nervous system; the brain, nerve-cords and ganglia of the
+butterfly or bluebottle are the direct outcome of those of the
+caterpillar or maggot. More than seventy years ago, <a href="#Newport1839">Newport (1839)</a>
+traced the <a name="Page_45" id="Page_45"></a>rapid but continuous changes, which, during the early pupal
+period, convert the elongate nerve-cord of the caterpillar with its
+relatively far-separated ganglia into the shortened, condensed
+nerve-cord of the Tortoise-shell butterfly (<i>Vanessa urticae</i>) with
+several of the ganglia coalesced. In many Diptera, on the other hand,
+the nervous system of the larva is more concentrated than that of the
+imago.</p>
+
+<p>The tubular heart also of a winged insect is the directly modified
+survival of the larval heart.</p>
+
+<p>Similarly the reproductive organs undergo a gradual, continuous
+development throughout an insect's life-story. Their rudiments appear in
+the embryo, often at a very early stage; they are recognisable in the
+larva, and the matured structures in the imago are the result of their
+slow process of growth, the details of which must be reckoned beyond the
+scope of this book. For a full summary of the subject the reader is
+referred to L.&nbsp;F. Henneguy's work <a href="#Henneguy1904">(1904)</a> containing references to much
+important modern literature, which cannot be mentioned here.</p>
+
+<p>On the other hand, the digestive system of insects that undergo a
+metamorphosis, passes through a profound crisis of dissolution and
+rebuilding. This is not surprising when we remember that there is often
+a great difference between larva and imago in the nature of the food.
+The digestive canal of a caterpillar runs a fairly straight course
+through the <a name="Page_46" id="Page_46"></a>body and consists of a gullet, stomach (mid-gut),
+intestine, and rectum; it is adapted for the digestion of solid food. In
+the butterfly there is one outgrowth of the gullet in the head&mdash;a
+pharyngeal sac adapted for sucking liquids; and another outgrowth at the
+hinder end of the gullet (which is much longer than in the larva)&mdash;a
+crop or food-reservoir lying in the abdomen. The intestine of the
+butterfly also is longer than that of the larva, being coiled or
+twisted. Towards the end of the last larval stage, the cells of the
+inner coat (epithelium) lining the stomach begin to undergo
+degeneration, small replacing cells appearing between their bases and
+later giving rise to the more delicate epithelium that lines the mid-gut
+of the imago. The larval cells are shed into the cavity of the stomach
+and become completely broken down. <a href="#Anglas1902">J. Anglas (1902)</a>, describing these
+microscopic changes in the transformations of wasps and bees, has shown
+that the tiny replacing cells can be recognised in sections through the
+digestive canal of a very young larva; they may be regarded as
+representing imaginal buds of the adult gastric epithelium. In the
+transformations of two-winged flies of the bluebottle group, <a href="#Kowalevsky1887">A.
+Kowalevsky (1887)</a> has shown that these replacing cells are aggregated in
+little masses scattered at different points along the stomach and thus
+corresponding rather closely to the imaginal discs of the legs and
+wings.</p>
+
+<p><a name="Page_47" id="Page_47"></a>The gullet, crop, and gizzard of an insect, which lie in front of the
+stomach, are lined by cells derived from the outer skin (ectoderm) which
+is pushed in to form what is called the 'fore-gut.' Similarly the
+intestine and rectum, behind the stomach, are lined with ectodermal
+cells which arise from the inpushed 'hind-gut.' The larval fore- and
+hind-guts are broken down at the end of larval life and their lining is
+replaced by fresh tissue derived from two imaginal bands which surround
+the cavity of the digestive tube, one at the hinder end of the fore-gut,
+and the other at the front end of the hind-gut. The larval salivary
+glands in connection with the gullet are also broken down, and fresh
+glands are formed for the imago.</p>
+
+<p>A large part of the substance of an insect larva consists of muscular
+tissue, surrounding the digestive tube, and forming the great muscles
+that move the various parts of the body, and of fat, surrounding the
+organs and serving as a store of food-material. Very many of the
+muscle-fibres and the fat-cells also become disintegrated during the
+late larval and pupal stages, and the corresponding tissues of the adult
+are new formations derived from special groups of imaginal cells, though
+some muscles may persist from the larva to the adult. Similarly the
+complex air-tube or tracheal system of the larva is broken down and a
+fresh set of tubes is developed, <a name="Page_48" id="Page_48"></a>adapted to the altered body-form of
+pupa and imago.</p>
+
+<p>The destruction of larval tissue and the development of replacing organs
+from special groups of cells, derived of course from the embryo, and
+carrying on the continuity of cell-lineage to the adult, are among the
+most remarkable facts connected with the life-story of insects. The
+process of tissue-destruction is known as 'histolysis'; the rebuilding
+process is called 'histogenesis.' Considerable difference of opinion has
+existed as to factors causing histolysis, and for a summary of the
+conflicting or complementary theories, the reader is referred to the
+work of L.&nbsp;F. Henneguy (<a href="#Henneguy1904">1904</a>, pp. 677-684). In the histolysis of the
+two-winged flies, wandering amoeboid cells&mdash;like the white corpuscles or
+leucocytes of vertebrate blood&mdash;have been observed destroying the larval
+tissues that need to be broken down, as they destroy invading
+micro-organisms in the body. But students of the internal changes that
+accompany transformation in insects of other orders have often been
+unable to observe such devouring activity of these 'phagocytes,' and
+attribute the dissolution of the larval tissues to internal chemical
+changes. The fact that in all insect transformation a part, and in many
+a large part, of the larval organs pass over to the pupa and imago,
+suggests that only those structures whose work is done are broken down
+through <a name="Page_49" id="Page_49"></a>the action of internally formed destructive substances, and one
+function of the phagocytes is to act as scavengers by devouring what has
+become effete and useless.</p>
+
+
+
+
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI<br />
+LARVAE AND THEIR ADAPTATIONS</h2>
+
+
+<p>Among the insects that undergo a complete transformation, there is, as
+we have seen in the preceding chapter, an amount of inward change, of
+dissolution and rebuilding of tissues, that varies in its completeness
+in members of different orders. It is now advisable to consider the
+various outward forms assumed by the larvae of these insects, or rather
+by a few examples chosen from a vast array of well-nigh 'infinite
+variety.'</p>
+
+<p>In comparing the transformations of endopterygote insects of different
+orders, it is worthy of notice that in some cases all the members of an
+order have larvae remarkably constant in their main structural features,
+while in others there is great variety of larval form within the order.
+For example, the caterpillars of all Lepidoptera are fundamentally <a name="Page_50" id="Page_50"></a>much
+alike, while the grubs of beetles of different families diverge widely
+from one another. A review of a selected series of beetle-larvae will
+therefore serve well to introduce this branch of the subject.</p>
+
+<div class="center">
+   <a id="fig12" name="fig12"></a>
+   <img src="images/12fig.png" height="350"
+      alt="Carrion-beetle (Silpha) and larva."
+      title="Carrion-beetle (Silpha) and larva." />
+   <div class="caption"><p>Fig. 12. <i>a</i>, Carrion-beetle (<i>Silpha</i>) with its larva,
+<i>b</i>. Magnified, <i>a</i> 3 times, and <i>b</i> 4 times.</p></div>
+</div>
+<div class="center">
+   <a id="fig13" name="fig13"></a>
+   <img src="images/13fig.png" height="400"
+      alt="Larva of a Ground-beetle (Aepus)."
+      title="Larva of a Ground-beetle (Aepus)." />
+   <div class="caption"><p>Fig. 13.
+Larva of a Ground-beetle (<i>Aepus</i>). Magnified 6 times. After Westwood,
+<i>Modern Classification of Insects</i>.</p></div>
+</div>
+
+<p>Beetles are as a rule remarkable among insects for the firm consistency
+of their chitinous cuticle, the various pieces (<i>sclerites</i>) of which
+are fitted together with admirable precision. In some families of
+beetles the larva also is furnished with a complete chitinous armour,
+the sclerites, both dorsal and ventral, of the successive body-segments
+being hard and firm, while the relatively long legs possess well-defined
+segments and are often spiny. Such a larva is evidently far less unlike
+its parent beetle than a caterpillar is unlike a butterfly. Perhaps of
+all beetle larvae, the woodlouse-like grub (<a href="#fig12">fig.&nbsp;12</a> <i>b</i>) of a
+carrion-beetle (Silpha) or of a semi-aquatic dascillid such as Helodes
+shows the least amount of difference from the typical adult, on account
+of the conspicuous jointed feelers. The larval glow-worm, however, is of
+the same woodlouse-like aspect, and in this case, where the female never
+acquires wings, but becomes mature in a form which does not differ
+markedly from that of the larva, the exceptional resemblance is closer
+still. In all beetle-grubs the legs are simplified, there being only one
+segment (a combined shin and foot) below the knee-joint, whereas in the
+adult there is a shin followed by five, four, or at least three
+<a name="Page_51" id="Page_51"></a>distinct tarsal segments. The foot of an adult beetle bears two claws
+at its tip, while the larval foot in the great majority of families has
+only one claw. In one section of the order, however, the Adephaga
+comprising the predaceous terrestrial and aquatic beetles, the larval
+foot has, like that of the adult, two claws. Some adephagous larvae,
+notably those of the large carnivorous water-beetles (Dyticus), often
+<a name="Page_52" id="Page_52"></a>destructive to tadpoles and young fish, have completely armoured bodies
+as well as long jointed legs. More commonly, as with most of the
+well-known Ground-beetles (Carabidae), the cuticle is less consistently
+hard, firm sclerites segmentally arranged alternating with considerable
+tracts of cuticle which remain feebly chitinised and flexible. Most of
+the adephagous larvae (<a href="#fig13">fig.&nbsp;13</a>) have a pair of stiff processes on the
+ninth abdominal segment, and the insect, from its general likeness to a
+bristle-tail of the genus Campodea, is often called a <i>campodeiform</i>
+larva (Brauer, 1869). From such as these, a series of forms can be
+traced among larvae of beetles, showing an increasing divergence from
+the imago. The well-known wireworms&mdash;grubs of the Click-beetles
+(Elateridae)&mdash;that eat the roots of farm crops, have well-armoured
+bodies, but their shape is elongate, cylindrical, worm-like; and their
+legs are relatively short, the build of the insect being adapted for
+rapid motion through the soil. The grubs of the Chafers (Scarabaeidae)
+are also root-eaters, but they are less active in their habits than the
+wireworms, <a name="Page_53" id="Page_53"></a>and the cuticle of their somewhat stout bodies is, for the
+most part, pale and flexible; only the head and legs are hard and horny.
+Usually an evident correspondence can be traced between the outward form
+of any larva and its mode of life. For example, in the family of the
+Leaf-beetles (Chrysomelidae) some larvae feed openly on the foliage of
+trees or herbs, while others burrow into the plant tissues. The exposed
+larvae of the Willow-beetles (Phyllodecta, <a href="#fig14">fig.&nbsp;14</a>) have their somewhat
+abbreviated body segments protected by numerous spine-bearing, firm
+tubercles. But the grub of the 'Turnip Fly' (Phyllotreta) <a name="Page_54" id="Page_54"></a>which feeds
+between the upper and lower skins of a leaf, or of <i>Psylliodes
+chrysocephala</i> (<a href="#fig15">fig.&nbsp;15</a>), which burrows in stalks, has a pale, soft
+cuticle like that of a caterpillar.</p>
+
+<div class="center">
+   <a id="fig14" name="fig14"></a>
+   <img src="images/14fig.png" height="250"
+      alt="Willow-beetle (Phyllodecta vulgatissima) and larva."
+      title="Willow-beetle (Phyllodecta vulgatissima) and larva." />
+   <div class="caption"><p>Fig. 14. (<i>a</i>) Willow-beetle (<i>Phyllodecta vulgatissima</i>)
+and its larva (<i>b</i>). Magnified 5 times. After Carpenter, <i>Econ. Proc. R.
+Dublin Soc</i>. vol. I.</p></div>
+</div>
+<div class="center">
+   <a id="fig15" name="fig15"></a>
+   <img src="images/15fig.png" height="350"
+      alt="Cabbage-beetle (Psylliodes chrysocephala) and larva."
+      title="Cabbage-beetle (Psylliodes chrysocephala) and larva." />
+   <div class="caption"><p>Fig. 15. (<i>a</i>) Cabbage-beetle
+(<i>Psylliodes chrysocephala</i>) magnified 5 times, and its larva (<i>b</i>)
+magnified 12 times.</p></div>
+</div>
+
+<p>In the larvae of the little timber-beetles and their allies (Ptinidae),
+including the 'death-watches' whose tapping in old furniture is often
+heard, a marked shortening of the legs and reduction in the size of the
+head accompany the whitening and softening of the <a name="Page_55" id="Page_55"></a>cuticle. This
+shortening of the legs is still more marked in the larvae of the
+Longhorn Beetles (Cerambycidae) burrowing in the wood of trees or felled
+trunks; here the legs are reduced to small vestiges.</p>
+
+<div class="center">
+   <a id="fig16" name="fig16"></a>
+   <img src="images/16fig.png" height="500"
+      alt="Corn Weevil (Calandra) and larva."
+      title="Corn Weevil (Calandra) and larva." />
+   <div class="caption"><p>Fig. 16. <i>a</i>, Grain Weevil (<i>Calandra granaria</i>); <i>b</i>,
+larva; <i>c</i>, pupa. Magnified 7 times. After Chittenden, <i>Yearbook U.S.
+Dept. Agric.</i> 1894.</p></div>
+</div>
+<p>Finally in the large family of the Weevils
+(Curculionidae, <a href="#fig16">fig.&nbsp;16</a>) and the Bark-beetles (Scolytidae), the grubs,
+eating underground root or stem structures, mining <a name="Page_56" id="Page_56"></a>in leaves or seeds,
+or tunnelling beneath the bark of trees, have no legs at all, the place
+of these limbs being indicated only by tiny tubercles on the thoracic
+segments. Such larvae as these latter are examples of the type called
+<i>eruciform</i> by <a href="#Packard1898">A.&nbsp;S. Packard (1898)</a> who as well as other writers has laid
+stress on the series of transitional steps from the campodeiform to the
+eruciform type afforded by the larvae of the Coleoptera.</p>
+
+<p>A fact of much importance in the transformations of beetles as pointed
+out by <a href="#Brauer1869">Brauer (1869)</a> is that in a few families, the first larval instar
+is campodeiform, while the subsequent instars are eruciform. We may take
+as an example of such 'hypermetamorphosis' the life-story of the Oil or
+Blister-beetles (Meloidae) as first described by <a href="#Fabre1857">J.&nbsp;H. Fabre (1857)</a>, and
+later with more elaboration by <a href="#Beauregard1890">H. Beaur&eacute;gard (1890)</a>. From the egg of one
+of these beetles is hatched a minute armoured larva, with long feelers,
+legs, and cerci, whose task is, for example, to seize hold of a bee in
+order that the latter may carry it, an uninvited guest, to her nest.
+Safely within the nest, the little 'triungulin' beetle-grub moults; the
+second instar has a soft cuticle and relatively shorter legs, which, as
+the larva, now living as a cuckoo-parasite, proceeds to gorge itself
+with honey, soon appear still further abbreviated. Later comes a stage
+during which legs are entirely wanting, the larva then resting and
+<a name="Page_57" id="Page_57"></a>taking no food. The last larval instar again has short legs like the
+grub of the second period. In connection with this life-history we
+notice that the newly-hatched larva is not in the neighbourhood of its
+appropriate food. Hence the preliminary armoured and active instar is
+necessary in order to reach the feeding place; this journey
+accomplished, the eruciform condition is at once assumed.</p>
+
+<p>In all cases indeed we may say that the particular larval form is
+adapted to the special conditions of life. A few examples from other
+orders of endopterygote insects will illustrate this point. The
+campodeiform type is relatively unusual, but most of the Neuroptera have
+larvae of this kind, active, armoured creatures with long legs, though
+devoid of the tail-processes often associated with similar larvae among
+the Coleoptera. Such are the 'Ant-lions,' larvae of the exotic lacewing
+flies, which hunt small insects, digging a sandy pit for their unwary
+steps in the case of the best-known members of the group, some of which
+are found as far north as Paris. In our own islands the 'Aphis-lions,'
+larvae of Hemerobius and Chrysopa, prowl on plants infested with
+'green-fly' which they impale on their sharp grooved mandibles, sucking
+out the victims' juices, and then, in some cases, using the dried
+cuticle to furnish a clothing for their own bodies. Among these insects,
+while the mouth of the imago is of the <a name="Page_58" id="Page_58"></a>normal mandibulate type adapted
+for eating solid food, the larval mouth is constricted and the slender
+mandibles are grooved for the transmission of liquid food.</p>
+
+<p>Turning to eruciform types of larva, we find the <i>caterpillar</i> (<a href="#fig1">fig.&nbsp;1</a>
+<i>b</i>, <i>c</i>, <i>d</i>) distinguished by its elongate, usually cylindrical body
+with feeble cuticle, short thoracic legs and a variable number of pairs
+of abdominal pro-legs, universal among the moths and butterflies forming
+the great order Lepidoptera, and usual among the saw-flies, which belong
+to the Hymenoptera. The vast majority of caterpillars feed on the leaves
+of plants and their long worm-like bodies with the series of paired
+pro-legs, are excellently adapted for their habit of clinging to twigs,
+and crawling along shoots or the edges of leaves as they go in search of
+food. Of great importance to a caterpillar is its power of spinning
+silk, consisting of fine threads solidified from the secretion of
+specially modified salivary glands whose ducts open in the insect's
+mouth at the tip of the tubular tongue which forms a spinneret.</p>
+
+<p>On the same bush caterpillars of moths and of saw-flies may often be
+seen feeding together. The lepidopterous caterpillar, in our countries
+at least, has never more than five pairs of pro-legs, situated on the
+third, fourth, fifth, sixth, and tenth abdominal segments; each of these
+pro-legs bears a number of <a name="Page_59" id="Page_59"></a>minute hooklets, arranged in a circular or
+crescentic pattern, which assist the caterpillar in clinging to its
+food-plant. The saw-fly caterpillar, on the other hand, may have as many
+as eight pairs of pro-legs, the series beginning on the second abdominal
+segment; here, however, the pro-legs have no hooklets. Among the
+Lepidoptera, we notice a reduction in the number of pro-legs in the
+'looper' caterpillars of Geometrid moths. Here only two pairs are
+present, those on the sixth and tenth abdominal segments. Consequently,
+as the caterpillar can cling only by the thorax and by the hinder region
+of the abdomen, the middle region of the body is first straightened out
+and then bent into an arch-like form, as the insect makes its progress
+by alternate movements of stretching and 'looping.'</p>
+
+<div class="center">
+   <a id="fig17" name="fig17"></a>
+   <img src="images/17fig.png" height="180"
+      alt="Ruby Tiger Moth (Phragmatobia fuliginosa)."
+      title="Ruby Tiger Moth (Phragmatobia fuliginosa)." />
+   <div class="caption"><p>Fig. 17. <i>c</i>, Ruby Tiger Moth (<i>Phragmatobia
+fuliginosa</i>); <i>a</i>, caterpillar; <i>b</i>, cocoon. After Lugger, <i>Insect
+Life</i>, vol. II.</p></div>
+</div>
+
+<p>Caterpillars, with their relatively soft bodies, feeding openly on the
+leaves of plants, are exposed to the attacks of many enemies, and the
+various ways in which they obtain protection are well worth studying. A
+clothing of hairs<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> or spines is often present, and it is interesting
+to find that many species of our native Tiger and Eggar Moths (Arctiadae
+and Lasiocampidae) which pass the winter in the larval stage, have
+caterpillars with an especially <a name="Page_60" id="Page_60"></a>dense hairy covering (<a href="#fig17">fig.&nbsp;17</a>).
+Experiments have shown that hairy and spiny insects are distasteful to
+birds and other creatures that prey readily on smooth-skinned species, a
+conclusion that might well have been expected. Certain smooth
+caterpillars however appear to be protected by producing some nauseous
+secretion, which renders them unpalatable. Many of these, as the
+familiar cream yellow and black larva of the Magpie Moth (<i>Abraxas
+grossulariata</i>), are very conspicuously adorned, and furnish examples of
+what is known as 'warning coloration,' on the supposition that the gaudy
+aspect of such insects serves as an advertisement that they are not fit
+to eat, and that birds and other possible devourers thus learn to leave
+them alone. On the other hand, smooth caterpillars which are readily
+eaten by birds are usually 'protectively' coloured, so as to resemble
+their surroundings and remain hidden except to careful seekers. Many
+such caterpillars are green, the upper surface, which is naturally
+exposed to the light, being darker than the lower which is in shadow.
+When the caterpillar is large, the green area is often broken up by pale
+lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or
+oblique, as on the great caterpillars of most Hawk Moths (Sphingidae).
+Such an arrangement tends to make the insect less easily seen than were
+it to display a continuous area of the same colour. The 'looper'
+caterpillars <a name="Page_61" id="Page_61"></a>mentioned above afford remarkable examples of 'protective'
+resemblance, for many of them show a marvellous likeness to the twigs of
+their food-plant, tubercles on the insect's body resembling closely the
+little outgrowths of the plant's cortex. It has been shown by <a href="#Poulton1892">E.&nbsp;B.
+Poulton (1892)</a> that many caterpillars are, in their early stages,
+directly responsive to their surroundings as regards colour. Usually
+green when hatched, they remain green if kept among leaves or young
+shoots of plants, while they turn red, brown, or blackish if placed
+among twigs of these respective hues. This effect appears to be due to a
+direct response of the subcutaneous tissue to the rays of light
+reflected from the surrounding objects. The sensitiveness dies away as
+the caterpillar grows older, since little or no change of hue in
+response to a change of environment could be induced after the
+penultimate moult.</p>
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span> </a> The 'hairs' of an insect are not in the least comparable to
+the hairs of mammals, being in truth, modified portions of the cuticle,
+secreted by special cells.</p></div>
+
+<p>Among those families of the Lepidoptera which <a name="Page_62" id="Page_62"></a>are usually regarded as
+low in the scale of organisation, caterpillars are very generally
+protected by the habit of feeding in some concealed situation. For
+example, the great larvae of the Goat Moth (Cossus) and the whitish
+caterpillars of the Clearwing Moths (Sesiidae) burrow through the wood
+of trees, eating the timber as they go. The little irritable
+caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the
+edges together with silk, and thus make for themselves a shelter; or
+they bore their way into seeds or fruits, like the larva of the Codling
+Moth that is the cause of 'worm-eaten' apples, too well-known to
+orchard-keepers. Very many small caterpillars mine between the two skins
+of a leaf, eating out the soft green tissue, and giving rise to a
+characteristic blister in form of a spreading patch or a narrow sinuous
+track through the leaf. The caterpillars of the Clothes-moths (Tineidae)
+make for themselves garments out of their own excrement, the particles
+fastened together by silk. In such curious cylindrical cases they wander
+over the wool or fur, feeding and indirectly supplying themselves with
+clothing at the same time.</p>
+
+<p>The case-forming habit of the Clothes-moth caterpillars leads us
+naturally to consider the similar habit adopted by their allies the
+Caddis-larvae which live in the waters of ponds and streams, for the
+Caddis-flies (Trichoptera) have much in common with the <a name="Page_63" id="Page_63"></a>more primitive
+Lepidoptera. The caddis-larva is as a rule of the eruciform type, but
+with well-developed thoracic legs, and with hook-like tail-appendages;
+by means of the latter it anchors itself to the extremity of its curious
+'house.' It is of interest to note that in the earlier stages of some
+caddises lately described and figured by <a href="#Siltala1907">A.&nbsp;J. Siltala (1907)</a>, the legs
+are relatively very long, and the larva is quite campodeiform in aspect.
+Some of these caddis-grubs retain the campodeiform condition and do not
+shelter permanently in cases, as their relations do. Different genera of
+caddises differ in their mode of building. Some fasten together
+fragments of water-weeds and plant refuse, others take tiny particles of
+stone, of which they make firmly compacted walls, others again lay hold
+of water-snail shells, which may even contain live inhabitants, and bind
+these into a limy rampart behind which their bodies are in safe hiding.</p>
+
+<p>The silk with which the 'caddis-worms' fasten together the materials for
+their houses is produced from spinning-glands which like those of the
+Lepidoptera open into the mouth.</p>
+
+<p>The survey of the various types of beetle-larvae enumerated above (<a href="#Page_50">pp.
+50-56</a>) concluded with a short description of the <i>legless grub</i>, which
+is the young form of a weevil or a bark-beetle. This is a larva in which
+the head alone has its cuticle firm and hard; the rest of the body is
+covered with a pale, flexible <a name="Page_64" id="Page_64"></a>cuticle, so that the grub is often
+described as 'fleshy.' This type of larva is by no means confined to
+certain families of the beetles, it is frequently met with, in more or
+less modified form, in two other important orders of insects, the
+Hymenoptera and the Diptera. Among the Hymenoptera this is indeed the
+predominant larval type. We have just seen that a caterpillar is the
+usual form of larva among the saw-flies, but in all other families of
+the Hymenoptera we find the legless grub. A grub of this order may
+usually be distinguished from the larva of a weevil or other beetle, by
+its relatively smaller head and smoother, less wrinkled cuticle; it
+strikes the observer as a feebler, more helpless creature than a
+beetle-grub. And it is of interest to note that this somewhat degraded
+type of larva is remarkably constant through a great series of
+families&mdash;gall-flies, ichneumon-flies, wasps, bees (<a href="#fig18">fig.&nbsp;18</a>), ants&mdash;that
+vary widely in the details of their structure and in their habits and
+mode of life. Almost without exception, however, they make in some way
+abundant provision for their young. The feeble, helpless, larva is in
+every case well sheltered and well fed; it has not to make its own way
+in the world, as the active armoured larva of a ground-beetle or the
+caterpillar of a butterfly is obliged to do.</p>
+
+<div class="center">
+   <a id="fig18" name="fig18"></a>
+   <img src="images/18fig.png" height="250"
+      alt="Larvae and Pupa of Hive-bee (Apis mellifica)."
+      title="Larvae and Pupa of Hive-bee (Apis mellifica)." />
+   <div class="caption"><p>Fig. 18. Young Larva (<i>FL</i>), Full-grown Larva (<i>SL</i>) and
+Pupa (<i>N</i>) of Hive-bee (<i>Apis mellifica</i>). <i>co</i>, cocoon; <i>sp</i>,
+spiracles; <i>ce</i>, eye; <i>an</i>, feeler; <i>m</i>, mandible; <i>l</i>, labium.
+Magnified 4 times. After Cheshire, <i>Bees</i>.</p></div>
+</div>
+
+<p>Among those saw-flies whose larvae feed throughout life in a concealed
+situation, we find an interesting <a name="Page_65" id="Page_65"></a>transition between the caterpillar
+and the legless grub. For example, the giant saw-flies (so called
+'Wood-wasps') have larvae that burrow in timber, and these larvae
+possess relatively large heads, somewhat flattened bodies with pointed
+tail-end, and very greatly reduced legs. The feeble legless grub,
+characteristic of the remaining families of the Hymenoptera, is provided
+for in a well-nigh endless variety of ways. The female imago among these
+insects is furnished with an elaborate and beautifully formed
+ovipositor, and the act of egg-laying is usually in itself a provision
+for the offspring. Gall-flies pierce plant-tissues within which their
+grubs find shelter and food, the plant responding to the irritation due
+<a name="Page_66" id="Page_66"></a>to the presence of the larva by forming a characteristic growth, the
+<i>gall</i>, pathological but often regular and shapely, in whose hollow
+chamber the grub lives and eats. Ichneumon-flies and their allies pierce
+the skin of caterpillars and other insect-larvae, laying their eggs
+within the victims' bodies, which their grubs proceed to devour
+internally. Some very small members of these families are content to lay
+their eggs within the eggs of larger insects, thus obtaining rich
+food-supply and effective protection for their tiny larvae. In
+Platygaster and other genera of the family Proctotrypidae, <a href="#Ganin1869">M. Ganin
+(1869)</a> showed the occurrence of hypermetamorphosis somewhat like that
+already described as occurring among the Oil-beetles (Meloidae). The
+larva of Platygaster is at first rather like a small Copepod crustacean,
+with prominent spiny tail-processes; after a moult this form changes
+into the legless grub characteristic of the Hymenoptera, among which
+larvae even approaching the campodeiform type are very exceptional. The
+species of Platygaster pass their larval stages within the larvae of
+gall-midges.</p>
+
+<p>Wasps, bees and ants, have the ovipositor of the female modified into a
+sting, which is often used for the purpose of providing food for the
+helpless grubs. Thus the digging wasps (Sphegidae and Pompilidae) hunt
+for caterpillars, spiders, and other creatures which they can paralyse
+with their stings, and bury <a name="Page_67" id="Page_67"></a>them alongside their eggs to furnish a
+food-supply for the newly-hatched young. The social wasps and many ants
+sting and kill flies and other insects, which they break up so as to
+feed their grubs within the nest. It is well known that the labour of
+tending the larvae in these insect societies is performed for the most
+part not by the mother ('Queen') but by the modified infertile females
+or 'workers.' Other ants and the bees feed their grubs (<a href="#fig18">fig.&nbsp;18</a>), also
+sheltered in well-constructed nests, on honey elaborated from nectar
+within their own digestive canals. In all cases we see that the
+helplessness of the grub is associated with some kind of parental care.</p>
+
+<div class="center">
+   <a id="fig19" name="fig19"></a>
+   <img src="images/19fig.png" height="500"
+      alt="Larva of Gall-midge (Contarinia nasturtii)."
+      title="Larva of Gall-midge (Contarinia nasturtii)." />
+   <div class="caption"><p>Fig. 19. Larva of Gall-midge (<i>Contarinia nasturtii</i>),
+ventral view showing anchor process (<i>a</i>), and spiracles projecting at
+sides. Magnified 30 times. From Carpenter, <i>Journ. Econ. Biol</i>, vol.
+VI.</p></div>
+</div>
+
+<p>From the Hymenoptera we may pass on to the Diptera or Two-winged Flies,
+an order of which the vast number of species and in many cases the
+myriads of individuals force themselves on the observer's notice. <a href="#Brauer1863">F.
+Brauer (1863)</a> divided the Diptera into two sub-orders<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a>; of the first
+of these a Crane-fly or 'Daddy-long-legs' may be taken as typical, of
+the second an ordinary House-fly or Bluebottle. All the larvae of the
+Diptera are legless, those of the Crane-fly group have well-developed
+hard heads, with biting mandibles, but in the House-fly section the
+larva is of the degraded <i>vermiculiform</i> <a name="Page_68" id="Page_68"></a>type known as the <i>maggot</i>,
+not only legless, but without a definite head, the front end of the
+creature usually tapering to the mouth, where there are a pair of strong
+hooks, used for tearing up the food. A few examples of each of these
+types must suffice in the present brief survey. A few pages back (<a href="#Page_66">p. 66</a>)
+reference was made to the production of galls on various plants, through
+the activity of larvae of the hymenopterous family Cynipidae. Many
+plant-galls are due, however, to the presence of grubs of tiny dipterous
+insects, the Cecidomyidae or Gall-midges. A cecid grub (<a href="#fig19">fig.&nbsp;19</a>) has an
+elongate body with flexible, wrinkled cuticle, tapering somewhat at the
+two ends. The head, if rather narrow, is distinct, and beneath the
+prothorax is a characteristic sclerite known as the <a name="Page_69" id="Page_69"></a>'anchor process' or
+'breast bone.' Along either side of the body is a series of paired
+spiracles, each usually situated at the tip of a little tubular
+outgrowth of <a name="Page_70" id="Page_70"></a>the cuticle; the hindmost spiracles are often larger than
+the others. These little grubs live in family communities, their
+presence leading to some deformation of the plant that serves to shelter
+them. A shrivelled fruit or an arrested and swollen shoot, such as may
+be due respectively to the Pear-midge (<i>Diplosis pyrivora</i>) or the
+Osier-midge (<i>Rhabdophaga heterobia</i>), is a frequent result of the
+irritation set up by these little grubs. In a larva of the crane-fly
+family (Tipulidae, <a href="#fig20">fig.&nbsp;20</a>) living underground and eating plant-roots,
+like the well-known 'leather-jacket' grubs of the large
+'Daddy-long-legs' (Tipula) or burrowing into a rotting turnip or swollen
+fungus, like the more slender grub of a 'Winter Gnat' (Trichocera), the
+student notices a somewhat tough cuticle, a relatively small but
+distinct head, and frequently prominent finger-like processes on the
+tail-segment. Further examination shows a striking modification in the
+arrangement of the spiracles. Instead of a paired series on most of the
+body-segments, as in caterpillars and the vast majority of insects
+whether larval or adult, there are two large spiracles surrounded by the
+prominent tail-processes, and a pair of very small ones on the
+prothorax, the latter possibly closed up and useless. This restriction
+of the breathing-holes to a front and hind pair (amphipneustic
+condition) or to a hind pair only (metapneustic type) is highly
+characteristic of the larvae of Two-winged flies.</p>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span> </a> Known as the Orthorrhapha and the Cyclorrhapha; these terms
+are derived from the manner in which the larval or pupal cuticle splits,
+as will be explained in the next chapter (<a href="#Page_88">p. 88</a>).</p></div>
+
+<div class="center">
+   <a id="fig20" name="fig20"></a>
+   <img src="images/20fig.png" height="500"
+      alt="Crane-fly (Tipula oleracea) and larva."
+      title="Crane-fly (Tipula oleracea) and larva." />
+   <div class="caption"><p>Fig. 20. Crane-fly (<i>Tipula oleracea</i>), <i>a</i>, female; <i>b</i>,
+larva ('leather-jacket' grub). Magnified twice.</p></div>
+</div>
+
+<p><a name="Page_71" id="Page_71"></a></p>
+<div class="center">
+   <a id="fig21" name="fig21"></a>
+   <img src="images/21fig.png" height="250"
+      alt="Maggot of House-fly (Musca domestica)."
+      title="Maggot of House-fly (Musca domestica)." />
+   <div class="caption"><p>Fig. 21. Maggot of House-fly (<i>Musca domestica</i>), <i>a</i>,
+side-view, magnified 5 times; <i>b</i>, prothoracic spiracle; <i>c</i>, feeler;
+<i>d</i>, hind-region with posterior spiracles; <i>e</i>, <i>f</i>, head-region with
+mouth-hooks; <i>g</i>, head-region of young maggot; <i>h</i>, eggs. All magnified.
+After Howard, <i>Entom. Bull.</i> 4, <i>U.S. Dept. Agric.</i></p></div>
+</div>
+
+<p>Turning now to the <i>maggot</i>, characteristic of the House-fly section
+(<a href="#fig21">fig.&nbsp;21</a>) of the Diptera, we see the greatest contrast between the larva
+and the imago that can be found throughout the whole class of the
+insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer
+time on exposed meat, not unnaturally arouse feelings of disgust, yet
+they are the prelude to one of the most marvellous of all insect
+life-stories. The fly&mdash;with its large globular head, bearing the
+extensive compound eyes, the highly modified feelers with their
+exquisitely feathered slender sensory tips, and the complex suctorial
+jaws; with its compact thorax bearing the glassy fore-wings <a name="Page_72" id="Page_72"></a>alone used
+for flight, though the hind-wings modified into tiny drumstick-like
+'halters' are the organs of a fine equilibrating sense&mdash;is perhaps the
+most specialised, structurally the 'highest' of all insects. Yet in a
+week or two this swift, alert, winged creature is developed from the
+degraded maggot, white, legless, headless, that buries itself in putrid
+flesh, 'feeding on corruption.'</p>
+
+<p>The broad end of the maggot is the tail, while the narrow extremity
+marks the position of the mouth. Above this are a pair of very short
+feelers (<a href="#fig21">fig.&nbsp;21</a> <i>c</i>), while from the aperture project the tips of the
+mouth-hooks (<a href="#fig21">fig.&nbsp;21</a> <i>e</i>, <i>f</i>), formidable, black, claw-like structures,
+articulated to the strong pharyngeal sclerites and moved by powerful
+muscles, tearing up the fibres of the flesh. On either side of the
+prothorax is an anterior spiracle, a curious branching or fan-like
+outgrowth (<a href="#fig21">fig.&nbsp;21</a> <i>b</i>), with a variable number of tiny openings which
+are probably of little use for the admission of air to the tubes. In
+many maggots the mouth-hooks and the front spiracles become more and
+more complex in form in the successive instars. The cuticle, white and
+smooth to the unaided eye, is seen on microscopic study to be set with
+rows of tiny spines which assist the maggot's movements through its
+food-mass. At the tail-end the large hind spiracles are conspicuous on a
+flattened dorsal area of the ninth abdominal segment; each shows a hard
+<a name="Page_73" id="Page_73"></a>brown plate, traversed by three slits. And as we watch this curious
+degraded larva thrusting its narrow head-end into the depths of its
+ofttimes loathsome food-supply, we understand the advantage of access to
+the air-tube system being mainly confined to the hinder end of the body.</p>
+
+<p>Maggots, differing from that of the Bluebottle only in minor details,
+are the larval forms of a vast multitude of allied species and display
+great variation in the nature of their food. Most, however, hide their
+soft defenceless bodies in some substance which affords shelter as well
+as food. The Bluebottle maggot burrows into flesh, that of the House-fly
+into horse-dung or vegetable refuse. The maggot of the Cabbage-fly eats
+its way into the roots of cruciferous plants, that of the Mangel-fly
+works out a broad blister between the two skins of a leaf, into which
+the newly-hatched larva crawls directly from the egg. A large number of
+species, forming an entire subfamily (the Tachininae) have larvae that
+feed as parasites within the bodies of other insects.</p>
+
+<p>The habit of parasitism by maggots in back-boned animals has led to some
+remarkable modifications of the larva and to curious adventures in the
+course of the life-story. The Bot-fly of the Horse (<i>Gastrophilus equi</i>)
+and the Warble-fly of the Ox (<i>Hypoderma bovis</i>, <a href="#fig22">fig.&nbsp;22</a>) lay eggs
+attached to the hairs of grazing animals, which, at least in the case of
+<a name="Page_74" id="Page_74"></a>Gastrophilus, lick the newly-hatched larvae into their mouths. The
+'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach;
+often a whole family attach themselves by their mouth-hooks to a small
+patch of the mucous coat of that organ. The maggot is relatively short
+and stout, with rows of strong spicules surrounding the segments, and
+with spiracles capable of withdrawal through a cup-like inpushing of the
+tail-region of the body, so that the parasite is preserved from drowning
+when the host drinks water. The young maggot of Hypoderma (<a href="#fig22">fig.&nbsp;22</a> <i>e</i>)
+is elongate and slender, spends its first two stages burrowing in the
+gullet wall and then wandering through the dorsal tissues of its host;
+ultimately it arrives beneath the skin of the back and assumes for its
+third and fourth instars a broad barrel-like form (<a href="#fig22">fig.&nbsp;22</a> <i>b</i>). The
+supply of free oxygen within the ox's tissues being now insufficient,
+the warble-maggot bores a circular hole through the skin and rests with
+the tail spiracles directed upwards towards the outer air. When fully
+grown the maggot works its way through the hole in the host's skin, and
+falling to the ground pupates in some sheltered spot, the life cycle
+occupying about a year. Similarly the Horse-bot escapes from the host's
+intestine with the excrement, and pupates on the ground.</p>
+
+<p>A curious modification of the maggot is noticeable in the larva of the
+Hover-flies (Syrphus). These, <a name="Page_75" id="Page_75"></a>unlike most of their allies, live exposed
+on the foliage of plants, where they feed by preying on aphids.</p>
+
+<div class="center">
+   <a id="fig22" name="fig22"></a>
+   <img src="images/22fig.png" height="500"
+      alt="Ox Warble-fly (Hypoderma bovis) with egg, larva, and puparium."
+      title="Ox Warble-fly (Hypoderma bovis) with egg, larva, and puparium." />
+   <div class="caption"><p>Fig. 22. Ox Warble-fly (<i>Hypoderma bovis</i>), <i>a</i>, female;
+<i>b</i>, full-grown maggot from back of ox, dorsal view; <i>c</i>, egg; <i>d</i>,
+empty puparium, ventral view; <i>e</i>, young maggot from gullet, ventral
+view. Magnified (lines show natural size). <i>a-d</i>, after Theobald, <i>2nd
+Report Econ. Zool.</i> (<i>Brit. Mus.</i>).</p></div>
+</div>
+
+<p><a name="Page_76" id="Page_76"></a>In agreement with this manner of life, the cuticle is roughly
+granulated, often greenish or reddish in hue, and the maggot, despite
+its want of definite head and sense organs, moves actively and
+purposefully about, often rearing up on its broad tail-end with an aphid
+victim impaled on its mouth-hooks.</p>
+
+<p>In a previous chapter reference was made to the exopterygote insects,
+stone-flies, dragon-flies, and may-flies, whose preparatory stages live
+in the water. Among the endopterygote orders many Neuroptera and
+Coleoptera, all Trichoptera, a very few Lepidoptera and many Diptera,
+have aquatic larvae. One or two examples of the adaptations of dipteran
+larvae to life in the water may well bring the present chapter to a
+close. Many members of the hover-fly family (Syrphidae) have maggots
+with the tail-spiracles situated at the end of a prominent tubular
+process. Among the best-known of syrphid flies are the drone-flies
+(Eristalis), often seen hovering over flowers, and presenting a curious
+likeness to hairy bees. The larva of Eristalis is one of the most
+remarkable in the whole order, the 'Rat-tailed maggot' found in the
+stagnant water of ditches and pools. It has a cylindrical body with the
+hinder end drawn out into a long telescopic tube, a more slender
+terminal section being capable of withdrawal into, or protrusion from, a
+thicker basal portion. At the extremity of the slender tube is a crown
+of sharp processes, forming <a name="Page_77" id="Page_77"></a>a stellate guard to the spiracles. These
+processes can pierce the surface-film of the water, and place the
+tracheal system of the maggot in touch with the pure upper air; while
+its mouth may be far down, feeding among the foul refuse of the ditch,
+it can still reach out to the medium in which the end of its life-story
+must be wrought out.</p>
+
+<p>Reverting to the first great division of the Diptera, we find varied
+adaptations to aquatic life among many grubs that possess a definite
+head. The larva of a Gnat (Culex<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>) has projecting from the hind region
+of the abdomen a long tubular outgrowth, at the end of which are the
+spiracles, guarded by three pointed flaps forming a valve. When closed
+these pierce the surface-film of the water in which the larva lives;
+when opened a little cup-like depression is formed in the surface-film,
+from which the larva hangs. Or having accumulated a supply of air, it
+can disengage itself from the surface-film and dive through the water,
+its tracheal system safely closed. Another mode of breathing is found in
+the 'Blood-worms' and allied larvae of the Harlequin-midges
+(Chironomidae) whose transformations are described in detail by <a href="#Miall1900">Miall
+and Hammond (1900)</a>. These larvae have two pairs of cylindrical,
+spine-bearing pro-legs&mdash;one on the prothorax and the other on the
+hindmost abdominal segment; the latter structures serve <a name="Page_78" id="Page_78"></a>to fix the
+larva in the muddy tube which it inhabits at the bottom of its native
+pond. The penultimate abdominal segment has four long hollow outgrowths,
+which contain blood, and have the function of gills, while the hindmost
+segment has four shorter outgrowths of the same nature. Enabled thus to
+breathe dissolved air, the Chironomus larva needs not, like the Culex or
+the Eristalis, to find contact with the atmosphere beyond the
+surface-film.</p>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span> </a> See <a href="#frontis"><i>Frontispiece</i></a>, A.</p></div>
+
+<p>Most remarkable, in many respects, of all aquatic larvae are the grubs
+of the Sand-midges (Simulium). These live entirely submerged and, having
+no special gills, carry out an exchange of gases through the general
+surface of the cuticle between the dissolved air in the water and the
+cavities of the air-tube system. The body is shaped like a flask swollen
+slightly at the hinder end and possesses a median pro-leg just behind
+the head, also another at the tail, which serves to attach the larva to
+a stone or to the leaf of an aquatic plant. The head has, in addition to
+feelers and jaws, a pair of processes with wonderful fringes which by
+their motion set up currents in the water, and bring food particles
+within reach of the mouth. A number of the larvae usually live in a
+community. Their power of spinning silken threads by which they can work
+their way back when accidentally dislodged from their resting-place, has
+been vividly described by <a href="#Miall1895">Miall (1895)</a>.</p>
+
+<p><a name="Page_79" id="Page_79"></a>Examples might be multiplied, but enough have been given to enforce the
+conclusion that the forms of insect-larvae are wondrously varied, and
+that frequently, within the limits of the same order or even family,
+modifications of type may be found which are suited to various modes of
+life adopted by different insects. A survey of the multitudes of insect
+larvae&mdash;grubs, caterpillars, maggots&mdash;living on land, on plants,
+underground, in the water; feeding on leaves, in stems, on roots, on
+carrion, on refuse; by hunting or by lurking after prey; as parasites or
+as scavengers, brings home to us most strongly the conclusion that each
+larva is fitted to some little niche in the vast temple of life, each is
+specially adapted to its part in the great drama of being.</p>
+
+
+
+
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII<br />
+PUPAE AND THEIR MODIFICATIONS</h2>
+
+
+<p>The pupal stage is characteristic of the life-story of those insects
+whose larvae have wing-rudiments in the form of inpushed imaginal discs,
+and in all these insects there is, as we have seen, considerable
+divergence in form between larva and imago. In <a name="Page_80" id="Page_80"></a>the pupa the wings and
+other characteristically adult structures are, for the first time,
+visible outwardly; it is the instar which marks the great crisis in
+transformation. The pupa rests, as a rule, in a quiescent condition, and
+during the early period of this stage the needful internal changes, the
+breaking down of many larval tissues, and their replacement by imaginal
+organs, go on. Both outwardly and inwardly therefore, the insect
+undergoes, at the pupal stage, a reconstruction necessitated by the
+differences in form and often in habit, between the larva and the winged
+adult; and the greater these differences, the more profound must be the
+changes that mark the pupal stage.</p>
+
+<p>From the prominence of imaginal structures in the pupa, it is at once
+seen that the pupa of any insect must resemble the adult more nearly
+than it resembles the larva. But in different groups of insects we find
+different degrees of likeness between pupa and imago. In a beetle pupa
+(see <a href="#fig16">fig.&nbsp;16</a> <i>c</i>), the appendages&mdash;feelers, jaws, legs, wings&mdash;stand out
+from the body as do those of the perfect insect. This type is called a
+<i>free</i> pupa. The pupal cuticle has to be shed for the emergence of the
+imago, but the pupa is already a somewhat reduced model of the final
+instar, with abbreviated wings and doubled-up legs. A free pupa is
+characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera
+and many <a name="Page_81" id="Page_81"></a>Diptera. In some cases the pupa requires to be specially
+adapted for a peculiar mode of life; for example, a special arrangement
+of breathing organs may be necessary for life under water, and there
+must needs be temporary pupal structures, not represented in the imago.</p>
+
+<p>On the other hand, in the pupae of most Lepidoptera and of some Diptera,
+there is more or less coalescence between the cuticle of the appendages
+and the cuticle of the body generally, so that the appendages do not
+stand out, but being, as it were, glued down to the body, are somewhat
+masked (see <a href="#fig1">fig.&nbsp;1</a> <i>e</i> and <a href="#fig23">fig.&nbsp;23</a>). Consequently the <i>obtect</i> pupa, as
+this type is called, does not resemble its imago as fully as a free pupa
+does. The outline of the wings for example in a butterfly's pupa can in
+some cases be traced only with difficulty. T.&nbsp;A. Chapman has shown <a href="#Chapman1893">(1893)</a>
+that the completely obtect pupa characterises the more highly developed
+families of Lepidoptera, while in the more primitive families the pupa
+is incompletely obtect. If the pupa of a butterfly or moth be lifted and
+held in the hand, a bending or wriggling motion of the abdomen can be
+observed. In the incompletely obtect pupa, this motion is evident in a
+greater number of segments than in the completely obtect, the number
+concerned varying from five to two in different families. In the
+nymphalid butterflies, the pupa is often called a <a name="Page_82" id="Page_82"></a>'chrysalis' on
+account of the golden hue displayed by the cuticle, and the term
+'chrysalis' is sometimes bestowed indiscriminately on any kind of pupa.
+It has been shown by <a href="#Poulton1892">Poulton (1892)</a> and others, that the colour of a
+butterfly pupa is to some extent affected by the surroundings of the
+caterpillar just before its last moult.</p>
+
+<p>Reference has been made (<a href="#Page_58">p. 58</a>) to the power of spinning silk possessed
+by many larvae; often the principal use of this silk is to form some
+protection for the pupa, the larva before its last moult constructing a
+<i>cocoon</i> within which the pupa may rest safely. Many larvae bury
+themselves in the earth, and the pupa lies in an earthen chamber, the
+lining particles of soil fastened together by fine silken threads.
+Larvae that feed in wood, like the caterpillar of the Goat-moth (Cossus)
+make a cocoon of splinters spun together, while hairy caterpillars, such
+as those of the Tiger-moths, work some of their hairs in with the silk
+to make a firm cocoon (<a href="#fig17">fig.&nbsp;17</a> <i>b</i>). On the other hand, those
+caterpillars known as 'silkworms' make a dense cocoon of pure silk,
+consisting of two layers, the outer of coarse and the inner of fine
+threads. Silken cocoons very similar in appearance are spun by the
+larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed
+of a few interlacing threads, as for example the conspicuous black and
+yellow banded pupa of the Magpie-moth <a name="Page_83" id="Page_83"></a>(<i>Abraxas grossulariata</i>) and the
+pupae of various leaf-beetles. Others again spin together the edges of
+leaves with connecting silken threads. The grubs of bees and wasps which
+are reared in the comb-chambers of their nests seal up the opening of
+the chamber with a lid, partly silk (<a href="#fig18">fig.&nbsp;18</a> <i>co</i>) and partly excretion,
+when ready to pass into the pupal state. An additional external
+'capping' may be also supplied by the workers.</p>
+
+<p>The pupae of butterflies are especially interesting, as illustrating the
+extreme reduction of the silken cocoon. The pupa of a 'swallowtail'
+(Papilionid) or a 'white' (Pierid) butterfly (<a href="#fig23">fig.&nbsp;23</a>) may be found
+attached to a twig of its food-plant or to a wall, in an upright
+position, its tail fastened to a pad of silk and a slender silken girdle
+encircling its thorax. The pupa of a 'Tortoiseshell' or 'Admiral'
+(Nymphalid) butterfly hangs head downwards from a twig, supported only
+by the tail-pad of silk, which, useless as a shelter, serves only for
+attachment. The pupa is fastened to this pad by a spiny hook or process,
+the <i>cremaster</i> (<a href="#fig23">fig.&nbsp;23</a> <i>cr</i>), on the last abdominal segment. The
+cremaster is a characteristic structure in the pupa of a moth or
+butterfly. <a href="#Riley1880">C.&nbsp;V. Riley (1880)</a> and <a href="#Hatchett1890">W. Hatchett-Jackson (1890)</a> have shown
+that it corresponds with a spiny area, the suranal plate, which lies
+above the opening of the caterpillar's intestine. The means by which the
+suspended pupa of a <a name="Page_84" id="Page_84"></a>nymphalid butterfly attaches its cremaster to the
+silken pad which the larva has spun in preparation for pupation, is
+worthy of brief attention. The caterpillar, hanging head downwards, is
+attached to the silken pad by its hindmost pair of pro-legs or claspers
+and by the suranal plate, and the cuticle is slowly worked off from
+before backwards, so as to expose the pupa. Were the process of moulting
+to be simply completed while the insect hangs by the claspers, the pupa
+would of course fall to the ground. But there is enough adhesion between
+the pupal and larval cuticles at the hinder end of the body, especially
+by means of the everted lining of the hind-gut, for the pupa to be
+supported while it jerks its cremaster out of the larval cuticle and
+works it into the meshes of the silken pad. The moult is thus completed
+and the pupa hangs securely all the time. In the numerous cases where
+the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa
+to the surrounding silk. <a href="#Chapman1893">Chapman (1893)</a> has drawn attention to the fact
+that among the more highly organised moths the pupa remains in the
+cocoon, the emergence being entirely left to the imago, while the pupae
+of the more primitive moths work their way partly out of the cocoon
+before the final moult begins. In the latter case, the cremaster is
+anchored by a strand of silk which allows a certain degree of emergence,
+and the pupa has rows of spines on its <a name="Page_85" id="Page_85"></a>abdominal segments, of which a
+greater number <a name="Page_86" id="Page_86"></a>retain the power of mutual motion than in those pupae
+which do not come out of their cocoons.</p>
+
+<div class="center">
+   <a id="fig23" name="fig23"></a>
+   <img src="images/23fig.png" height="500"
+      alt="Pupa of White Butterfly (Pieris)."
+      title="Pupa of White Butterfly (Pieris)." />
+   <div class="caption"><p>Fig. 23. Pupa of White Butterfly (<i>Pieris</i>), side view;
+<i>f</i>, feeler; <i>w</i>, wing; <i>sp</i>, spiracle; <i>p</i>, anal pro-leg; <i>cr</i>,
+cremaster. Magnified 8 times. In part after Hatchett-Jackson, <i>Trans.
+Linn. Soc.</i> 1900, and Tutt's <i>British Butterflies</i>.</p></div>
+</div>
+
+<p>While the pupa on the whole resembles the imago that is to emerge from
+it, there are not a few cases in which a special structure necessary for
+some contingency in pupal life is retained or adopted in this stage. A
+butterfly pupa, like the imago, has no mandibles, but in the case of the
+Caddis-flies (Trichoptera) and two families of small moths, the most
+primitive of all Lepidoptera, the pupa, like the larva, has
+well-developed mandibles. These enable the caddis pupa to bite its way
+out of the shortened larval case in which it has pupated, and then to
+swim upwards through the water ready for the caddis-fly's emergence into
+the air. Pupae that are submerged require special breathing-organs. In
+the previous chapter (<a href="#Page_77">p. 77</a>) mention was made of the gnat's aquatic
+larva with its tail-spiracles adapted for procuring atmospheric air
+through the surface-film. The pupa of the gnat<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> also has 'respiratory
+trumpets' serving the same purpose, but these are a pair of processes on
+the prothorax, so that the pupa, which is fairly active, hangs from the
+surface-film with its abdomen pointing downwards through the water. This
+change of position is correlated with the necessity for the imago to
+emerge into the air; were the pupa to hang head downwards as the larva
+does, the gnat would <a name="Page_87" id="Page_87"></a>perforce have to dive into the water. With the
+beautifully adapted transfer of the functional spiracles, their position
+is appropriately arranged for the gnat's emergence at the surface, and
+the empty pupal cuticle floats serving the insect as a raft. On this it
+rests securely and the crumpled wings have opportunity to expand and
+harden before the insect takes to flight.</p>
+
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span> </a> See <a href="#frontis"><i>Frontispiece</i></a>, B.</p></div>
+
+<p>The aquatic pupae of other Diptera, many species of the midges
+Chironomus and Simulium for example, breathe dissolved air by means of
+tufts of thread-like gills, which arise on either side of the prothorax.
+The pupae of Simulium rest in their curious little cup-like dwellings,
+attached to submerged stones or plants. The Chironomus pupa is usually
+found in an elongate gelatinous case adhering to a stone. From this case
+the pupa rises to the surface of the water, that the midge may emerge
+into the air. <a href="#Miall1900">Miall and Hammond (1900)</a> describe the arrangement by
+which, when the pupal stage ends, and these gills are no longer
+required, their connection with the air-tube system is severed 'without
+undue violence.' The walls of the fine air-tubes that pass into the
+gills are specially strengthened, but just below the pupal cuticle these
+walls are exceedingly thin and delicate. Thus when the pupal cuticle is
+cast, they are readily broken there, and the cuticle of the midge
+forming beneath has a spiracular opening into the main air-trunk, ready
+for use during the insect's aerial life.</p>
+
+<p><a name="Page_88" id="Page_88"></a>Among those Diptera whose larva is the headless maggot a most
+remarkable arrangement for protecting the pupa is to be found. The last
+larval cuticle, instead of being as usual worked off and cast, after
+separation from the underlying structures, becomes hard and firm,
+forming a protective case (<i>puparium</i>) within which by the processes of
+histolysis and histogenesis already described the organs of the pupa and
+imago are built up. This puparium (<a href="#fig22">fig.&nbsp;22</a> <i>d</i>) is usually dark in
+colour, often brown and barrel-shaped, and a subcircular lid splits off
+from it at the head-end to allow the emergence of the fly<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a>. While the
+maggot breathes by its tail-spiracles, the functional spiracles of the
+puparium (connected with the tracheal system of the enclosed pupa) are
+far forward, and these may be situated at the tips of long sometimes
+branching processes, which recall the thoracic gills of the aquatic
+pupae mentioned a few pages above. Adaptations, various and beautiful,
+to special modes of life, are thus seen to characterise pupae as well as
+larvae.</p>
+
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span> </a> The presence of this sub-circular lid characterises
+Brauer's suborder Cyclorrhapha. Those Diptera in which the pupal cuticle
+splits in the normal, longitudinal manner are included in the
+Orthorrhapha (see <a href="#Page_67">p. 67</a>).</p></div>
+
+
+
+<p><a name="Page_89" id="Page_89"></a></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII<br />
+THE LIFE-STORY AND THE SEASONS</h2>
+
+
+<p>A number of interesting questions are associated with the seasonal cycle
+of an insect's life-history. In a previous chapter (<a href="#CHAPTER_IV">IV</a>. pp. <a href="#Page_30">30</a>, <a href="#Page_34">34</a>)
+reference has been made to the contrast between the long aquatic life of
+the larval dragon-fly or may-fly, extending over several years, and the
+short aerial existence of the winged adult restricted in the case of the
+may-flies to a few hours. Here we see that the feeding activities of the
+insect are carried on during the larval stage only; the may-fly in its
+winged condition takes no food, pairing and egg-laying form the whole of
+its appointed task. A similar though less extreme shortening of the
+imaginal life may be noticed in many endopterygote insects. For example,
+the bot- and warble-flies have the jaws so far reduced that they are
+unable to feed, and the parasitic life of the maggot (see <a href="#Page_74">p. 74</a>)
+extending over eight or nine months in the body of the horse or ox,
+prepares for a winged existence of probably but a few days. Again in
+many moths the jaws are reduced or vestigial so that no food can be
+taken in the winged state, as for example in the 'Eggars'
+(Lasiocampidae) <a name="Page_90" id="Page_90"></a>and the 'Tussocks' (Lymantriidae). It is noteworthy
+that in these short-lived insects the male is often provided with
+elaborate sense-organs which, we may believe, assist him to find a mate
+with as little delay as possible; the male may-fly has especially
+complex eyes, while the feelers of the male silk-moth or eggar are
+comb-like or feathery, the branches bearing thousands of sensory hairs.
+A box with a captive living female of one of these moths, if taken into
+a wood haunted by the species becomes rapidly surrounded by a swarm of
+would-be suitors, attracted by the odour emitted from the prisoner's
+scent-glands.</p>
+
+<p>Very exceptionally the imaginal stage may be omitted from the life-story
+altogether. Nearly fifty years ago <a href="#Wagner1865">N. Wagner (1865)</a> made the remarkable
+discovery that in the larvae of certain gall-midges (Cecidomyidae) the
+ovaries might become precociously mature and unfertilised eggs might be
+developed into small larvae observable within the body of the
+mother-larva; ultimately these abnormally reared young break their way
+out. In this case therefore there may be a series of larval generations,
+neither pupa nor imago being formed. Extended observations on the
+precocious reproductive processes of these midges have lately been
+published by <a href="#Kahle1908">W. Kahle (1908)</a>. A less extreme instance of an abbreviated
+life-story was made known by <a name="Page_91" id="Page_91"></a><a href="#Grimm1870">O. Grimm (1870)</a> who saw pupae of
+Harlequin-midges (Chironomus) lay unfertilised eggs, which developed
+into larvae. Here the imaginal stage only is omitted from the
+life-history. Not always however is it the imaginal stage of the
+life-history which is shortened. Reference (<a href="#Page_18">p. 18</a>) has already been made
+to the case of the virgin female aphids, whose eggs develop within the
+mother's body, so that active, formed young are brought forth. Among the
+Diptera it is not unusual to find similar cases, the female fly giving
+birth to young maggots instead of laying eggs. Such is the habit of the
+great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.)
+whose larvae live as parasites on other insects, and occasionally of the
+Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a
+shortened larval period, and Brace's investigations in 1895, summarised
+by <a href="#Austen1911">E.&nbsp;E. Austen (1911)</a>, have shown that females of the dreaded African
+Tsetse flies (Glossinia) bring forth nearly mature larvae, which pupate
+soon after birth. In another group of Diptera, the blood-sucking
+parasites of the Hippoboscidae and allied families, the whole larval
+development is passed through within the mother's body, and a full-grown
+larva is born the cuticle of which hardens and darkens immediately to
+form a puparium; hence these flies are often called, though incorrectly,
+Pupipara. Still more astonishing is the mode of reproduction in the
+allied family of <a name="Page_92" id="Page_92"></a>the Termitoxeniidae, curious, degraded, wingless
+'guests' of the termites, or 'white ants,' lately made known through the
+researches of <a href="#Wasmann1901">E. Wasmann (1901)</a>. Here the individual is hermaphrodite&mdash;a
+most exceptional condition among insects&mdash;and lays a large egg, whence
+is usually hatched a fully-developed adult! Here then we find that all
+the early stages, usual in the higher insects, are omitted from the
+life-story.</p>
+
+<p>Interesting comparison may be made between the total duration of various
+insect life-stories. To some extent at least, the length of an insect's
+life is correlated with its size, its food, the season of the year when
+it breeds. Small insects have, as a rule, shorter lives than large ones;
+those whose larvae devour highly nutritive food generally develop more
+quickly than those which have to live on dry, poor, substances;
+life-cycles follow one another most rapidly in summer weather when
+temperature is high and food plentiful.</p>
+
+<p>In early chapters we have already noticed the long aquatic life of the
+larva and nymph of a dragon-fly, relatively a large insect, and the
+rapid multiplication of the repeated summer broods of virgin aphids (p.
+18). Within the one order of the Coleoptera it is instructive to compare
+the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose
+larvae mine in the green tissues, and complete <a name="Page_93" id="Page_93"></a>their transformations so
+rapidly that several successive broods appear in the spring and early
+summer, with the larger click-beetles whose larvae, the equally
+notorious 'wireworms,' feed on roots for three or four years before they
+become fully grown. Among the Diptera, the 'leather-jacket' grub of the
+crane-fly, feeding like the wireworm on roots, has a larval life
+extending through the greater part of a year, while the maggot of the
+bluebottle, feeding on a rich meat diet, becomes mature in a few days.
+As examples of excessively long life-cycles the 'thirteen-year' and
+'seventeen-year' cicads of North America, described by <a href="#Marlatt1898">C.&nbsp;L. Marlatt
+(189<ins class="correction" title="Transcriber's note: The Bibliography lists this item with year 1898 not 1895.">5</ins>)</a>, are noteworthy. Certain specially populous 'broods' of these
+insects are known and localised, so that the appearance of the imagos in
+future years can be accurately predicted. Here again we have to do with
+bulky insects whose subterranean larvae and nymphs feed on comparatively
+innutritious roots.</p>
+
+<p>In our own climate, it is of interest to notice the variation among
+insects as to the stage which carries the race over the winter. The
+click-beetles, mentioned just above, emerge from their buried pupae in
+summer, hibernate under stones or clods, and lay eggs among the herbage
+next spring. At the same time of course, owing to the extended term of
+the larval life, many more individuals of the species are wintering
+underground as 'wireworms' of various <a name="Page_94" id="Page_94"></a>ages, and these, except in very
+severe frosts, can continue their occupation of feeding on roots. But in
+the case of the 'turnip-flies' the food-supply is cut off in winter, and
+all those beetles of the latest summer brood that survive hibernate in
+some sheltered spot, waiting for the return of spring, that they may lay
+their eggs, and start the life-cycle once again. Among the Diptera, most
+species pass the winter as pupae, the sheltering puparium being a good
+protection against most adverse conditions, or as flies. But where there
+is a prolonged parasitic larval life, as with the bot- and warble-flies,
+the maggot, warm and well-fed within the body of its mammalian host,
+affords an appropriate wintering stage.</p>
+
+<p>Among the Hymenoptera an especially interesting seasonal life-cycle is
+afforded by the alternation of summer and winter generations in many
+Gall-flies (Cynipidae) as H. Adler (<a href="#Adler1881">1881</a>, <a href="#Adler1896">1896</a>) demonstrated for most of
+our common species. The well-known 'oak-apples' are tenanted in summer
+by grubs, which after pupation develop into winged males and wingless
+females. The latter, after pairing, burrow underground and lay their
+eggs in the roots, the larvae causing the presence there of globular
+swellings or root-galls within which they live, pass through their
+transformations and develop into wingless virgin females. These shelter
+until February or March in <a name="Page_95" id="Page_95"></a>their underground chambers, then climb up
+the tree and lay on the shoots eggs, from which will be hatched the
+grubs destined to grow within the oak-apples into the summer sexual
+brood of flies.</p>
+
+<p>The Lepidoptera afford examples of hibernation in all stages of the
+life-history. In this order a few large moths with wood-boring
+caterpillars, the 'Goat' (Cossus) for example, undergo a development
+extending over several years, while at the other extreme a few small
+species may have three or more complete cycles within the twelve months.
+But in the vast majority of Lepidoptera we find either one or two
+generations, definitely seasonal, within the year; the insect is either
+'single-brooded' or 'double-brooded.'</p>
+
+<p>Almost every winter one or more letters may be read in some newspaper
+recording the writer's surprise at seeing on a sunny day during the cold
+season, one of our common gaily-coloured butterflies of the Vanessa
+group, a 'Tortoiseshell' or 'Red Admiral,' flitting about. Surprise
+might be greater did the observers realise that the imaginal is the
+normal hibernating stage for these species. Emerging from the pupa in
+late summer or autumn, they shelter during winter in hollow trees, under
+thatched eaves, in outbuildings or in similar situations, coming out in
+spring to lay their eggs on the leaves of their caterpillars'
+food-plants. The larvae feed and grow <a name="Page_96" id="Page_96"></a>through the early summer months,
+in the case of the Small Tortoiseshell (<i>Vanessa urticae</i>) pupating
+before midsummer and developing into a July brood of butterflies whose
+offspring after a late summer life-cycle, hibernate; while for the
+larger species of the group there is, in our islands, only one complete
+life-cycle in the year, though the same insects in warmer countries may
+be double-brooded. C.&nbsp;G. Barrett records (<a href="#Barrett1893">1893</a>, vol. I. pp. 153-4) how in
+the August of 1879 hundreds and thousands of 'Painted Ladies' (<i>Pyrameis
+cardui</i>) migrated into the south of England from the European continent
+where in many places great swarms had been observed early in the summer.
+'These August butterflies, the progeny of the June swarms, coming from a
+warmer climate, had no intention of hibernating, but paired and laid
+eggs. Some of the larvae were collected and reared indoors [butterflies]
+emerging in November and December, but out of doors all must have been
+destroyed by damp or frost, in either the larva or pupa state, for no
+freshly emerged specimens were noticed in the spring, and no trace of
+the great migration remained.'</p>
+
+<p>In September and October the pedestrian, even in a suburban square, may
+see moths with pretty brown, white-spotted wings flying around trees.
+These are males of the common 'Vapourer' (<i>Orgyia antiqua</i>), in search
+of the females which, wingless and helpless, rest on the cocoons
+surrounding the pupae whence <a name="Page_97" id="Page_97"></a>they have just emerged, the cocoons being
+attached to the branches of the trees where the caterpillars have fed.
+After pairing, the female lays her eggs among the silk of the cocoon,
+partly covering them with hairs shed from her body, and then dies. The
+eggs thus protected remain through the winter, the larvae not being
+hatched till springtide, when the young leaves begin to sprout forth.
+The caterpillars, adorned and probably protected by their 'tussocks' of
+black or coloured bristles, feed vigorously. Their activity and habit of
+occasional migration from one tree to another, compensates, to some
+extent, as <a href="#Miall1908">Miall (1908)</a> has pointed out, for the females' enforced
+passivity; only in the larval state can moths with such wingless females
+extend their range. The caterpillars spin their cocoons towards the end
+of summer, and then pupate, the moths emerging in the autumn and the
+eggs, as we have seen, furnishing the winter stage.</p>
+
+<p>After midsummer, the conspicuous cream, black and yellow-spotted
+'Magpie' moth (<i>Abraxas grossulariata</i>) is common in gardens. The female
+lays her eggs on a variety of shrubby plants; gooseberry and currant
+bushes are often chosen. From the eggs caterpillars are hatched in
+autumn, but these, instead of beginning to feed, seek almost at once for
+rolled-up leaves, cracks in walls, crannies of bark, or similar places,
+which may afford winter shelters. Here they <a name="Page_98" id="Page_98"></a>remain until the spring,
+when they come out to feed on the young foliage and grow rapidly into
+the conspicuous cream, yellow and black 'looper' caterpillars mentioned
+in a previous chapter (<a href="#Page_60">p. 60</a>). These, when fully-grown, spin among the
+twigs of the food-plant a light cocoon, in which the black and
+yellow-banded wasp-like pupa spends its short summer term before the
+emergence of the moth.</p>
+
+<p>An equally familiar garden insect, the common 'Tiger' moth (<i>Arctia
+caia</i>) with its 'woolly bear' caterpillar, affords a life-cycle slightly
+differing from that of the 'Magpie.' The gaudy winged insects are seen
+in July and August, and lay their eggs on a great variety of plants. The
+larvae hatched from these eggs begin to feed at once, and having moulted
+once or twice and attained about half their full size, they rest through
+the winter, the dense hairy covering wherewith they are provided forming
+an effective protection against the cold. At the approach of spring they
+begin to feed again, and the fully-grown 'woolly bear' is a common
+object on garden paths in May and June. Before midsummer it has usually
+spun its yellow cocoon under some shelter on the ground and changed into
+a pupa.</p>
+
+<p>Another modification with respect to seasonal change is shown by the
+Turnip moth (<i>Agrotis segetum</i>) and other allied Noctuidae (Owl-moths).
+These are insects with brown-coloured wings, flying after dark <a name="Page_99" id="Page_99"></a>in June.
+The dull greyish larvae feed on many kinds of low-growing plants,
+usually hiding in the earth by day and wandering along the surface of
+the ground by night, biting off the farmer's ripening corn, or burrowing
+into his turnips or potatoes. On account of the burrowing habits of this
+insect it can feed throughout the winter, except when a hard frost puts
+a temporary stop to its activity. By April it has become fully grown and
+pupates in an earthen chamber a few inches below the surface. The Turnip
+moth in our countries is partially double-brooded, a minority of the
+autumn caterpillars growing more rapidly than their comrades so that
+they pupate, and a second brood of moths appear in September. These pair
+and lay eggs, the resulting caterpillars going as Barrett suggests
+(<a href="#Barrett1893">1896</a>, vol. III. p. 291) 'to reinforce the great army of wintering
+larvae.'</p>
+
+<p>Such underground caterpillars, to a great extent protected from cold,
+can continue to feed through the winter. With other species we find that
+the larva becomes fully grown in autumn, yet lives through the winter
+without further change. This is the case with the Codling moth
+(<i>Carpocapsa pomonella</i>), a well-known orchard pest, which in our
+countries is usually single-brooded. The moth is flying in May and lays
+her eggs on the shoots or leaves of apple-trees, more rarely on the
+fruitlets, into which however the caterpillar always bores by <a name="Page_100" id="Page_100"></a>the upper
+(calyx) end. Here it feeds, growing with the growth of the fruit,
+feeding on the tissue around the cores, ultimately eating its way out
+through a lateral hole, and crawling upwards if its apple-habitation has
+fallen, downwards if it still remains on the bough, to shelter under a
+loose piece of bark where it spins its cocoon about midsummer and
+hibernates still in the larval condition. Not until spring is the pupal
+form assumed, and then it quickly passes into the imaginal state. In the
+south of England, as <a href="#Theobald1909">F.&nbsp;V. Theobald (1909)</a> has lately shown, and also in
+southwestern Ireland, this species may be double-brooded, the usual
+condition on the European continent and in the United States of America.
+There the midsummer larvae pupate at once and the moths of an August
+brood lay eggs on the hanging or stored fruit; in this case, again,
+however, the full-grown larva, quickly fed-up within the developed
+apples, is the wintering stage.</p>
+
+<p>Several of the insects mentioned in this survey, like the last-named
+codling moth, are occasionally double-brooded. As an example of the many
+Lepidoptera, which in our islands have normally two complete life-cycles
+in the year, we may take the very familiar White butterflies (Pieris) of
+which three species are common everywhere. The appearance of the first
+brood of these butterflies on the wing in late April or May is hailed as
+a sign of advanced <a name="Page_101" id="Page_101"></a>spring-time. They pair and lay their eggs on
+cabbages and other plants, and the green hairy caterpillars feed in June
+and July, after which the spotted pupae may be found on fences and
+walls, attached by the silken tail-pad and supported by the
+waist-girdle. In August and September butterflies of the second brood
+have emerged from these and are on the wing; their offspring are the
+autumn caterpillars which feed in some seasons as late as November,
+doing often serious damage to the late cruciferous crops before they
+pupate. The pupae may be seen during the winter months, waiting for the
+spring sunshine to call out the butterflies whose structures are being
+formed beneath the hard cuticle.</p>
+
+<p>Reviewing the small selection of life-stories of various Lepidoptera
+just sketched, we notice an interesting and suggestive variety in the
+wintering stage. The vanessid butterflies hibernate as imagos; the
+'vapourer' winters in the egg, the magpie as a young ungrown larva, the
+'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
+winter, growing all the time; the codling caterpillar completes its
+growth in the autumn, and winters as a full-size resting larva; lastly,
+the 'whites' hibernate in the pupal state. And in every case it is
+noteworthy that the form or habit of the wintering stage is well adapted
+for enduring cold.</p>
+
+<p>Our native 'whites' afford illustration of another <a name="Page_102" id="Page_102"></a>interesting feature
+often to be noticed in the life-story of double-brooded Lepidoptera. The
+butterflies of the spring brood differ slightly but constantly from
+their summer offspring, affording examples of what is called <i>seasonal
+dimorphism</i>. All three species have whitish wings marked with black
+spots, larger and more numerous in the female than in the male. In the
+spring butterflies these spots tend towards reduction or replacement by
+grey, while in the summer insects they are more strongly defined, and
+the ground colour of the wings varies towards yellowish. In the
+'Green-veined' white (<i>Pieris napi</i>) the characteristic greenish-grey
+lines of scaling beneath the wings along the nervures, are much broader
+and more strongly marked in the spring than in the summer generation,
+whose members are distinguished by systematic entomologists under the
+varietal name <i>napaeae</i>. The two forms of this insect were discussed by
+A. Weismann in his classical work on the Seasonal Dimorphism of
+butterflies <a href="#Weismann1876">(1876)</a>. He tried the effect of artificially induced cold
+conditions on the summer pupae of <i>Pieris napi</i>, and by keeping a batch
+for three months at the temperature of freezing water, he succeeded in
+completely changing every individual of the summer generation into the
+winter form. The reverse of this experiment also was attempted by
+Weismann. He took a female of <i>bryoniae</i>, an alpine and arctic variety
+of <i>Pieris napi</i>, showing in an <a name="Page_103" id="Page_103"></a>intensive degree the characters of the
+spring brood. This female laid eggs the caterpillars from which fed and
+pupated. The pupae although kept through the summer in a hothouse all
+produced typical <i>bryoniae</i>, and none of these with one exception
+appeared until the next year, for in the alpine and arctic regions this
+species is only single-brooded. Weismann experimented also with a small
+vanessid butterfly, <i>Araschnia levana</i>, common on the European
+continent, though unknown in our islands, which is double (or at times
+treble) brooded, its spring form (<i>levana</i>) alternating with a larger
+and more brightly coloured summer form (<i>prorsa</i>). Here again by
+refrigerating the summer pupae, butterflies were reared most of which
+approached the winter pattern, but it was impossible by heating the
+winter pupae to change <i>levana</i> into <i>prorsa</i>. Experiments with North
+American dimorphic species have given similar results. Weismann argued
+from these experiments that the winter form of these seasonally
+dimorphic species is in all cases the older, and that the butterflies
+developing within the summer pupae can be made to revert to the
+ancestral condition by repeating the low-temperature stimulus which
+always prevailed during the geologically recent Ice Age. On the other
+hand, a high temperature stimulus applied to one generation of the
+winter pupae cannot induce the change into the summer pattern, which has
+been <a name="Page_104" id="Page_104"></a>evolved still more recently by slow stages, as the continental
+climate has become more genial. In tropical countries where instead of
+an alternation of winter and summer, alternate dry and rainy seasons
+prevail, somewhat similar seasonal dimorphism has been observed among
+many butterflies. Not a few forms of Precis, an African and Indian genus
+allied to our Vanessa, that had long been considered distinct species
+are now known, thanks to the researches of <a href="#Marshall1898">G.&nbsp;A.&nbsp;K. Marshall (1898)</a>, to be
+alternating seasonal forms of the same insect. The offspring when adult
+does not closely resemble the parent; its appearance is modified by the
+climatic environment of the pupa. The experiments of Weismann just
+sketched in outline show at least that the same principle holds for our
+northern butterflies.</p>
+
+<p>We are thus led to see from the life-story of such insects, that the
+course of the story is not rigidly fixed; the creature in its various
+stages is plastic, open to influence from its surroundings, capable of
+marked change in the course of generations. And so the seasonal changes
+in the history of the individual from egg to imago point us to changes
+in the age-long history of the race.</p>
+
+
+
+<p><a name="Page_105" id="Page_105"></a></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX<br />
+PAST AND PRESENT; THE MEANING OF THE STORY</h2>
+
+
+<p>In the previous chapter we recognised how the seasonal changes in
+various species of butterflies as observable in two or three
+generations, indicate changes in the history of the race as it might be
+traced through innumerable generations. The endless variety in the form
+and habits of insect-larvae and their adaptations to various modes of
+life, which have been briefly sketched in this little book, suggest
+vaster changes in the class of insects, as a whole, through the long
+periods of geological time. Every student of life, influenced by the
+teaching of Charles <a href="#Darwin1859">Darwin (1859)</a> and his successors, now regards all
+groups of animals from the evolutionary standpoint, and believes that
+comparisons of facts of structure and life-history of orders and classes
+evidently akin to each other, furnish at least some indications of the
+course of development in the greater systematic divisions, even as the
+facts of seasonal dimorphism, mentioned in the last chapter, give hints
+as to the course of development in those restricted groups that we call
+species or varieties. A brief discussion <a name="Page_106" id="Page_106"></a>of the main outlines of the
+life-story of insects in the wide, evolutionary sense may thus fitly
+conclude this book.</p>
+
+<p>In the first place we turn to the 'records' of those rocks, in whose
+stratified layers<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> are entombed remains, often fragmentary and
+obscure, of the insects of past ages of the earth's history. Compared
+with the thousands of extinct types of hard-shelled marine animals, such
+as the Mollusca, fossil insects are few, as could only be expected,
+seeing that insects are terrestrial and aerial creatures with slight
+chance of preservation in sediments formed under water. Yet a number of
+insect remains are now known to naturalists, who are, in this
+connection, more particularly indebted to the researches of <a href="#Scudder1885">S.&nbsp;H. Scudder
+(1885)</a>, <a href="#Brongniart1894">C. Brongniart (1894)</a>, and <a href="#Handlirsch1906">A. Handlirsch (1906)</a>.</p>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span> </a> See Table of Geological Systems, <a href="#Page_123">p. 123</a>.</p></div>
+
+<p>We are now considering insects from the standpoint of their
+life-histories, and the individual life-story of an insect of which we
+possess but a few fragments of wings or body, entombed in a rock formed
+possibly before the period of the Coal Measures, can only be a matter of
+inference. Still it may safely be inferred that when the structure of
+these remains clearly indicates affinity to some existing order or
+family, the life-history of the extinct creature must have resembled, on
+the whole, <a name="Page_107" id="Page_107"></a>that of its nearest living allies. And all the fossil
+insects known can be either referred to existing orders, or shown to
+indicate definite relationship to some existing group.</p>
+
+<p>Passing over some doubtful remains of Silurian age, we find in rocks
+usually regarded as Devonian<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> the most ancient fossils that can be
+certainly referred to the insects, while from beds of the succeeding
+Carboniferous period, a number of insect remains have been disinterred.
+These Palaeozoic insects were frequently of large size, and they show
+distinct affinities with our recent may-flies, dragon-flies,
+stone-flies, and cockroaches. In the Permian period, the latest of the
+divisions of the Palaeozoic, lived Eugereon, an insect with hemipteroid
+jaws and orthopteroid wings. All these insects must have been
+exopterygote in their life-history, if we may trust the indications of
+affinity furnished by their structure. In the Mesozoic period, however,
+insects with complete transformations must have been fairly abundant.
+Rocks of Triassic age have yielded beetles and lacewing-flies, while
+from among Jurassic fossils specimens have been described as
+representing most of our existing orders, including Lepidoptera,
+Hymenoptera and Diptera. In Cainozoic rocks fossil insects of nearly six
+thousand species have been found, which are easily <a name="Page_108" id="Page_108"></a>referable to
+existing families and often to existing genera. We may conclude then,
+imperfect though our knowledge of extinct insects is, that some of the
+most complex of insect life-stories were being worked out before the
+dawn of the Cainozoic era. Some instructive hints as to differences in
+the rate of change among different insect groups may be drawn from the
+study of parasites. For example, <a href="#Kellogg1913">V.&nbsp;L. Kellogg (1913)</a> points out that an
+identical species of the Mallophaga (Bird-lice) infests an Australian
+Cassowary and two of the South American Rheas; while two species of the
+same genus (Lipeurus) are common to the African Ostrich and a third kind
+of South American Rhea. These parasites must have been inherited
+unchanged by the various members of these three families of flightless
+birds from their common ancestors, that is from early Cainozoic times at
+latest. On the other hand, the various kinds of such highly specialised
+parasites as the warble-flies of the oxen and deer, must have become
+differentiated during those later stages of the Cainozoic period which
+witnessed the evolution of their respective mammalian hosts.</p>
+
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span> </a> The 'Little River' beds of St John, New Brunswick, Canada,
+by some modern geologists however considered as Carboniferous.</p></div>
+
+<p>The foregoing brief outline of our knowledge of the geological
+succession of insects shows that the exopterygote preceded, in time, the
+endopterygote type of life-history. We have already seen that those
+insects undergoing little change in the life-cycle, and with visible,
+external wing-rudiments, <a name="Page_109" id="Page_109"></a>are on the whole less specialised in structure
+than those which pass through a complete transformation. These two
+considerations, taken together, suggest strongly that in the evolution
+of the insect class, the simpler life-history preceded the more complex.
+Such a conclusion seems reasonable and what might have been expected,
+but we are confronted with the difficulty that if the most highly
+organised insects pass through the most profound transformations, then
+insects present a remarkable and puzzling exception to the general rules
+of development among animals, as has already been pointed out in the
+first chapter of this volume (<a href="#Page_7">p. 7</a>). A few students of insect
+transformation have indeed supposed that the crawling caterpillar or
+maggot must be regarded as a larval stage which recalls the worm-like
+nature of the supposed far-off ancestors of insects generally. Even in
+Poulton's classical memoir (<a href="#Poulton1891">1891</a>, p. 190), this view finds some support,
+and it may be hard to give up the seductive idea that the worm-like
+insect-larva has some phylogenetic meaning. But the weight of evidence,
+when we take a comprehensive survey of the life-story of insects, must
+be pronounced to be strongly in favour of the view put forward by <a href="#Brauer1869">Brauer (1869)</a>, and since supported by the great majority of naturalists who
+have discussed the subject, that the caterpillar or the maggot is itself
+a specialised product of the evolutionary <a name="Page_110" id="Page_110"></a>process, adapted to its own
+particular mode of larval life.</p>
+
+<p>The explanation of insect transformation is, in brief, to be found in an
+increasing amount of divergence between larva and imago. The most
+profound metamorphosis is but a special type of growth, accompanied by
+successive castings and renewings of the chitinous cuticle, which
+envelopes all arthropods. In the simplest type of insect life-story,
+there is no marked difference in form between the newly-hatched young
+and the adult, and in such cases we find that the young insect lives in
+the same way as the adult, has the same surroundings, eats the same
+food. This is the rule (see Chapters II and III) with the Apterygota,
+the Orthoptera, and most of the Hemiptera. In the last-named order,
+however, we find in certain families marked divergence between larva and
+imago, for example in the cicads, whose larvae live underground, while
+in the coccids, whose males are highly specialised and females degraded,
+there succeeds to the larva&mdash;very like the young stage in allied
+families&mdash;a resting instar, which in the case of the male, suggests
+comparison with the pupa of a moth or beetle.</p>
+
+<p>Turning to the stone-flies, dragon-flies and may-flies, whose
+life-stories have been sketched in Chapter IV, we find that the early
+stages are passed in water, whence before the final moult, the insects
+<a name="Page_111" id="Page_111"></a>emerge to the upper air. Except for the possession of tufted gills,
+adapting them to an aquatic life, the stone-fly nymphs differ but
+slightly from the adults; the grubs of the dragon-flies and may-flies,
+however, are markedly different from their parents. In connection with
+these comparisons, it is to be noted that the dragon-flies and may-flies
+are more highly specialised insects than stone-flies, divergent
+specialisation of the adult and larva is therefore well illustrated in
+these groups, which nevertheless have, like the Hemiptera and
+Orthoptera, visible external wing-rudiments.</p>
+
+<p>From the vast array of insects that show internal wing-growth and a true
+pupal stage, a few larval types were chosen for description in Chapter
+VI, and a review of these suggests again the thought of increasing
+divergence between larva and imago. Reference has been made previously
+to the many instances in which the former has become pre-eminently the
+feeding, and the latter the breeding stage in the life-cycle. It seems
+impossible to avoid the conclusion that the active, armoured
+campodeiform grub differing less from its parent than an eruciform larva
+differs from its parent, is as a larval type more primitive than the
+caterpillar or maggot. A. Lameere has indeed, while admitting the
+adaptive character of insect larvae generally, argued <a href="#Lameere1899">(1899)</a> with much
+ingenuity that the eruciform or vermiform type must <a name="Page_112" id="Page_112"></a>have been primitive
+among the Endopterygota, believing that the original environment of the
+larvae of the ancestral stock of all these insects must have been the
+interior of plant tissues. He is thus forced to the necessity of
+suggesting that the campodeiform larvae of ground-beetles or lacewings
+must be regarded as due to secondarily acquired adaptations; 'they
+resemble Thysanura and the larvae of Heterometabola only as whales
+resemble fishes.' There are two considerations which render these
+theories untenable. The Neuroptera and Coleoptera among which
+campodeiform larvae are common, are less specialised than Lepidoptera,
+Hymenoptera, and Diptera, in which they are unknown. And among the
+Coleoptera which as we have seen (<a href="#Page_50">pp. 50<i>f</i></a>.) display a most
+interesting variety of larval structure, the legless, eruciform larva
+characterises families in which the imago shows the greatest
+specialisation, while in the same life-story, as in the case of the
+oil-beetles (<a href="#Page_56">pp. 56-7</a>), the newly-hatched grub may be campodeiform,
+changing to the eruciform type as soon as it finds itself within reach
+of its host's rich store of food.</p>
+
+<p>A certain amount of difficulty may be felt with regard to the theory of
+divergent evolution between imago and larva, in the case of those
+insects with complete transformation whose grubs and adults live in much
+the same conditions. By turning over stones the naturalist may find
+ground-beetles in <a name="Page_113" id="Page_113"></a>company with the larvae of their own species. On the
+leaves of a willow tree he may observe leaf-beetles (Phyllodecta and
+Galerucella) together with their grubs, all greedily eating the foliage;
+or lady-bird beetles (Coccinella) and their larvae hunting and devouring
+the 'greenfly.' All of these insects are, however, Coleoptera, and the
+adult insects of this order are much more disposed to walk and crawl and
+less disposed to fly than other endopterygote insects. Their heavily
+armoured bodies and their firm shield-like forewings render them less
+aerial than other insects; in many genera the power of flight has been
+altogether lost. It is not surprising, therefore, that many beetles,
+even when adult, should live as their larvae do; since the acquirement
+of complete metamorphosis they have become modified towards the larval
+condition, and an extreme case of such modification is afforded by the
+wingless grub-like female Glow-worm (Lampyris).</p>
+
+<p>With most insects, however, the larva must be regarded as the more
+specially modified, even if degraded, stage. <a href="#Miall1895">Miall (1895)</a> has pointed
+out that the insect grub is not a precociously hatched embryo, like the
+larvae of multitudes of marine animals, but that it exhibits in a
+modified form the essential characters of the adult. Comparison for
+example can be readily made between the parts of the caterpillar and the
+butterfly, whose story was sketched in the first <a name="Page_114" id="Page_114"></a>chapter of this book,
+widely different though caterpillar and butterfly may appear at a
+superficial glance. And the survey of variety in form, food, and habit
+of insect larvae given in Chapter VI enforces surely the conclusion that
+the larva is eminently plastic, adaptable, capable of changing so as to
+suit the most diverse surroundings. In a most suggestive recent
+discussion on the transformation of insects <a href="#Deegener1909">P. Deegener (1909)</a> has
+claimed that the larva must be regarded as the more modified stage,
+because while all the adult's structures are represented in the larva,
+even if only as imaginal buds, there are commonly present in the larva
+special adaptive organs not found in the imago, for example the pro-legs
+of caterpillars or the skin-gills of midge-grubs. The correspondence of
+parts in butterfly and caterpillar just referred to, may still be
+traced, though less easily, in bluebottle and maggot. The latter is an
+extreme example of degenerative evolution, and its contrast with the
+elaborately organised two-winged fly marks the greatest divergence
+observable between the larva and imago. With this divergence the resting
+pupal stage, during which more or less dissolution and reconstruction of
+organs goes on, becomes a necessity, and it has already been pointed out
+how the amount of this reconstruction is greatest where the divergence
+between the larval and perfect stages is most marked. Whatever
+differences of opinion may prevail on points <a name="Page_115" id="Page_115"></a>of detail, the general
+explanation of insect metamorphosis as the result of divergent evolution
+in the two active stages of the life-story must assuredly be accepted.
+No other explanation accords with the increasing degree of divergence to
+be observed as we pass from the lower to the higher insect orders.</p>
+
+<p>The successive incidents of the life-story of most insects are largely
+connected with the acquisition of wings. Wings, and the power of flight
+wherewith they endow their possessors, are evidently beneficial to the
+race in giving power of extending the range during the breeding period
+and thus ensuring a wide distribution of the eggs. In no case are wings
+fully developed until the closing stage of the insect's life, they are
+always acquired after hatching or birth. We have already noticed (<a href="#Page_40">p. 40</a>)
+how <a href="#Sharp1899">Sharp (1899)</a> has laid stress on the essential difference between the
+exopterygote and endopterygote insects, the wing-rudiments of the former
+growing outwards throughout life while those of the latter remain hidden
+until the pupal instar. Sharp considers that there is some difficulty in
+bridging, in thought, the gap between these two methods of wing-growth,
+and has put forward an ingenious suggestion to meet it <a href="#Sharp1902">(1902)</a>. Reference
+has already been made to insects of various orders in which one sex is
+wingless, the Vapourer Moth (<a href="#Page_96">p. 96</a>) for example, or all the individuals
+of both sexes are wingless, as the aberrant cockroaches <a name="Page_116" id="Page_116"></a>mentioned in
+Chapter II (<a href="#Page_15">p. 15</a>), or certain generations of virgin females are
+wingless, for example aphids (<a href="#Page_18">pp. 18-19</a>) and gall-flies (<a href="#Page_94">pp. 94-5</a>).
+Insects may thus become secondarily wingless, that is to say be
+manifestly the offspring of winged parents, and such wingless forms may
+on the other hand give rise to offspring or descendants with
+well-developed wings. Frequently, as in the case of the aphids, many
+wingless generations intervene between two winged generations. A
+striking illustration of this fact is afforded by an aquatic bug, <i>Velia
+currens</i>, commonly to be seen skating over the surface of running water.
+The adults of Velia are nearly always wingless, but now and then the
+naturalist meets with a specimen provided with functional wings, the
+possession of which enables the insect to make its way to a fresh
+stream. Moreover there are whole orders of parasitic insects, such as
+the lice and fleas, which, showing clear affinity to orders of winged
+insects, are believed to be secondarily wingless. These orders are
+designated by Sharp 'Anapterygota.' And from the analogy of the periodic
+loss and recovery of wings in various generations of the same species,
+he has concluded that the gap between the exopterygote and the
+endopterygote method of development may have been bridged by an
+anapterygote condition; that the ancestors of those insects with
+complete transformations were the wingless descendants of primitive
+<a name="Page_117" id="Page_117"></a>insects which grew their wings from visible external rudiments, and
+that in later times re-acquiring wings, they developed these organs in a
+new way, from inwardly directed rudiments or imaginal buds.</p>
+
+<p>This theory of Sharp's is original, daring, and ingenious, but the loss
+and re-acquisition of wings which it presupposes is difficult to imagine
+in large groups during a prolonged evolutionary history, while the
+sudden appearance of a totally new mode of wing-growth in the offspring
+of wingless insects would be an extreme example of discontinuity in
+development.</p>
+
+<p>On the whole the most probable suggestion which can be made as to the
+origin of 'complete' transformation in insects is that the instar in
+which wings were first visible externally became later and later in the
+course of the evolution of the more highly organised groups. In this way
+a gradual transition from the exopterygote to the endopterygote type of
+life-story is at least conceivable. It will be remembered that a may-fly
+(<a href="#Page_33">p. 33</a>) undergoes a moult after acquiring functional wings, emerging
+into the air as a 'sub-imago.' In not a few endopterygote insects, the
+pupa shows more or less activity, swimming through water intermittently
+(gnats) or just before the imago has to emerge (caddis-flies); working
+its way out of the ground (crane-flies) or coming half-way out of its
+cocoon (many moths). The pupa <a name="Page_118" id="Page_118"></a>of the higher insects almost certainly
+corresponds with the may-fly's sub-imago, and the facts just recalled as
+to remnants of pupal activity suggest that in the ancestors of
+endopterygote insects what is now the pupal instar was represented by an
+active nymphal or sub-imaginal stage, possibly indeed by more than one
+stage, as Packard and other writers have stated that pupae of bees and
+wasps undergo two or three moults before the final exposure of the
+imago. Such an early pupal instar has been defined as a 'pro-nymph' or a
+'semi-pupa.' Examples have been given of the exceptional passive
+condition of the penultimate instar in Exopterygota. The instars
+preceding this presumably had originally outward wing-rudiments in all
+insect life-histories, and the endopterygote condition was attained by
+the postponement of the outward appearance of these to successively
+later stages. The leg and wing rudiments of the male coccid (<a href="#Page_20">pp. 20-1</a>)
+beneath the cuticle of the second instar are strictly comparable to
+imaginal buds, and these are present in one instar of what is generally
+regarded as an exopterygote life-history. The first instar in all
+insects has no visible wing-rudiments, but when they grow outwardly from
+the body, they necessarily become covered with cuticle, so that they
+must be visible after the first moult. There is no supreme difficulty in
+supposing that the important change was for these <a name="Page_119" id="Page_119"></a>early rudiments to
+become sunk into the body, so that the cuticle of the second, and,
+later, of the third and succeeding instars, showed no outward sign of
+their presence. This suggestion is confirmed by Heymons' (<a href="#Heymons1896">1896</a>, <a href="#Heymons1907">1907</a>)
+observation of the occasional appearance of outward wing-rudiments on
+the thoracic segments of a mealworm, the larva of the beetle <i>Tenebrio
+molitor</i>, and by F. Silvestri's discovery <a href="#Silvestri1905">(1905)</a> of a 'pro-nymph' stage
+with short external wing-rudiments between the second larval and the
+pupal instars of the small ground-beetle <i>Lebia scapularis</i>. Whatever
+may be the exact explanation of these abnormalities, they show that in
+the life-story of the higher insects outward wing-rudiments may even yet
+appear before the pupal stage, confirming our belief that such
+appearance is an ancestral character. The inward growth of these
+wing-rudiments may well have been correlated with a difference in form
+between the newly-hatched insect and its parent. As this difference
+persisted until a constantly later stage, and the pre-imaginal instar
+became necessarily a stage for reconstruction, the present condition of
+complete metamorphosis in the more highly organised orders was finally
+attained.</p>
+
+<p>To explain satisfactorily these complex life-stories is however
+admittedly a difficult task. The acquisition of wings is, as we have
+seen, a dominating feature in them all, but if we try to go yet a step
+farther back <a name="Page_120" id="Page_120"></a>and speculate on the origin of wings in the most primitive
+exopterygote insects, the task becomes still more difficult. Many years
+ago <a href="#Gegenbaur1878">Gegenbaur (1878)</a> was struck by the correspondence of insect wings to
+the tracheal gills of may-fly larvae, which are carried on the abdominal
+segments somewhat as wings are on the thoracic segments. But B&ouml;rner has
+recently <a href="#Boerner1909b"><ins class="correction" title="Transcriber's note: It is not clear which of the two B&ouml;rner 1909 entries in the Bibliography is meant here.">(1909)</ins></a> brought forward evidence that these abdominal gills
+really correspond serially with legs. Moreover Gegenbaur's theory
+suggests that the ancestral insects were aquatic, whereas the presence
+of tubes for breathing atmospheric air in well-nigh all members of the
+class, and the fact that aquatic adaptations, respiratory and otherwise,
+in insect-larvae are secondary force the student to regard the ancestral
+insects as terrestrial. It is indeed highly probable that insects had a
+common origin with aquatic Crustacea, but all the evidence points to the
+ancestors of insects having become breathers of atmospheric air before
+they acquired wings. How the wings arose, what function their precursors
+performed before they became capable of supporting flight, we can hardly
+even guess.</p>
+
+<p>Our study of the life-story of insects, therefore, while it has taught
+us something of what is going on around us to-day, and has given us
+hints of the course of a few threads of that long life-story which runs
+through the ages, brings us face to face with the <a name="Page_121" id="Page_121"></a>most instructive, if
+humbling fact that 'there are many more things of which we are
+ignorant.' The passage from creeping to flight, as the caterpillar
+becomes transformed into the butterfly, was a mystery to those who first
+observed it, and many of its aspects remain mysterious still. Perhaps
+the most striking result of the study of insect transformation is the
+appreciation of the divergent specialisation of larva and imago, and it
+is a suggestive thought that of the two the larva has in many cases
+diverged the more from the typical condition. The caterpillar crawling
+over the leaf, or the fly-grub swimming through the water, may thus be
+regarded as a creature preparing for a change to the true conditions of
+its life. It is a strange irony that the preparation is often far longer
+than the brief hours of achievement. But the light which research has
+thrown on the nature of these wonderful life-stories, the demonstration
+of the unseen presence and growth within the insect, during its time of
+preparation among strange surroundings, of the organs required for
+service in the coming life amid its native air, confirm surely the
+intuition of the old-time students, who saw in these changes, so
+familiar and yet so wonderful, a parable and a prophecy of the higher
+nature of man.</p>
+
+
+
+<p><a name="Page_122" id="Page_122"></a></p>
+<h2><a name="OUTLINE_CLASSIFICATION_OF_INSECTS" id="OUTLINE_CLASSIFICATION_OF_INSECTS"></a>OUTLINE CLASSIFICATION OF INSECTS<br /><small>Class INSECTA or HEXAPODA.</small></h2>
+
+<h3>Sub-class A, <span class="smcap">Apterygota</span>.</h3>
+
+<ul class="preol">
+<li><span class="lalign">Order</span>&nbsp;</li>
+</ul>
+<ol class="postul">
+<li><i>Thysanura</i> (Bristle-tails).</li>
+<li><i>Collembola</i> (Spring-tails).</li>
+</ol>
+<h3>Sub-class B, <span class="smcap">Exopterygota</span>.</h3>
+
+<ul class="preol">
+<li><span class="lalign">Order</span>&nbsp;</li>
+</ul>
+<ol class="postul">
+<li><i>Dermaptera</i> (Earwigs).</li>
+<li><i>Orthoptera</i> (Cockroaches, Grasshoppers, Crickets).</li>
+<li><i>Plecoptera</i> (Stone-flies).</li>
+<li><i>Isoptera</i> (Termites or 'White Ants').</li>
+<li><i>Corrodentia</i>
+<ol>
+<li><i>Copeognatha</i> (Book-lice).</li>
+<li><i>Mallophaga</i> (Biting-lice).</li>
+</ol></li>
+<li><i>Ephemeroptera</i> (May-flies).</li>
+<li><i>Odonata</i> (Dragon-flies).</li>
+<li><i>Thysanoptera</i> (Thrips).</li>
+<li><i>Hemiptera</i>
+<ol><li><i>Heteroptera</i> (Bugs, Pond-skaters)</li>
+<li><i>Homoptera</i> (Cicads, 'Greenfly,' Scales).</li>
+</ol></li>
+<li><i>Anoplura</i> (Lice).</li>
+</ol>
+
+<h3>Sub-class C, <span class="smcap">Endopterygota</span>.</h3>
+
+<ul class="preol">
+<li><span class="lalign">Order</span>&nbsp;</li>
+</ul>
+<ol class="postul">
+<li><i>Neuroptera</i> (Alder-flies, Ant-lions, Lacewings).</li>
+<li><i>Coleoptera</i> (Beetles).</li>
+<li><i>Mecaptera</i> (Scorpion-flies).</li>
+<li><i>Trichoptera</i> (Caddis-flies).</li>
+<li><i>Lepidoptera</i> (Moths and Butterflies).</li>
+<li><i>Diptera</i> (Two-winged flies)
+<ol><li><i>Orthorrhapha</i> (Crane-flies, Midges, Gnats)</li>
+<li><i>Cyclorrhapha</i> (Hover-flies, House-flies, Bot-flies, &amp;c).</li>
+</ol></li>
+<li><i>Siphonaptera</i> (Fleas).</li>
+<li><i>Hymenoptera</i>
+<ol><li><i>Symphyta</i> (Saw-flies)</li>
+<li><i>Apocrita</i> (Gall-flies, Ichneumon-flies, Wasps, Bees, Ants).</li>
+</ol></li>
+</ol>
+
+
+
+<p><a name="Page_123" id="Page_123"></a></p>
+<h2><a name="TABLE_OF_GEOLOGICAL_SYSTEMS" id="TABLE_OF_GEOLOGICAL_SYSTEMS"></a>TABLE OF GEOLOGICAL SYSTEMS</h2>
+
+
+<p>These names, given by geologists to the various divisions of rocks, as
+indicated by the fossils entombed in them, are arranged in 'descending'
+order, the more recent formations above, the more ancient below, as
+newer deposits necessarily lie over older beds.</p>
+
+
+
+<h3 class="smcap">Calnozoic or Tertiary Group.</h3>
+<ul class="TGS">
+<li>Pleistocene.</li>
+<li>Pliocene.</li>
+<li>Miocene.</li>
+<li>Eocene.</li>
+</ul>
+
+<h3 class="smcap">Mesozoic or Secondary Group.</h3>
+<ul class="TGS">
+<li>Cretaceous.</li>
+<li>Jurassic.</li>
+<li>Triassic.</li>
+</ul>
+
+<h3 class="smcap">Palaeozoic or Primary Group.</h3>
+<ul class="TGS">
+<li>Permian.</li>
+<li>Carboniferous.</li>
+<li>Devonian.</li>
+<li>Silurian.</li>
+<li>Cambrian.</li>
+</ul>
+
+
+
+<p><a name="Page_124" id="Page_124"></a></p>
+<h2><a name="BIBLIOGRAPHY" id="BIBLIOGRAPHY"></a>BIBLIOGRAPHY</h2>
+
+
+<p>The following list of some books and papers, referred to in this little
+volume or of especial service to the author in its preparation, is
+needless to say very far from exhaustive. To save space, titles are
+often abbreviated. Most of the works in the general list (A) contain
+extensive lists of literature on insects and their transformations,
+these should be consulted by the serious student.</p>
+
+
+<h3>A. GENERAL WORKS.</h3>
+
+
+
+<p class="bib"><a name="Boerner1909a" id="Boerner1909a"></a>
+1909. C. B&ouml;rner. Die Verwandlungen der Insekten. <i>Sitzb.
+d. Gesellsch. naturforsch. Freunde, Berlin</i>.</p>
+
+<p class="bib"><a name="Brauer1869" id="Brauer1869"></a>
+1869. F. Brauer. Betrachtung &uuml;ber die Verwandlung der Insekten.
+<i>Verhandl. der K.K. zool.-bot. Gesellschaft in Wien</i>. XIX.</p>
+
+<p class="bib"><a name="Carpenter1899" id="Carpenter1899"></a>
+1899. G.&nbsp;H. Carpenter. Insects, their Structure and Life.
+London.</p>
+
+<p class="bib"><a name="Darwin1859" id="Darwin1859"></a>
+1859. C. Darwin. The Origin of Species. London.</p>
+
+<p class="bib"><a name="Deegener1909" id="Deegener1909"></a>
+1909. P. Deegener. Die Metamorphose der Insekten. Leipzig.</p>
+
+<p class="bib"><a name="Folsom1906" id="Folsom1906"></a>
+1906. J.&nbsp;W. Folsom. Entomology. London.</p>
+
+<p class="bib"><a name="Gegenbaur1878" id="Gegenbaur1878"></a>
+1878. C. Gegenbaur. Grundriss der Vergleichende Anatomie.
+Leipzig.</p>
+
+<p class="bib"><a name="Handlirsch1906" id="Handlirsch1906"></a>
+1906. A. Handlirsch. Die fossilen Insekten. Leipzig.</p>
+
+<p class="bib"><a name="Henneguy1904" id="Henneguy1904"></a>
+1904. L.&nbsp;F. Henneguy. Les Insectes. Paris.</p>
+
+<p class="bib"><a name="Heymons1907" id="Heymons1907"></a>
+1907. R. Heymons. Die verschiedenen Formen der Insectenmetamorphose.
+<i>Ergebnisse der Zoologie</i>. I.</p>
+
+<p class="bib"><a name="Lameere1899" id="Lameere1899"></a>
+1899. A. Lameere. La raison d'&ecirc;tre des Metamorphoses chez
+les Insectes. <i>Ann. Soc. Entom. Bruxelles</i>. XLIII.</p>
+
+<p class="bib"><a name="Lubbock1874" id="Lubbock1874"></a>
+1874. J. Lubbock. The Origin and Metamorphoses of Insects.<a name="Page_125" id="Page_125"></a>
+London.</p>
+
+<p class="bib"><a name="Miall1895" id="Miall1895"></a>
+1895. L.&nbsp;C. Miall. (<i>a</i>) The Transformations of Insects. <i>Nature</i>.
+LIII.</p>
+<p class="bib"><a name="Miall1895b" id="Miall1895b"></a>
+1895. &mdash;&mdash; (<i>b</i>) The Natural History of Aquatic Insects.
+London.</p>
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+1908. &mdash;&mdash; Injurious and Useful Insects. 2nd edition. London.</p>
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+1839. G. Newport. Insects. <i>Todd Cyclopaedia</i>. II. London.</p>
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+1898. A.&nbsp;S. Packard. Text book of Entomology. New York.</p>
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+1893, &amp;c. C.&nbsp;G. Barrett. Lepidoptera of the British Islands.
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+1890. H. Beaur&eacute;gard. Les Insectes V&eacute;sicants. Paris.</p>
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+1909. C. B&ouml;rner. Die Tracheenkiemen der Ephemeriden.
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+1863. F. Brauer. Monographie der Oestriden. Wien.</p>
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+1894. C. Brongniart. R&eacute;cherches pour servir &agrave; l'histoire des
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+1893. T.&nbsp;A. Chapman. Structure of Pupae of Heterocerous
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+1891. H. Dewitz. Das geschlossene Tracheensystem bei Insektenlarven.
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+1857-8. J.&nbsp;H. Fabre. L'Hyperm&eacute;tamorphose et les M&oelig;urs des
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+1869. M. Ganin. Die Entwicklungsgeschichte bei den Insekten.
+<i>Zeitsch. f. wissensch. Zoolog.</i> xix.</p>
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+1894. J. Gonin. La M&eacute;tamorphose des Lepidopt&egrave;res. <i>Bull.
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+1870. O. Grimm. Die ungeschechtliche Fortpflanzung einer
+Chironomus. <i>Mem. Acad. Imp&eacute;r. St P&eacute;tersbourg</i>
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+1890. W. Hatchett-Jackson. Morphology of the Lepidoptera.
+<i>Trans. Linn. Soc. (Zool.) Lond</i>. (2). v.</p>
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+1896. R. Heymons. Fl&uuml;gelbildung bei der Larve von Tenebrio
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+1906. &mdash;&mdash; Ueber die ersten Jugendformen von Machilis alternata.
+<i>Ib.</i></p>
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+1908. W. Kahle. Die Paedogenesis der Cecidomyiden. <i>Zoologica</i>.
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+1913. V.&nbsp;L. Kellogg. Distribution and Species-forming of Ectoparasites.
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+1887. A. Kowalevsky. Die nachembryonale Entwicklung der<a name="Page_127" id="Page_127"></a>
+Musciden. <i>Zeitsch. f. wissensch. Zool.</i> XLV.</p>
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+1904. O.&nbsp;H. Latter. Natural History of Common Animals
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+1890-95. B.&nbsp;T. Lowne. The Blowfly, 2 vols. London.</p>
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+1863. J. Lubbock. Development of Chloeon. <i>Trans. Linn. Soc.
+Lond.</i> XXIII.</p>
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+1762. P. Lyonet. Trait&eacute; anatomique de la Chenille. Haag.</p>
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+1669. M. Malpighi. De Bombyce. London.</p>
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+<p class="bib"><a name="Marlatt1898" id="Marlatt1898"></a>189<ins class="correction" title="Transcriber's note: The citation of this work gives the year 1895 not 1898.">8</ins>.
+C.&nbsp;L. Marlatt. The periodical Cicada. <i>Entom. Bull.</i> 14,
+<i>U.S. Dept. Agric.</i></p>
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+1898. G.&nbsp;A.&nbsp;K. Marshall. Seasonal Dimorphism in Butterflies.
+<i>Ann. Mag. Nat. Hist.</i> (7). II.</p>
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+1900. L.&nbsp;C. Miall and A.&nbsp;B. Hammond. The Harlequin Fly.
+Oxford.</p>
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+1901-3. R. Newstead. Coccidae of the British Isles. London.</p>
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+1877. J.&nbsp;A. Palm&eacute;n. Zur Morphologie des Tracheensystems.
+Leipzig.</p>
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+1891. E.&nbsp;B. Poulton. External Morphology of the Lepidopterous
+Pupa. <i>Trans. Linn. Soc. Zool.</i> (2). V.</p>
+<p class="bib"><a name="Poulton1892" id="Poulton1892"></a>
+1892. &mdash;&mdash; Colour-relation between Lepidopterous Larvae &amp;c.
+and their surroundings. <i>Trans. Entom. Soc. Lond.</i></p>
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+1880. C.&nbsp;V. Riley. Pupation of Butterflies. <i>Proc. Amer. Assoc.</i>
+XXVIII.</p>
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+1902. E.&nbsp;D. Sanderson. Report of Entomologist. Delaware.
+U.S.A.</p>
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+1885. E.&nbsp;O. Schmidt. Metamorphose und Anatomie des m&auml;nnlichen
+Aspidiotus. <i>Archiv f. Naturgeschichte</i>. LI.</p>
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+1885. S.&nbsp;H. Scudder. Insekten in Zittel's Paleontologie. II.</p>
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+1907. A.&nbsp;J. Siltala. Die postembryonale Entwicklung der Trichopteren-Larven.
+<i>Zoolog. Jahrb. Suppl.</i> IX.</p>
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+1905. F. Silvestri. Metamorfosi e Costumi della Lebia scapularis.
+<i>Redia</i>. II.</p>
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+1900. J.&nbsp;B. Smith. The Apple Plant-louse. <i>New Jersey Agric. Exp. Station Bull.</i> 143.<a name="Page_128" id="Page_128"></a></p>
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+1888. J. Van Rees. Die innere Metamorphose von Musca. <i>Zoolog. Jahrb. Anat.</i> III.</p>
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+1911. K.&nbsp;W. Verhoeff. Ueber Felsenspringer, Machiloidea. <i>Zoolog. Anz.</i> XXXVIII.</p>
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+1865. N. Wagner. Die viviparen Gallm&uuml;ckenlarven. <i>Zeitsch. f. wissensch. Zoolog.</i> XV.</p>
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+1901. E. Wasmann. Termitoxenia. <i>Zeitsch. f. wissensch. Zoolog.</i> LXX.</p>
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+1865. &mdash;&mdash; Die Metamorphose von Corethra. <i>Ib.</i> XVI.</p>
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+1876. &mdash;&mdash; Studien zur Descendenz-Theorie. Leipzig. (English Translation by R. Meldola, London, 1882.)</p>
+
+
+
+<p><a name="Page_129" id="Page_129"></a></p>
+<h2><a name="INDEX" id="INDEX"></a>INDEX</h2>
+<table style="margin-left:15%; width:70%; text-align:center; margin-bottom:2em; border-collapse:collapse;" border="1" summary="index navigation">
+		<tr>
+			<td><a href="#IX_A">A</a></td>
+			<td><a href="#IX_B">B</a></td>
+			<td><a href="#IX_C">C</a></td>
+			<td><a href="#IX_D">D</a></td>
+			<td><a href="#IX_E">E</a></td>
+			<td><a href="#IX_F">F</a></td>
+			<td><a href="#IX_G">G</a></td>
+			<td><a href="#IX_H">H</a></td>
+			<td><a href="#IX_I">I</a></td>
+			<td><a href="#IX_J">J</a></td>
+			<td><a href="#IX_K">K</a></td>
+			<td><a href="#IX_L">L</a></td>
+			<td><a href="#IX_M">M</a></td>
+		</tr>
+		<tr>
+			<td><a href="#IX_N">N</a></td>
+			<td><a href="#IX_O">O</a></td>
+			<td><a href="#IX_P">P</a></td>
+			<td>Q</td>
+			<td><a href="#IX_R">R</a></td>
+			<td><a href="#IX_S">S</a></td>
+			<td><a href="#IX_T">T</a></td>
+			<td>U</td>
+			<td><a href="#IX_V">V</a></td>
+			<td><a href="#IX_W">W</a></td>
+			<td>X</td>
+			<td>Y</td>
+			<td>Z</td>
+		</tr>
+</table>
+
+
+
+<ul class="IX">
+<li><a name="IX_A" id="IX_A"></a><i>Abraxas grossulariata</i>, <a href="#Page_60">60</a>, <a href="#Page_83">83</a>, <a href="#Page_97">97-8</a></li>
+<li>Adaptation of larvae, <a href="#Page_57">57</a>, <a href="#Page_79">79</a>, <a href="#Page_114">114</a></li>
+<li>Adephaga, <a href="#Page_51">51</a></li>
+<li>Adler, H., <a href="#Page_94">94</a></li>
+<li>Aeschnidae, <a href="#Page_27">27</a>, <a href="#Page_29">29</a>, <a href="#Page_31">31</a></li>
+<li>Agrionidae, <a href="#Page_27">27</a>, <a href="#Page_28">28</a></li>
+<li><i>Agrotis segetum</i>, <a href="#Page_98">98</a></li>
+<li><a name="airtubes" id="airtubes"></a>Air-tubes, <a href="#Page_2">2</a>, <a href="#Page_11">11</a>, <a href="#Page_23">23</a>, <a href="#Page_47">47</a>, <a href="#Page_70">70</a>, <a href="#Page_77">77</a>, <a href="#Page_87">87</a>, <a href="#Page_120">120</a></li>
+<li>Alternation of generations, <a href="#Page_17">17</a>, <a href="#Page_94">94</a></li>
+<li>Ametabola, <a href="#Page_11">11</a>, <a href="#Page_35">35</a></li>
+<li>Anapterygota, <a href="#Page_116">116</a></li>
+<li>Anglas, J., <a href="#Page_46">46</a></li>
+<li>Ant-lions, <a href="#Page_57">57</a></li>
+<li>Ants, <a href="#Page_64">64</a>, <a href="#Page_66">66</a></li>
+<li>Aphidae, <a href="#Page_17">17-20</a>, <a href="#Page_116">116</a></li>
+<li><i>Aphis pomi</i>, <a href="#Page_18">18-19</a></li>
+<li>Aphis-lion, <a href="#Page_57">57</a></li>
+<li>Apterygota, <a href="#Page_41">41</a>, <a href="#Page_110">110</a></li>
+<li><a name="aquatic" id="aquatic"></a>Aquatic insects, <a href="#Page_23">23-34</a>, <a href="#Page_76">76-9</a>, <a href="#Page_120">120</a></li>
+<li><i>Araschnia levana</i> and var. <i>prorsa</i>, <a href="#Page_103">103</a></li>
+<li><i>Arctia caia</i>, <a href="#Page_98">98</a></li>
+<li>Arctiadae, <a href="#Page_59">59</a></li>
+<li>Arthropoda, <a href="#Page_9">9</a></li>
+<li>Austen, E.&nbsp;E., <a href="#Page_91">91</a></li>
+<li>Avebury, Lord, <i>see</i> <a href="#lubbock">Lubbock, J.</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_B" id="IX_B"></a>Balfour-Browne, F., <a href="#Page_28">28</a></li>
+<li>Bark-beetles, <a href="#Page_55">55</a></li>
+<li>Barrett, C.&nbsp;G., <a href="#Page_96">96</a>, <a href="#Page_99">99</a></li>
+<li>Beaur&eacute;gard, H., <a href="#Page_56">56</a></li>
+<li>Bees, <a href="#Page_40">40</a>, <a href="#Page_46">46</a>, <a href="#Page_64">64</a>, <a href="#Page_83">83</a></li>
+<li>Beetles, <a href="#Page_40">40</a>, <a href="#Page_50">50-7</a>, <a href="#Page_80">80</a>, <a href="#Page_107">107</a>, <a href="#Page_112">112-3</a>, <a href="#Page_119">119</a></li>
+<li>Bell Moths, <a href="#Page_62">62</a></li>
+<li>Bird-lice, <a href="#Page_108">108</a></li>
+<li><a name="birth" id="birth"></a>Birth, <a href="#Page_18">18</a>, <a href="#Page_91">91</a></li>
+<li><i>Blatta orientalis</i>, <a href="#Page_15">15</a></li>
+<li>Blister-beetles, <a href="#Page_56">56</a></li>
+<li><a name="blowfly" id="blowfly"></a>Blowfly or Bluebottle, <a href="#Page_43">43</a>, <a href="#Page_44">44</a>, <a href="#Page_46">46</a>, <a href="#Page_67">67</a>, <a href="#Page_71">71-3</a>, <a href="#Page_93">93</a>, <a href="#Page_114">114</a></li>
+<li>B&ouml;rner, C., <a href="#Page_32">32</a>, <a href="#Page_120">120</a></li>
+<li>Bot-flies, <a href="#Page_73">73-4</a>, <a href="#Page_89">89</a>, <a href="#Page_91">91</a></li>
+<li>Brain, <a href="#Page_44">44</a></li>
+<li>Brauer, F., <a href="#Page_6">6</a>, <a href="#Page_52">52</a>, <a href="#Page_56">56</a>, <a href="#Page_67">67</a>, <a href="#Page_109">109</a></li>
+<li>Bristle-tails, <a href="#Page_11">11</a></li>
+<li>Brongniart, C., <a href="#Page_106">106</a></li>
+<li>Butterflies, <a href="#Page_1">1</a>, <a href="#Page_83">83</a>, <a href="#Page_95">95-6</a>, <a href="#Page_114">114</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_C" id="IX_C"></a>Cabbage-butterflies, <a href="#Page_39">39</a>, <a href="#Page_41">41</a>, <a href="#Page_85">85</a>, <a href="#Page_100">100-1</a></li>
+<li>Cabbage-fly, <a href="#Page_73">73</a></li>
+<li>Caddis-flies, <a href="#Page_62">62-3</a>, <a href="#Page_86">86</a>, <a href="#Page_117">117</a><a name="Page_130" id="Page_130"></a></li>
+<li>Cainozoic insects, <a href="#Page_107">107</a></li>
+<li>Calliphora, <a href="#Page_43">43</a>.
+<i>See also</i> <a href="#blowfly">Blowfly</a></li>
+<li>Campodeiform larvae, <a href="#Page_52">52</a>, <a href="#Page_56">56</a>, <a href="#Page_111">111</a></li>
+<li>Carabidae, <a href="#Page_52">52</a></li>
+<li>Carboniferous insects, <a href="#Page_107">107</a></li>
+<li><i>Carpocapsa pomonella</i>, <a href="#Page_99">99-100</a></li>
+<li>Carrion-beetles, <a href="#Page_50">50</a></li>
+<li><a name="caterpillar" id="caterpillar"></a>Caterpillar, <a href="#Page_4">4</a>, <a href="#Page_36">36</a>, <a href="#Page_49">49</a>, <a href="#Page_58">58-62</a>, <a href="#Page_95">95-101</a>, <a href="#Page_109">109</a>, <a href="#Page_114">114</a></li>
+<li>Cecidomyidae, <a href="#Page_68">68-70</a>, <a href="#Page_90">90</a></li>
+<li>Cerambycidae, <a href="#Page_55">55</a></li>
+<li>Cercopods, <a href="#Page_12">12</a>, <a href="#Page_15">15</a></li>
+<li>Chafers, <a href="#Page_52">52</a></li>
+<li>Chapman, T.&nbsp;A., <a href="#Page_81">81</a>, <a href="#Page_84">84</a></li>
+<li>Chironomus, <a href="#Page_43">43</a>, <a href="#Page_77">77</a>, <a href="#Page_87">87</a>, <a href="#Page_91">91</a></li>
+<li>Chloeon, <a href="#Page_33">33</a></li>
+<li>Chrysalis, <a href="#Page_82">82</a>.
+<i>See also</i> <a href="#pupa">Pupa</a></li>
+<li>Chrysomelidae, <a href="#Page_53">53</a>.
+<i>See also</i> <a href="#leafbeetles">Leaf-beetles</a></li>
+<li>Chrysopa, <a href="#Page_57">57</a></li>
+<li>Cicads, <a href="#Page_22">22</a>, <a href="#Page_93">93</a>, <a href="#Page_110">110</a></li>
+<li>Classification, <a href="#Page_122">122</a></li>
+<li>Clearwing Moths, <a href="#Page_62">62</a></li>
+<li>Click-beetles, <a href="#Page_52">52</a>, <a href="#Page_93">93</a></li>
+<li>Clothes-moths, <a href="#Page_62">62</a></li>
+<li><a name="coccidae" id="coccidae"></a>Coccidae, <a href="#Page_20">20</a>, <a href="#Page_110">110</a>, <a href="#Page_118">118</a></li>
+<li>Coccinella, <a href="#Page_113">113</a></li>
+<li>Cockroaches, <a href="#Page_11">11</a>, <a href="#Page_14">14</a>, <a href="#Page_15">15</a>, <a href="#Page_107">107</a>, <a href="#Page_115">115</a></li>
+<li>Cocoons, <a href="#Page_82">82</a></li>
+<li>Codling Moth, <a href="#Page_62">62</a>, <a href="#Page_99">99</a></li>
+<li>Coleoptera, <a href="#Page_50">50-6</a>, <a href="#Page_80">80</a>, <a href="#Page_112">112</a>, <a href="#Page_119">119</a></li>
+<li>Collembola, <a href="#Page_11">11</a></li>
+<li>Complete transformation, <a href="#Page_35">35</a>, <a href="#Page_107">107</a>, <a href="#Page_119">119</a>.
+<i>See also</i> <a href="#endopt">Endopterygota</a></li>
+<li>Corethra, <a href="#Page_43">43</a></li>
+<li>Cossus, <a href="#Page_38">38</a>, <a href="#Page_62">62</a>, <a href="#Page_82">82</a>, <a href="#Page_95">95</a></li>
+<li>Crane-flies, <a href="#Page_67">67</a>, <a href="#Page_70">70</a>, <a href="#Page_93">93</a>, <a href="#Page_117">117</a></li>
+<li>Cremaster, <a href="#Page_83">83</a></li>
+<li>Crustacea, <a href="#Page_7">7</a>, <a href="#Page_120">120</a></li>
+<li><a name="culex" id="culex"></a>Culex, <a href="#Page_43">43</a>, <a href="#Page_77">77</a>, <a href="#Page_86">86</a></li>
+<li>Curculionidae, <a href="#Page_55">55</a></li>
+<li>Cuticle, <a href="#Page_2">2</a>, <a href="#Page_9">9</a>, <a href="#Page_29">29</a>, <a href="#Page_37">37</a>, <a href="#Page_40">40</a>, <a href="#Page_50">50</a>, <a href="#Page_81">81</a>, <a href="#Page_87">87</a>, <a href="#Page_110">110</a></li>
+<li>Cynipidae, <a href="#Page_94">94</a>.
+<i>See also</i> <a href="#gallflies">Gall-flies</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_D" id="IX_D"></a>Daddy-long-legs, <a href="#Page_69">69-70</a></li>
+<li>Darwin, C., <a href="#Page_105">105</a></li>
+<li>Deegener, P., <a href="#Page_6">6</a>, <a href="#Page_114">114</a></li>
+<li>Devonian insects, <a href="#Page_107">107</a></li>
+<li>Dewitz, H., <a href="#Page_28">28</a></li>
+<li>Digestive system, <a href="#Page_10">10</a>, <a href="#Page_45">45-7</a></li>
+<li><i>Diplosis pyrivora</i>, <a href="#Page_70">70</a></li>
+<li>Diptera, <a href="#Page_42">42</a>, <a href="#Page_64">64</a>, <a href="#Page_67">67-79</a>, <a href="#Page_81">81</a>, <a href="#Page_86">86-8</a>, <a href="#Page_91">91</a>, <a href="#Page_94">94</a>, <a href="#Page_107">107</a></li>
+<li>Divergence between larva and imago, <a href="#Page_110">110</a>, <a href="#Page_114">114</a>, <a href="#Page_121">121</a></li>
+<li>Double-brooded Lepidoptera, <a href="#Page_95">95</a>, <a href="#Page_100">100-4</a></li>
+<li><a name="dragonflies" id="dragonflies"></a>Dragon-flies, <a href="#Page_26">26-31</a>, <a href="#Page_107">107</a>, <a href="#Page_110">110</a></li>
+<li>Drone-flies, <a href="#Page_76">76</a></li>
+<li>Duration of life, <a href="#Page_34">34</a>, <a href="#Page_89">89</a>, <a href="#Page_92">92-3</a>, <a href="#Page_95">95</a></li>
+<li>Dyticus, <a href="#Page_51">51</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_E" id="IX_E"></a>Ecdysis, <a href="#Page_10">10</a>.
+<i>See also</i> <a href="#moult">Moult</a></li>
+<li>Ectoderm, <a href="#Page_9">9</a>, <a href="#Page_11">11</a>, <a href="#Page_47">47</a></li>
+<li>Eggar Moths, <a href="#Page_59">59</a>, <a href="#Page_89">89</a></li>
+<li>Eggs, <a href="#Page_6">6</a>, <a href="#Page_17">17-18</a>, <a href="#Page_26">26</a>, <a href="#Page_34">34</a>, <a href="#Page_65">65-7</a>, <a href="#Page_71">71</a>, <a href="#Page_90">90</a>, <a href="#Page_94">94-5</a>, <a href="#Page_97">97</a></li>
+<li>Elateridae, <a href="#Page_52">52</a></li>
+<li><a name="endopt" id="endopt"></a>Endopterygota, <a href="#Page_41">41</a>, <a href="#Page_49">49</a>, <a href="#Page_108">108</a>, <a href="#Page_112">112</a>, <a href="#Page_115">115-6</a></li>
+<li>Ephemeroptera, <a href="#Page_24">24</a>.
+<i>See also</i> <a href="#mayflies">May-flies</a></li>
+<li>Epidermis, <a href="#Page_9">9</a>, <a href="#Page_40">40</a></li>
+<li>Eristalis, <a href="#Page_76">76</a></li>
+<li>Eruciform larvae, <a href="#Page_56">56</a>, <a href="#Page_58">58-70</a>, <a href="#Page_111">111</a></li>
+<li>Evolution, <a href="#Page_16">16</a>, <a href="#Page_103">103</a>, <a href="#Page_105">105-21</a></li>
+<li>Exopterygota, <a href="#Page_41">41</a>, <a href="#Page_108">108</a>, <a href="#Page_115">115-6</a>, <a href="#Page_118">118</a></li>
+<li>Exoskeleton, <a href="#Page_9">9</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_F" id="IX_F"></a><a name="Page_131" id="Page_131"></a>Fabre, J.&nbsp;H., <a href="#Page_56">56</a></li>
+<li>Fat-body, <a href="#Page_47">47</a></li>
+<li>Feeding-period, <a href="#Page_27">27</a>, <a href="#Page_32">32</a>, <a href="#Page_36">36</a>, <a href="#Page_89">89</a>, <a href="#Page_111">111</a></li>
+<li>Feelers, <a href="#Page_1">1</a>, <a href="#Page_4">4</a>, <a href="#Page_42">42</a>, <a href="#Page_71">71</a></li>
+<li>Fleas, <a href="#Page_116">116</a></li>
+<li>Fore-gut, <a href="#Page_47">47</a></li>
+<li>Free pupa, <a href="#Page_80">80</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_G" id="IX_G"></a><a name="gallflies" id="gallflies"></a>Gall-flies, <a href="#Page_64">64-6</a>, <a href="#Page_94">94</a>, <a href="#Page_115">115</a></li>
+<li>Gall-midges, <a href="#Page_68">68-70</a>, <a href="#Page_90">90</a></li>
+<li>Ganin, M., <a href="#Page_66">66</a></li>
+<li><i>Gastrophilus equi</i>, <a href="#Page_73">73-4</a></li>
+<li>Gegenbaur, C., <a href="#Page_120">120</a></li>
+<li>Geological history, <a href="#Page_106">106-8</a>, <a href="#Page_123">123</a></li>
+<li>Geometridae, <a href="#Page_59">59</a></li>
+<li>Gills, <a href="#Page_24">24</a>, <a href="#Page_27">27</a>, <a href="#Page_32">32</a>, <a href="#Page_78">78</a>, <a href="#Page_87">87</a>, <a href="#Page_114">114</a>, <a href="#Page_120">120</a></li>
+<li>Glossinia, <a href="#Page_91">91</a></li>
+<li>Glow-worm, <a href="#Page_50">50</a>, <a href="#Page_113">113</a></li>
+<li><a name="gnats" id="gnats"></a>Gnats, <a href="#Page_43">43</a>, <a href="#Page_77">77</a>, <a href="#Page_86">86</a></li>
+<li>Goat Moth, <a href="#Page_38">38</a>, <a href="#Page_62">62</a>, <a href="#Page_82">82</a>, <a href="#Page_95">95</a></li>
+<li>Gonin, J., <a href="#Page_38">38</a>, <a href="#Page_41">41</a></li>
+<li>Grasshoppers, <a href="#Page_11">11</a>, <a href="#Page_14">14</a>, <a href="#Page_15">15</a></li>
+<li>Grimm, O., <a href="#Page_90">90</a></li>
+<li>Ground-beetles, <a href="#Page_52">52</a>, <a href="#Page_112">112</a></li>
+<li>Growth, <a href="#Page_9">9</a></li>
+<li>Grub, <a href="#Page_63">63-70</a>.
+<i>See also</i> <a href="#caterpillar">Caterpillar</a>, <a href="#larva">Larva</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_H" id="IX_H"></a>Hairs, <a href="#Page_59">59</a>, <a href="#Page_82">82</a>, <a href="#Page_98">98</a></li>
+<li>Hammond, A.&nbsp;R., <a href="#Page_43">43</a>, <a href="#Page_77">77</a>, <a href="#Page_87">87</a></li>
+<li>Handlirsch, A., <a href="#Page_106">106</a></li>
+<li>Harvey, William, <a href="#Page_7">7</a></li>
+<li>Hatchett-Jackson, W., <a href="#Page_83">83</a></li>
+<li>Hawk Moths, <a href="#Page_60">60</a></li>
+<li>Heart, <a href="#Page_45">45</a></li>
+<li>Helodes, <a href="#Page_50">50</a></li>
+<li>Hemerobius, <a href="#Page_57">57</a></li>
+<li>Hemimetabola, <a href="#Page_35">35</a></li>
+<li>Hemiptera, <a href="#Page_17">17</a>, <a href="#Page_110">110</a></li>
+<li>Henneguy, L.&nbsp;F., <a href="#Page_45">45</a>, <a href="#Page_48">48</a></li>
+<li>Heymons, R., <a href="#Page_6">6</a>, <a href="#Page_11">11</a>, <a href="#Page_119">119</a></li>
+<li>Hibernation. <i>See</i> <a href="#wintering">Wintering stages</a></li>
+<li>Hind-gut, <a href="#Page_47">47</a></li>
+<li>Hippoboscidae, <a href="#Page_91">91</a></li>
+<li>Histogenesis and Histolysis, <a href="#Page_48">48</a></li>
+<li>Holometabola, <a href="#Page_35">35</a></li>
+<li>House-fly, <a href="#Page_67">67</a>, <a href="#Page_71">71</a>, <a href="#Page_73">73</a></li>
+<li>Hover-flies, <a href="#Page_74">74-6</a></li>
+<li>Hymenoptera, <a href="#Page_58">58</a>, <a href="#Page_64">64</a>, <a href="#Page_94">94</a>, <a href="#Page_107">107</a></li>
+<li>Hypermetamorphosis, <a href="#Page_56">56</a></li>
+<li><i>Hypoderma bovis</i>, <a href="#Page_73">73-5</a></li>
+<li>Hypodermis, <a href="#Page_9">9</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_I" id="IX_I"></a>Ichneumon-flies, <a href="#Page_64">64</a>, <a href="#Page_66">66</a>, <a href="#Page_82">82</a></li>
+<li>Imaginal buds or discs, <a href="#Page_34">34-48</a>, <a href="#Page_114">114</a>, <a href="#Page_117">117-8</a></li>
+<li>Imago, <a href="#Page_24">24</a>, <a href="#Page_34">34</a>, <a href="#Page_114">114</a></li>
+<li>Instar, <a href="#Page_13">13</a>, <a href="#Page_33">33</a>, <a href="#Page_56">56</a>, <a href="#Page_117">117-9</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_J" id="IX_J"></a>Jaws of imago and larva, <a href="#Page_2">2</a>, <a href="#Page_4">4</a>, <a href="#Page_5">5</a>, <a href="#Page_32">32</a>, <a href="#Page_42">42</a>, <a href="#Page_89">89</a></li>
+<li>Jurassic insects, <a href="#Page_107">107</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_K" id="IX_K"></a>Kahle, W., <a href="#Page_90">90</a></li>
+<li>Kellogg, V.&nbsp;L., <a href="#Page_108">108</a></li>
+<li>Kowalevsky, A., <a href="#Page_46">46</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_L" id="IX_L"></a>Labium, <a href="#Page_2">2</a>, <a href="#Page_27">27</a></li>
+<li>Lacewing-flies, <a href="#Page_57">57</a>, <a href="#Page_107">107</a></li>
+<li>Ladybirds, <a href="#Page_113">113</a></li>
+<li>Lameere, A., <a href="#Page_111">111</a></li>
+<li>Lampyris, <a href="#Page_113">113</a></li>
+<li><a name="larva" id="larva"></a>Larva, <a href="#Page_4">4</a>, <a href="#Page_22">22</a>, <a href="#Page_26">26-7</a>, <a href="#Page_32">32</a>, <a href="#Page_49">49-79</a>, <a href="#Page_110">110-15</a></li>
+<li><a name="repro" id="repro"></a>Larval reproduction, <a href="#Page_90">90</a></li>
+<li>Lasiocampidae, <a href="#Page_59">59</a>, <a href="#Page_89">89</a></li>
+<li>Latter, O.&nbsp;H., <a href="#Page_28">28</a></li>
+<li><a name="leafbeetles" id="leafbeetles"></a>Leaf-beetles, <a href="#Page_53">53</a>, <a href="#Page_83">83</a>, <a href="#Page_92">92-3</a>, <a href="#Page_113">113</a></li>
+<li><i>Lebia scapularis</i>, <a href="#Page_119">119</a></li>
+<li>Lepidoptera, <a href="#Page_1">1</a>, <a href="#Page_36">36</a>, <a href="#Page_38">38</a>, <a href="#Page_49">49</a>, <a href="#Page_58">58</a>, <a href="#Page_81">81</a>, <a href="#Page_95">95-104</a>, <a href="#Page_107">107</a></li>
+<li>Libellulidae, <a href="#Page_27">27</a></li>
+<li>Lice, <a href="#Page_116">116</a><a name="Page_132" id="Page_132"></a></li>
+<li>Lipeurus, <a href="#Page_108">108</a></li>
+<li>Longhorn Beetles, <a href="#Page_55">55</a></li>
+<li>Looper caterpillars, <a href="#Page_59">59</a>, <a href="#Page_61">61</a></li>
+<li>Lowne, B.&nbsp;T., <a href="#Page_42">42</a></li>
+<li><a name="lubbock" id="lubbock"></a>Lubbock, J., <a href="#Page_6">6</a>, <a href="#Page_32">32</a></li>
+<li>Lymantriidae, <a href="#Page_90">90</a></li>
+<li>Lyonet, P., <a href="#Page_38">38</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_M" id="IX_M"></a>Machilis, <a href="#Page_11">11</a></li>
+<li>Maggot, <a href="#Page_44">44</a>, <a href="#Page_67">67</a>, <a href="#Page_71">71-6</a>, <a href="#Page_109">109</a>, <a href="#Page_114">114</a></li>
+<li>Magpie Moth, <a href="#Page_60">60</a>, <a href="#Page_82">82</a>, <a href="#Page_97">97-8</a></li>
+<li>Mallophaga, <a href="#Page_108">108</a></li>
+<li>Mandibles, <a href="#Page_4">4</a>, <a href="#Page_17">17</a>, <a href="#Page_26">26</a>, <a href="#Page_58">58</a>, <a href="#Page_67">67</a>, <a href="#Page_86">86</a></li>
+<li>Mangel-fly, <a href="#Page_73">73</a></li>
+<li>Marlatt, C.&nbsp;L., <a href="#Page_93">93</a></li>
+<li>Marshall, G.&nbsp;A.&nbsp;K., <a href="#Page_104">104</a></li>
+<li>Maxillae, <a href="#Page_2">2</a>, <a href="#Page_17">17</a>, <a href="#Page_37">37</a>, <a href="#Page_42">42</a></li>
+<li><a name="mayflies" id="mayflies"></a>May-flies, <a href="#Page_31">31-4</a>, <a href="#Page_107">107</a>, <a href="#Page_110">110</a>, <a href="#Page_117">117</a>, <a href="#Page_120">120</a></li>
+<li>Meloidae, <a href="#Page_56">56</a></li>
+<li>Mesozoic insects, <a href="#Page_107">107</a></li>
+<li>Metabola, <a href="#Page_35">35</a></li>
+<li><a name="metamorphosis" id="metamorphosis"></a>Metamorphosis
+<ul class="IX"><li>(in general), <a href="#Page_6">6</a>, <a href="#Page_109">109</a>;</li>
+<li>(degrees of in insects) <a href="#Page_8">8</a>, <a href="#Page_35">35</a>, <a href="#Page_109">109</a>, <a href="#Page_117">117-19</a></li></ul></li>
+<li>Miall, L.&nbsp;C., <a href="#Page_6">6</a>, <a href="#Page_28">28</a>, <a href="#Page_33">33</a>, <a href="#Page_43">43</a>, <a href="#Page_77">77</a>, <a href="#Page_78">78</a>, <a href="#Page_87">87</a>, <a href="#Page_97">97</a>, <a href="#Page_113">113</a></li>
+<li>Mosquito. <i>See</i> <a href="#culex">Culex</a>, <a href="#gnats">Gnats</a></li>
+<li>Moths, <a href="#Page_1">1</a>, <a href="#Page_58">58-62</a>, <a href="#Page_84">84</a>, <a href="#Page_95">95-100</a>, <a href="#Page_117">117</a></li>
+<li><a name="moult" id="moult"></a>Moult, <a href="#Page_10">10</a>, <a href="#Page_32">32</a>, <a href="#Page_36">36</a>, <a href="#Page_41">41</a></li>
+<li><i>Musca domestica</i>, <a href="#Page_71">71</a></li>
+<li>Muscidae, <a href="#Page_44">44</a></li>
+<li>Muscles, <a href="#Page_47">47</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_N" id="IX_N"></a>Nervous system, <a href="#Page_44">44-5</a></li>
+<li>Neuroptera, <a href="#Page_57">57</a>, <a href="#Page_80">80</a>, <a href="#Page_112">112</a></li>
+<li>Newport, G., <a href="#Page_41">41</a>, <a href="#Page_44">44</a></li>
+<li>Noctuidae, <a href="#Page_60">60</a>, <a href="#Page_98">98</a></li>
+<li>Nymph, <a href="#Page_15">15</a>, <a href="#Page_28">28</a>, <a href="#Page_33">33</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_O" id="IX_O"></a>Oak-apples, <a href="#Page_94">94</a></li>
+<li>Obtect pupa, <a href="#Page_81">81</a></li>
+<li>Odonata, <a href="#Page_24">24</a>.
+<i>See also</i> <a href="#dragonflies">Dragon-flies</a></li>
+<li><i>Oestrus ovis</i>, <a href="#Page_91">91</a></li>
+<li>Oil-beetles, <a href="#Page_56">56</a>, <a href="#Page_112">112</a></li>
+<li><i>Orgyia antiqua</i>, <a href="#Page_96">96-7</a></li>
+<li>Orthoptera, <a href="#Page_17">17</a>, <a href="#Page_35">35</a>, <a href="#Page_110">110</a></li>
+<li>Owl Moths, <a href="#Page_60">60</a>, <a href="#Page_98">98</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_P" id="IX_P"></a>Packard, A.&nbsp;S., <a href="#Page_56">56</a>, <a href="#Page_118">118</a></li>
+<li>Paedogenesis. <i>See</i> <a href="#repro">Larval reproduction</a></li>
+<li>Painted Lady Butterfly, <a href="#Page_96">96</a></li>
+<li>Palaeozoic insects, <a href="#Page_107">107</a></li>
+<li>Palm&eacute;n, J.&nbsp;A., <a href="#Page_25">25</a></li>
+<li>Parasitic insects, <a href="#Page_73">73-4</a>, <a href="#Page_108">108</a>, <a href="#Page_116">116</a></li>
+<li>Parental care, <a href="#Page_64">64-6</a></li>
+<li>Parthenogenesis, <a href="#Page_18">18</a></li>
+<li>Partial transformation, <a href="#Page_35">35</a>, <a href="#Page_37">37</a></li>
+<li>Perla, <a href="#Page_24">24</a></li>
+<li>Permian insects, <a href="#Page_107">107</a></li>
+<li>Phagocytes, <a href="#Page_48">48</a></li>
+<li>Phyllodecta, <a href="#Page_53">53</a>, <a href="#Page_113">113</a></li>
+<li>Phyllotreta, <a href="#Page_53">53</a></li>
+<li><i>Pieris brassicae</i>, <a href="#Page_39">39</a>, <a href="#Page_41">41</a>, <a href="#Page_85">85</a>, <a href="#Page_100">100</a></li>
+<li><i>Pieris napi</i> and var. <i>bryoniae</i>, <a href="#Page_102">102-3</a></li>
+<li>Platygaster, <a href="#Page_66">66</a></li>
+<li>Plecoptera, <a href="#Page_24">24</a>.
+<i>See also</i> <a href="#stoneflies">Stone-flies</a></li>
+<li>Pompilidae, <a href="#Page_66">66-7</a></li>
+<li>Poulton, E.&nbsp;B., <a href="#Page_61">61</a>, <a href="#Page_82">82</a>, <a href="#Page_109">109</a></li>
+<li>Precis, <a href="#Page_104">104</a></li>
+<li>Proctotrypidae, <a href="#Page_66">66</a></li>
+<li>Pro-legs, <a href="#Page_4">4</a>, <a href="#Page_58">58-9</a>, <a href="#Page_84">84</a>, <a href="#Page_114">114</a></li>
+<li>Pro-nymph, <a href="#Page_118">118</a>, <a href="#Page_119">119</a></li>
+<li>Protective coloration, <a href="#Page_60">60-1</a></li>
+<li><i>Psylliodes chrysocephala</i>, <a href="#Page_54">54</a></li>
+<li>Ptinidae, <a href="#Page_54">54</a></li>
+<li><a name="pupa" id="pupa"></a>Pupa, <a href="#Page_4">4</a>, <a href="#Page_37">37</a>, <a href="#Page_40">40</a>, <a href="#Page_79">79-88</a>, <a href="#Page_114">114</a>, <a href="#Page_117">117</a></li>
+<li>Puparium, <a href="#Page_88">88</a></li>
+<li>Pupipara, <a href="#Page_91">91</a></li>
+<li><i>Pyrameis cardui</i>, <a href="#Page_96">96</a><a name="Page_133" id="Page_133"></a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_R" id="IX_R"></a>Rat-tailed maggot, <a href="#Page_76">76</a></li>
+<li>R&eacute;aumur, R.&nbsp;A.&nbsp;F. de, <a href="#Page_8">8</a>, <a href="#Page_28">28</a>, <a href="#Page_33">33</a>, <a href="#Page_41">41</a></li>
+<li>Reproductive larvae, <a href="#Page_90">90</a>;
+pupae, <a href="#Page_91">91</a></li>
+<li>Reproductive organs, <a href="#Page_45">45</a></li>
+<li><i>Rhabdophaga heterobia</i>, <a href="#Page_70">70</a></li>
+<li>Riley, C.&nbsp;V., <a href="#Page_83">83</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_S" id="IX_S"></a>Sanderson, E.&nbsp;D., <a href="#Page_17">17</a></li>
+<li>Sand-midges, <a href="#Page_78">78</a></li>
+<li>Sarcophaga, <a href="#Page_91">91</a></li>
+<li>Saw-flies, <a href="#Page_58">58-9</a></li>
+<li>Scale-insects, <a href="#Page_20">20</a>.
+<i>See also</i> <a href="#coccidae">Coccidae</a></li>
+<li>Scarabaeidae, <a href="#Page_52">52</a></li>
+<li>Schmidt, E.&nbsp;O., <a href="#Page_21">21</a></li>
+<li>Scolytidae, <a href="#Page_55">55</a></li>
+<li>Scudder, S.&nbsp;H., <a href="#Page_106">106</a></li>
+<li>Seasonal changes, <a href="#Page_89">89-104</a></li>
+<li>Seasonal dimorphism, <a href="#Page_102">102</a></li>
+<li>Semi-pupa, <a href="#Page_118">118</a></li>
+<li>Sesiidae, <a href="#Page_62">62</a></li>
+<li>Sexual differences, <a href="#Page_15">15</a>, <a href="#Page_20">20-1</a>, <a href="#Page_90">90</a></li>
+<li>Sharp, D., <a href="#Page_13">13</a>, <a href="#Page_36">36</a>, <a href="#Page_40">40</a>, <a href="#Page_115">115</a></li>
+<li>Silk-spinning, <a href="#Page_58">58</a>, <a href="#Page_62">62-3</a>, <a href="#Page_82">82</a></li>
+<li>Silkworms, <a href="#Page_82">82</a></li>
+<li>Silpha, <a href="#Page_50">50</a></li>
+<li>Siltala, A.&nbsp;J., <a href="#Page_63">63</a></li>
+<li>Silvestri, F., <a href="#Page_119">119</a></li>
+<li>Simulium, <a href="#Page_78">78</a>, <a href="#Page_87">87</a></li>
+<li>Smith, J.&nbsp;B., <a href="#Page_17">17</a></li>
+<li>Sphegidae, <a href="#Page_66">66-7</a></li>
+<li>Sphingidae, <a href="#Page_60">60</a></li>
+<li>Spinneret, <a href="#Page_58">58</a></li>
+<li><a name="spiracles" id="spiracles"></a>Spiracles, <a href="#Page_2">2</a>, <a href="#Page_23">23</a>, <a href="#Page_70">70</a>, <a href="#Page_72">72</a>, <a href="#Page_77">77</a>, <a href="#Page_86">86</a>, <a href="#Page_87">87</a></li>
+<li>Spring-tails, <a href="#Page_11">11</a></li>
+<li><a name="stoneflies" id="stoneflies"></a>Stone-flies, <a href="#Page_24">24</a>, <a href="#Page_107">107</a>, <a href="#Page_110">110</a></li>
+<li>Sub-imago, <a href="#Page_33">33</a>, <a href="#Page_117">117</a></li>
+<li>Sucking insects, <a href="#Page_17">17</a></li>
+<li>Swammerdam, J., <a href="#Page_33">33</a></li>
+<li>Syrphus, <a href="#Page_74">74-6</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_T" id="IX_T"></a>Tachininae, <a href="#Page_73">73</a>, <a href="#Page_91">91</a></li>
+<li><i>Tenebrio molitor</i>, <a href="#Page_119">119</a></li>
+<li>Termitoxeniidae, <a href="#Page_92">92</a></li>
+<li>Theobald, F.&nbsp;V., <a href="#Page_100">100</a></li>
+<li>Thysanura, <a href="#Page_11">11</a></li>
+<li>Tiger Moths, <a href="#Page_59">59</a>, <a href="#Page_82">82</a>, <a href="#Page_98">98</a></li>
+<li>Timber-beetles, <a href="#Page_54">54</a></li>
+<li>Tineidae, <a href="#Page_62">62</a></li>
+<li>Tipulidae, <a href="#Page_70">70</a></li>
+<li>Tortoiseshell Butterfly, <a href="#Page_45">45</a>, <a href="#Page_95">95</a></li>
+<li>Tortricidae, <a href="#Page_62">62</a></li>
+<li>Tracheal system. <i>See</i> <a href="#airtubes">Air-tubes</a>, <a href="#spiracles">Spiracles</a></li>
+<li>Transformation. <i>See</i> <a href="#metamorphosis">Metamorphosis</a></li>
+<li>Triassic insects, <a href="#Page_107">107</a></li>
+<li>Trichocera, <a href="#Page_70">70</a></li>
+<li>Trichoptera, <a href="#Page_62">62-3</a>, <a href="#Page_76">76</a>, <a href="#Page_80">80</a>, <a href="#Page_86">86</a></li>
+<li>Tsetse Flies, <a href="#Page_91">91</a></li>
+<li>Turnip-fly, <a href="#Page_53">53</a>, <a href="#Page_92">92</a>, <a href="#Page_94">94</a></li>
+<li>Turnip Moth, <a href="#Page_98">98-9</a></li>
+<li>Tussock Moths, <a href="#Page_90">90</a>, <a href="#Page_97">97</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_V" id="IX_V"></a><i>Vanessa urticae</i>, <a href="#Page_45">45</a>, <a href="#Page_95">95</a></li>
+<li>Van Rees, J., <a href="#Page_42">42</a></li>
+<li>Vapourer Moth, <a href="#Page_96">96-7</a>, <a href="#Page_115">115</a></li>
+<li><i>Velia currens</i>, <a href="#Page_116">116</a></li>
+<li>Verhoeff, K.&nbsp;W., <a href="#Page_11">11</a></li>
+<li>Vermiculiform larvae, <a href="#Page_67">67</a>, <a href="#Page_71">71-6</a>, <a href="#Page_111">111</a></li>
+<li>Virgin stem-mothers, <a href="#Page_18">18</a></li>
+<li>Viviparous reproduction. <i>See</i> <a href="#birth">Birth</a></li>
+</ul>
+<ul class="IX">
+<li><a name="IX_W" id="IX_W"></a>Wagner, N., <a href="#Page_90">90</a></li>
+<li>Warble-fly, <a href="#Page_73">73-4</a>, <a href="#Page_89">89</a>, <a href="#Page_108">108</a></li>
+<li>Warning coloration, <a href="#Page_60">60</a></li>
+<li>Wasmann, E., <a href="#Page_92">92</a></li>
+<li>Wasps, <a href="#Page_46">46</a>, <a href="#Page_64">64</a>, <a href="#Page_66">66-7</a>, <a href="#Page_83">83</a></li>
+<li>Water-insects. <i>See</i> <a href="#aquatic">Aquatic insects</a><a name="Page_134" id="Page_134"></a></li>
+
+<li>Weevils, <a href="#Page_55">55</a></li>
+<li>Weismann, A., <a href="#Page_38">38</a>, <a href="#Page_42">42</a>, <a href="#Page_102">102</a></li>
+<li>White Butterflies, <a href="#Page_41">41</a>, <a href="#Page_83">83</a>, <a href="#Page_85">85</a>, <a href="#Page_100">100-3</a></li>
+<li>Willow-beetles, <a href="#Page_53">53</a></li>
+<li>Wingless insects, <a href="#Page_15">15</a>, <a href="#Page_18">18</a>, <a href="#Page_20">20</a>, <a href="#Page_96">96</a>, <a href="#Page_115">115</a></li>
+<li>Wing-rudiments, <a href="#Page_13">13</a>, <a href="#Page_18">18</a>, <a href="#Page_20">20</a>, <a href="#Page_22">22</a>, <a href="#Page_24">24</a>, <a href="#Page_28">28</a>, <a href="#Page_33">33</a>, <a href="#Page_36">36-8</a>, <a href="#Page_40">40</a>, <a href="#Page_111">111</a>, <a href="#Page_115">115</a>, <a href="#Page_117">117-19</a></li>
+<li>Wings, <a href="#Page_1">1</a>, <a href="#Page_14">14</a>, <a href="#Page_115">115</a>, <a href="#Page_119">119-20</a></li>
+<li>Winter broods, <a href="#Page_102">102-3</a></li>
+<li><a name="wintering" id="wintering"></a>Wintering stages, <a href="#Page_93">93-101</a></li>
+<li>Wireworms, <a href="#Page_52">52</a>, <a href="#Page_93">93</a></li>
+<li>Wood-wasps, <a href="#Page_65">65</a></li>
+</ul>
+
+
+
+
+<p  class="center" style="margin-top:3em;"><small>CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS</small></p>
+
+
+
+<p><a name="Page_135" id="Page_135"></a></p>
+<h2>THE<br />
+CAMBRIDGE MANUALS<br />
+OF SCIENCE AND LITERATURE</h2>
+
+<p class="center">Published by the Cambridge University Press</p>
+
+<p class="center">GENERAL EDITORS<br /><br />
+P. GILES, Litt.D.<br />
+Master of Emmanuel College<br /><br />
+and<br /><br />
+A.&nbsp;C. SEWARD, M.A., F.R.S.<br />
+Professor of Botany in the University of Cambridge</p>
+
+<h3>70 VOLUMES NOW READY</h3>
+
+<h4>HISTORY AND ARCHAEOLOGY</h4>
+<ul>
+<li>Ancient Assyria. By Rev. C.&nbsp;H.&nbsp;W. Johns, Litt.D.</li>
+<li>Ancient Babylonia. By Rev. C.&nbsp;H.&nbsp;W. Johns, Litt.D.</li>
+<li>A History of Civilization in Palestine. By Prof. R.&nbsp;A.&nbsp;S. Macalister,
+M.A., F.S.A.</li>
+<li>China and the Manchus. By Prof. H.&nbsp;A. Giles, LL.D.</li>
+<li>The Civilization of Ancient Mexico. By Lewis Spence.</li>
+<li>The Vikings. By Prof. Allen Mawer, M.A.</li>
+<li>New Zealand. By the Hon. Sir Robert Stout, K.C.M.G., LL.D., and J. Logan
+Stout, LL.B. (N.Z.).</li>
+<li>The Ground Plan of the English Parish Church. By A. Hamilton Thompson,
+M.A., F.S.A.</li>
+<li>The Historical Growth of the English Parish Church. By A. Hamilton
+Thompson, M.A., F.S.A.</li>
+<li>English Monasteries. By A.&nbsp;H. Thompson, M.A., F.S.A.</li>
+<li>Brasses. By J.&nbsp;S.&nbsp;M. Ward, B.A., F.R.Hist.S.</li>
+<li>Ancient Stained and Painted Glass. By F.&nbsp;S. Eden.</li>
+</ul>
+
+<h4>ECONOMICS</h4>
+<ul>
+<li>Co-partnership in Industry. By C.&nbsp;R. Fay, M.A.</li>
+<li>Cash and Credit. By D.&nbsp;A. Barker.</li>
+<li>The Theory of Money. By D.&nbsp;A. Barker.</li>
+</ul>
+
+<h4><a name="Page_136" id="Page_136"></a>LITERARY HISTORY</h4>
+<ul>
+<li>The Early Religious Poetry of the Hebrews. By the Rev. E.&nbsp;G. King, D.D.</li>
+<li>The Early Religious Poetry of Persia. By the Rev. Prof. J. Hope Moulton,
+D.D., D.Theol. (Berlin).</li>
+<li>The History of the English Bible. By John Brown, D.D.</li>
+<li>English Dialects from the Eighth Century to the Present Day. By W.&nbsp;W.
+Skeat, Litt.D., D.C.L., F.B.A.</li>
+<li>King Arthur in History and Legend. By Prof. W. Lewis Jones, M.A.</li>
+<li>The Icelandic Sagas. By W.&nbsp;A. Craigie, LL.D.</li>
+<li>Greek Tragedy. By J.&nbsp;T. Sheppard, M.A.</li>
+<li>The Ballad in Literature. By T.&nbsp;F. Henderson.</li>
+<li>Goethe and the Twentieth Century. By Prof. J.&nbsp;G. Robertson, M.A., Ph.D.</li>
+<li>The Troubadours. By the Rev. H.&nbsp;J. Chaytor, M.A.</li>
+<li>Mysticism in English Literature. By Miss C.&nbsp;F.&nbsp;E. Spurgeon.</li>
+</ul>
+
+<h4>PHILOSOPHY AND RELIGION</h4>
+<ul>
+<li>The Idea of God in Early Religions. By Dr F.&nbsp;B. Jevons.</li>
+<li>Comparative Religion. By Dr F.&nbsp;B. Jevons.</li>
+<li>Plato: Moral and Political Ideals. By Mrs A.&nbsp;M. Adam.</li>
+<li>The Moral Life and Moral Worth. By Prof. Sorley, Litt.D.</li>
+<li>The English Puritans. By John Brown, D.D.</li>
+<li>An Historical Account of the Rise and Development of Presbyterianism in
+Scotland. By the Rt Hon. the Lord Balfour of Burleigh, K.T., G.C.M.G.</li>
+<li>Methodism. By Rev. H.&nbsp;B. Workman, D.Lit.</li>
+</ul>
+
+<h4>EDUCATION</h4>
+<ul>
+<li>Life in the Medieval University. By R.&nbsp;S. Rait, M.A.</li>
+</ul>
+
+<h4>LAW</h4>
+<ul>
+<li>The Administration of Justice in Criminal Matters (in England and
+Wales). By G. Glover Alexander, M.A., LL.M.</li>
+</ul>
+
+<h4>BIOLOGY</h4>
+<ul>
+<li>The Coming of Evolution. By Prof. J.&nbsp;W. Judd, C.B., F.R.S.</li>
+<li>Heredity in the Light of Recent Research. By L. Doncaster, M.A.</li>
+<li>Primitive Animals. By Geoffrey Smith, M.A.</li>
+<li>The Individual in the Animal Kingdom. By J.&nbsp;S. Huxley, B.A.</li>
+<li>Life in the Sea. By James Johnstone, B.Sc.</li>
+<li>The Migration of Birds. By T.&nbsp;A. Coward.</li>
+<li><a name="Page_137" id="Page_137"></a>Spiders. By C. Warburton, M.A.</li>
+<li>Bees and Wasps. By O.&nbsp;H. Latter, M.A.</li>
+<li>House Flies. By C.&nbsp;G. Hewitt, D.Sc.</li>
+<li>Earthworms and their Allies. By F.&nbsp;E. Beddard, F.R.S.</li>
+<li>The Wanderings of Animals. By H.&nbsp;F. Gadow, F.R.S.</li>
+</ul>
+
+<h4>ANTHROPOLOGY</h4>
+<ul>
+<li>The Wanderings of Peoples. By Dr A.&nbsp;C. Haddon, F.R.S.</li>
+<li>Prehistoric Man. By Dr W.&nbsp;L.&nbsp;H. Duckworth.</li>
+</ul>
+
+<h4>GEOLOGY</h4>
+<ul>
+<li>Rocks and their Origins. By Prof. Grenville A.&nbsp;J. Cole.</li>
+<li>The Work of Rain and Rivers. By T.&nbsp;G. Bonney, Sc.D.</li>
+<li>The Natural History of Coal. By Dr E.&nbsp;A. Newell Arber.</li>
+<li>The Natural History of Clay. By Alfred B. Searle.</li>
+<li>The Origin of Earthquakes. By C. Davison, Sc.D., F.G.S.</li>
+<li>Submerged Forests. By Clement Reid, F.R.S.</li>
+</ul>
+
+<h4>BOTANY</h4>
+<ul>
+<li>Plant-Animals: a Study in Symbiosis. By Prof. F.&nbsp;W. Keeble.</li>
+<li>Plant-Life on Land. By Prof. F.&nbsp;O. Bower, Sc.D., F.R.S.</li>
+<li>Links with the Past in the Plant-World. By Prof. A.&nbsp;C. Seward.</li>
+</ul>
+
+<h4>PHYSICS</h4>
+<ul>
+<li>The Earth. By Prof. J.&nbsp;H. Poynting, F.R.S.</li>
+<li>The Atmosphere. By A.&nbsp;J. Berry, M.A.</li>
+<li>Beyond the Atom. By John Cox, M.A.</li>
+<li>The Physical Basis of Music. By A. Wood, M.A.</li>
+</ul>
+
+<h4>PSYCHOLOGY</h4>
+<ul>
+<li>An Introduction to Experimental Psychology. By Dr C.&nbsp;S. Myers.</li>
+<li>The Psychology of Insanity. By Bernard Hart, M.D.</li>
+</ul>
+
+<h4>INDUSTRIAL AND MECHANICAL SCIENCE</h4>
+<ul>
+<li>The Modern Locomotive. By C. Edgar Allen, A.M.I.Mech.E.</li>
+<li>The Modern Warship. By E.&nbsp;L. Attwood.</li>
+<li>Aerial Locomotion. By E.&nbsp;H. Harper, M.A., and Allan E. Ferguson, B.Sc.</li>
+<li>Electricity in Locomotion. By A.&nbsp;G. Whyte, B.Sc.</li>
+<li>Wireless Telegraphy. By Prof. C.&nbsp;L. Fortescue, M.A.</li>
+<li>The Story of a Loaf of Bread. By Prof. T.&nbsp;B. Wood, M.A.</li>
+<li>Brewing. By A. Chaston Chapman, F.I.C.</li>
+</ul>
+
+
+<p><a name="Page_138" id="Page_138"></a></p>
+<h3>SOME VOLUMES IN PREPARATION</h3>
+
+<h4>HISTORY AND ARCHAEOLOGY</h4>
+
+<ul>
+<li>The Aryans. By Prof. M. Winternitz.</li>
+<li>Ancient India. By Prof. E.&nbsp;J. Rapson, M.A.</li>
+<li>The Peoples of India. By J.&nbsp;D. Anderson, M.A.</li>
+<li>The Balkan Peoples. By J.&nbsp;D. Bourchier.</li>
+<li>Canada of the present day. By C.&nbsp;G. Hewitt, D.Sc.</li>
+<li>The Evolution of Japan. By Prof. J.&nbsp;H. Longford.</li>
+<li>The West Indies. By Sir Daniel Morris, K.C.M.G.</li>
+<li>The Royal Navy. By John Leyland.</li>
+<li>Gypsies. By John Sampson.</li>
+<li>A Grammar of Heraldry. By W.&nbsp;H. St John Hope, Litt.D.</li>
+<li>Celtic Art. By Joseph Anderson, LL.D.</li>
+</ul>
+
+<h4>ECONOMICS</h4>
+<ul>
+<li>Women's Work. By Miss Constance Smith.</li>
+</ul>
+
+<h4>LITERARY HISTORY</h4>
+<ul>
+<li>Early Indian Poetry. By A.&nbsp;A. Macdonell.</li>
+<li>The Book. By H.&nbsp;G. Aldis, M.A.</li>
+<li>Pantomime. By D.&nbsp;L. Murray.</li>
+<li>Folk Song and Dance. By Miss Neal and F. Kidson.</li>
+</ul>
+
+<h4>PHYSICS</h4>
+<ul>
+<li>The Natural Sources of Energy. By Prof. A.&nbsp;H. Gibson, D.Sc.</li>
+<li>The Sun. By Prof. R.&nbsp;A. Sampson.</li>
+<li>R&ouml;ntgen Rays. By Prof. W.&nbsp;H. Bragg, F.R.S.</li>
+</ul>
+
+<h4>BIOLOGY</h4>
+<ul>
+<li>The Life-story of Insects. By Prof. G.&nbsp;H. Carpenter.</li>
+<li>The Flea. By H. Russell.</li>
+<li>Pearls. By Prof. W.&nbsp;J. Dakin.</li>
+</ul>
+
+<h4>GEOLOGY</h4>
+<ul>
+<li>Soil Fertility. By E.&nbsp;J. Russell, D.Sc.</li>
+<li>Coast Erosion. By Prof. T.&nbsp;J. Jehu.</li>
+</ul>
+
+<h4>INDUSTRIAL AND MECHANICAL SCIENCE</h4>
+<ul>
+<li>Coal Mining. By T.&nbsp;C. Cantrill.</li>
+<li>Leather. By Prof. H.&nbsp;R. Procter.</li>
+</ul>
+<p class="center" style="margin-top:3em;">Cambridge University Press<br />
+C.&nbsp;F. Clay, Manager<br />
+London: Fetter Lane, E.C.<br />
+Edinburgh: 100, Princes Street</p>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
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+Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Life-Story of Insects
+
+Author: Geo. H. Carpenter
+
+Release Date: August 1, 2005 [EBook #16410]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE LIFE-STORY OF INSECTS ***
+
+
+
+
+Produced by Justin Kerk, Laura Wisewell and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+                 The Cambridge Manuals of Science and
+                              Literature
+
+
+
+                      THE LIFE-STORY OF INSECTS
+
+
+
+                      CAMBRIDGE UNIVERSITY PRESS
+                      London: FETTER LANE, E.C.
+                          C.F. CLAY, MANAGER
+
+                            [Illustration]
+
+                    Edinburgh: 100, PRINCES STREET
+             London: H.K. LEWIS, 136, GOWER STREET, W.C.
+            WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
+                       Berlin: A. ASHER AND CO.
+                       Leipzig: F.A. BROCKHAUS
+                     New York: G.P. PUTNAM'S SONS
+             Bombay and Calcutta: MACMILLAN AND CO., LTD.
+
+
+
+
+[Illustration: _Frontispiece._ Transformation of a Gnat (_Culex_).
+    Magnified 5 times.
+A.  Larva. (The head is directed downwards and the tail-siphon with
+    spiracle points upwards to the surface of the water.)
+B.  Pupal Cuticle from which the Imago is emerging. (The pair of
+    'respiratory trumpets' on the thorax of the pupa are conspicuous. The
+    wings of the Imago are crumpled, and the hind feet are not yet
+    withdrawn.)
+C.  Adult Gnat. Female.]
+
+
+
+                            [Illustration]
+
+
+
+                            THE LIFE-STORY
+
+                              OF INSECTS
+
+
+
+                                  BY
+
+                          GEO. H. CARPENTER
+
+                  Professor of Zoology in the Royal
+                      College of Science, Dublin
+
+                              Cambridge:
+                       at the University Press
+                              New York:
+                          G.P. Putnam's Sons
+                                 1913
+
+
+                              Cambridge:
+                      PRINTED BY JOHN CLAY, M.A.
+                       AT THE UNIVERSITY PRESS
+
+
+     With the exception of the coat of arms at the foot, the design on
+     the title page is a reproduction of one used by the earliest known
+     Cambridge printer John Siberch 1521
+
+
+
+
+PREFACE
+
+The object of this little book is to afford an outline sketch of the
+facts and meaning of insect-transformations. Considerations of space
+forbid anything like an exhaustive treatment of so vast a subject, and
+some aspects of the question, the physiological for example, are almost
+neglected. Other books already published in this series, such as Dr
+Gordon Hewitt's _House-flies_ and Mr O H. Latter's _Bees and Wasps_, may
+be consulted with advantage for details of special insect life-stories.
+Recent researches have emphasised the practical importance to human
+society of entomological study, and insects will always be a source of
+delight to the lover of nature. This humble volume will best serve its
+object if its reading should lead fresh observers to the brookside and
+the woodland.
+
+G.H.C.
+
+DUBLIN,
+
+_July_, 1913.
+
+
+
+
+CONTENTS
+
+CHAP.                                               PAGE
+
+   I.  Introduction                                     1
+
+  II.  Growth and Change                                8
+
+ III.  The Life-stories of some Sucking Insects        16
+
+  IV.  From Water to Air                               23
+
+   V.  Transformations, Outward and Inward             35
+
+  VI.  Larvae and their Adaptations                    49
+
+ VII.  Pupae and their Modifications                   79
+
+VIII.  The Life-story and the Seasons                  89
+
+  IX.  Past and Present--the Meaning of the Story     105
+
+       Outline Classification of Insects              122
+
+       Table of Geological Systems                    123
+
+       Bibliography                                   124
+
+       Index                                          129
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+Stages in the Transformations of a Gnat    _Frontispiece_
+
+FIG                                                  PAGE
+ 1. Stages of the Diamond-back Moth (_Plutella          3
+    cruciferarum_)
+
+ 2. Head of typical Moth                                5
+
+ 3. Head of Caterpillar                                 5
+
+ 4. Common Cockroach (_Blatta orientalis_)             12
+
+ 5. Nymph of Locust (_Schistocera americana_)          13
+
+ 6. _Aphis pomi_, winged and wingless females          19
+
+ 7. Mussel Scale-Insect (_Mytilaspis pomorum_)         21
+
+ 8. Emergence of Dragon-fly (_Aeschna cyanea_)      29-31
+
+ 9. Nymph of May-fly (_Chloeon dipterum_)              33
+
+10. Imaginal buds of Butterfly                         39
+
+11. Imaginal buds of Blow-fly                          43
+
+12. Carrion Beetle (_Silpha_) and larva                51
+
+13. Larva of Ground-beetle (_Aepus_)                   52
+
+14. Willow-beetle (_Phyllodecta_) and larva            53
+
+15. Cabbage-beetle (_Psylliodes_) and larva            54
+
+16. Corn Weevil (_Calandra_) and larva                 55
+
+17. Ruby Tiger Moth (_Phragmatobia fuliginosa_)        61
+
+18. Larvae and Pupa of Hive-bee (_Apis mellifica_)     65
+
+19. Larva of Gall-midge (_Contarinia nasturtii_)       68
+
+20. Crane-fly (_Tipula oleracea_) and larva            69
+
+21. Maggot of House-fly (_Musca domestica_)            71
+
+22. Ox Warble-fly (_Hypoderma bovis_) with egg,
+    larva, and puparium                                75
+
+23. Pupa of White Butterfly (_Pieris_)                 85
+
+
+
+
+CHAPTER I
+
+INTRODUCTION
+
+
+Among the manifold operations of living creatures few have more strongly
+impressed the casual observer or more deeply interested the thoughtful
+student than the transformations of insects. The schoolboy watches the
+tiny green caterpillars hatched from eggs laid on a cabbage leaf by the
+common white butterfly, or maybe rears successfully a batch of silkworms
+through the changes and chances of their lives, while the naturalist
+questions yet again the 'how' and 'why' of these common though wondrous
+life-stories, as he seeks to trace their course more fully than his
+predecessors knew.
+
+[Illustration: Fig. 1. _a_, Diamond-back Moth (_Plutella
+cruciferarum_); _b_, young caterpillar, dorsal view; _c_, full-grown
+caterpillar, dorsal view; _d_, side view; _e_, pupa, ventral view.
+Magnified 6 times. From _Journ. Dept. Agric. Ireland_, vol. I.]
+
+Everyone is familiar with the main facts of such a life-story as that of
+a moth or butterfly. The form of the adult insect (fig. 1 _a_) is
+dominated by the wings--two pairs of scaly wings, carried respectively
+on the middle and hindmost of the three segments that make up the
+_thorax_ or central region of the insect's body. Each of these three
+segments carries a pair of legs. In front of the thorax is the head on
+which the pair of long jointed feelers and the pair of large,
+sub-globular, compound eyes are the most prominent features. Below the
+head, however, may be seen, now coiled up like a watch-spring, now
+stretched out to draw the nectar from some scented blossom, the
+butterfly's sucking trunk or proboscis, situated between a pair of short
+hairy limbs or palps (fig. 2). These palps belong to the appendages of
+the hindmost segment of the head, appendages which in insects are
+modified to form a hind-lip or _labium_, bounding the mouth cavity below
+or behind. The proboscis is made up of the pair of jaw-appendages in
+front of the labium, the _maxillae_, as they are called. Behind the
+thorax is situated the _abdomen,_ made up of nine or ten recognisable
+segments, none of which carry limbs comparable to the walking legs, or
+to the jaws which are the modified limbs of the head-segments. The whole
+cuticle or outer covering of the body, formed (as is usual in the group
+of animals to which insects belong) of a horny (chitinous) secretion of
+the skin, is firm and hard, and densely covered with hairy or scaly
+outgrowths. Along the sides of the insect are a series of paired
+openings or spiracles, leading to a set of air-tubes which ramify
+throughout the body and carry oxygen directly to the tissues.
+
+[Illustration: Fig. 2. A. Head of a typical Moth, showing proboscis
+formed by flexible maxillae (_g_) between the labial palps (_p_); _c_,
+face; _e_, eye; the structure _m_ has been regarded as the vestige of a
+mandible. B. Basal part (_b_) of maxilla removed from head, with
+vestigial palp (_p_). Magnified.]
+
+Such a butterfly as we have briefly sketched lays an egg on the leaf of
+some suitable food-plant, and there is hatched from it the well-known
+crawling larva[1] (fig. 1 _b, c, d_) called a caterpillar, offering in
+many superficial features a marked contrast to its parent. Except on the
+head, whose surface is hard and firm, the caterpillar's cuticle is as a
+rule thin and flexible, though it may carry a protective armature of
+closely set hairs, or strong sharp spines. The feelers (fig. 3 _At_) are
+very short and the eyes are small and simple. In connection with the
+mouth, there are present in front of the maxillae a pair of _mandibles_
+(fig. 3 _Mn_), strong jaws, adapted for biting solid food, which are
+absent from the adult butterfly, though well developed in cockroaches,
+dragon-flies, beetles, and many other insects. The three pairs of legs
+on the segments of the thorax are relatively short, and as many as five
+segments of the abdomen may carry short cylindrical limbs or pro-legs,
+which assist the clinging habits and worm-like locomotion of the
+caterpillar. No trace of wings is visible externally. The caterpillar,
+therefore, differs markedly from its parent in its outward structure, in
+its mode of progression, and in its manner of feeding; for while the
+butterfly sucks nectar or other liquid food, the caterpillar bites up
+and devours solid vegetable substances, such as the leaves of herbs or
+trees. It is well-known that between the close of its larval life and
+its attainment of perfection as a butterfly, the insect spends a
+period as a _pupa_ (fig. 1 _e_) unable to move from place to place, and
+taking no food.
+
+[1] The term _larva_ is applied to any young animal which differs
+markedly from its parent.
+
+[Illustration: Fig. 3. Head of Caterpillar of Goat-moth (_Cossus_) seen
+from behind. _At_, feeler; _Mn_, mandible; _Mx_, maxilla; _Lm_, labium,
+spinneret projecting beyond it. Magnified. After Lyonet from Miall and
+Denny's _Cockroach_.]
+
+Such, in brief, is the course of the most familiar of insect
+life-stories. For the student of the animal world as a whole, this
+familiar transformation raises some startling problems, which have been
+suggestively treated by F. Brauer (1869), L.C. Miall (1895), J. Lubbock
+(1874), R. Heymons (1907), P. Deegener (1909) and other writers[2]. To
+appreciate these problems is the first step towards learning the true
+meaning of the transformation.
+
+[2] The dates in brackets after authors' names will facilitate reference
+to the Bibliography (pp. 124-8).
+
+The butterfly's egg is absolutely and relatively of large size, and
+contains a considerable amount of yolk. As a rule we find that young
+animals hatched from such eggs resemble their parents rather closely and
+pass through no marked changes during their lives. A chicken, a
+crocodile, a dogfish, a cuttlefish, and a spider afford well-known
+examples of this rule. Land-animals, generally, produce young which are
+miniature copies of themselves, for example horses, dogs, and other
+mammals, snails and slugs, scorpions and earthworms. On the other hand,
+metamorphosis among animals is associated with eggs of small size, with
+aquatic habit, and with relatively low zoological rank. The young of a
+starfish, for example, has hardly a character in common with its parent,
+while a marine segmented worm and an oyster, unlike enough when adult,
+develop from closely similar larval forms. If we take a class of
+animals, the Crustacea, nearly allied to insects, we find that its more
+lowly members, such as 'water-fleas' and barnacles, pass through far
+more striking changes than its higher groups, such as lobsters and
+woodlice. But among the Insects, a class of predominantly terrestrial
+and aerial creatures producing large eggs, the highest groups undergo,
+as we shall see, the most profound changes. The life-story of the
+butterfly, then, well-known as it may be, furnishes a puzzling exception
+to some wide-reaching generalisations concerning animal development. And
+the student of science often finds that an exception to some rule is the
+key to a problem of the highest interest.
+
+During many centuries naturalists have bent their energies to explain
+the difficulties presented by insect transformations. Aristotle, the
+first serious student of organised beings whose writings have been
+preserved for us, and William Harvey, the famous demonstrator of the
+mammalian blood circulation two thousand years later, agreed in
+regarding the pupa as a second egg. The egg laid by a butterfly had not,
+according to Harvey, enough store of food to provide for the building-up
+of a complex organism like the parent; only the imperfect larva could be
+produced from it. The larva was regarded as feeding voraciously for the
+purpose of acquiring a large store of nutritive material, after which it
+was believed to revert to the state of a second but far larger egg, the
+pupa, from which the winged insect could take origin. Others again,
+following de Reaumur (1734), have speculated whether the development of
+pupa within larva, and of winged insect within pupa might not be
+explained as abnormal births. But a comparison of the transformation of
+butterflies with simpler insect life-stories will convince the enquirer
+that no such heroic theories as these are necessary. It will be realised
+that even the most profound transformation among insects can be
+explained as a special case of growth.
+
+
+
+
+CHAPTER II
+
+GROWTH AND CHANGE
+
+
+The caterpillar differs markedly from the butterfly. As we pursue our
+studies of insect growth and transformation we shall find that in some
+cases the difference between young and adult is much greater--as for
+example between the maggot and the house-fly, in others far less--as
+between the young and full-grown grasshopper or plant-bug. It is
+evidently wise to begin a general survey of the subject with some of
+those simpler cases in which the differences between the young and
+adult insect are comparatively slight. We shall then be in a position to
+understand better the meaning of the more puzzling and complex cases in
+which the differences between the stages are profound.
+
+In the first place it is necessary to realise that the changes which any
+insect passes through during its life-story are essentially
+accompaniments of its growth. The limits of this little book allow only
+slight reference to features of internal structure; we must be content,
+in the main, to deal with the outward form. But there is an important
+relation between this outward form and the underlying living tissues
+which must be clearly understood. Throughout the great race of
+animals--the Arthropoda--of which insects form a class, the body is
+covered outwardly by a _cuticle_ or secretion of the underlying layer of
+living cells which form the outer skin or _epidermis_[3] (see fig. 10
+_ep_, _cu_, p. 39). This cuticle has regions which are hard and firm,
+forming an _exoskeleton_, and, between these, areas which are relatively
+soft and flexible. The firm regions are commonly segmental in their
+arrangement, and the intervening flexible connections render possible
+accurate motions of the exoskeletal parts in relation to each other,
+the motions being due to the contraction of muscles which are attached
+within the exoskeleton.
+
+[3] The term 'hypodermis' frequently applied to this layer is
+misleading. The layer is the true outer skin--ectoderm or epidermis.
+
+Now this jointed exoskeleton--an admirably formed suit of armour though
+it often is--has one drawback: it is not part of the insect's living
+tissues. It is a cuticle formed by the solidifying of a fluid secreted
+by the epidermal cells, therefore without life, without the power of
+growth, and with only a limited capacity for stretching. It follows,
+therefore, that at least during the period through which the insect
+continues to grow, the cuticle must be periodically shed. Thus in the
+life-story of an insect or other arthropod, such as a lobster, a spider,
+or a centipede, there must be a succession of cuticle-castings--'moults'
+or _ecdyses_ as they are often called.
+
+When such a moult is about to take place the cuticle separates from the
+underlying epidermis, and a fluid collects beneath. A delicate new
+cuticle (see fig. 10 _cu'_) is then formed in contact with the
+epidermis, and the old cuticle opens, usually with a slit lengthwise
+along the back, to allow the insect in its new coat to emerge. At first
+this new coat is thin and flabby, but after a period of exposure to the
+air it hardens and darkens, becoming a worthy and larger successor to
+that which has been cast. The cuticle moreover is by no means wholly
+external. The greater part of the digestive canal and the whole
+air-tube system are formed by inpushings of the outer skin (ectoderm)
+and are consequently lined with an extension of the chitinous cuticle
+which is shed and renewed at every moult.
+
+In all insects these successive moults tend to be associated with change
+of form, sometimes slight, sometimes very great. The new cuticle is
+rarely an exact reproduction of the old one, it exhibits some new
+features, which are often indications of the insect's approach towards
+maturity. Even in some of those interesting and primitive insects the
+Bristle-tails (Thysanura) and Spring-tails (Collembola), in which wings
+are never developed, perceptible differences in the form and arrangement
+of the abdominal limbs can be traced through the successive stages, as
+R. Heymons (1906) and K.W. Verhoeff (1911) have shown for Machilis. But
+the changes undergone by such insects are comparatively so slight, that
+the creatures are often known as 'Ametabola' or insects without
+transformation in the life-history. Now there are a considerable number
+of winged insects--cockroaches and grasshoppers for example--in which
+the observable changes are also comparatively slight. We will sketch
+briefly the main features of the life-story of such an insect.
+
+[Illustration: Fig. 4. Common Cockroach (_Blatta orientalis_). _a_,
+female; _b_, male; _c_, side view of female; _d_, young. After Marlatt,
+_Entom. Bull._ 4, _U.S. Dept. Agric._]
+
+The young creature is hatched from the egg in a form closely resembling,
+on the whole, that of its parent, so that the term 'miniature adult'
+sometimes applied to it, is not inappropriate. The baby cockroach (fig.
+4 _d_) is known by its flattened body, rounded prothorax, and stiff,
+jointed tail-feelers or cercopods; the baby grasshopper by its strong,
+elongate hind-legs, adapted, like those of the adult, for vigorous
+leaping. During the growth of the insect to the adult state there may be
+four or five moults, each preceded and succeeded by a characteristic
+instar[4]. The first instar differs, however, from the adult in one
+conspicuous and noteworthy feature, it possesses no trace of wings. But
+after the first or the second moult, definite wing-rudiments are visible
+in the form of outgrowths on the corners of the second and third
+thoracic segments. In each succeeding instar these rudiments become more
+prominent, and in the fourth or the fifth stage, they show a branching
+arrangement of air-tubes, prefiguring the nervures of the adult's wing
+(fig. 5). After the last moult the wings are exposed, articulated to the
+segments that bear them, and capable of motion. Having been formed
+beneath the cuticle of the wing-rudiments of the penultimate instar, the
+wings are necessarily abbreviated and crumpled. But during the process
+of hardening of the cuticle, they rapidly increase in size, blood and
+air being forced through the nervures, so that the wings attaining their
+full expanse and firmness, become suited for the function of flight.
+
+[4] The convenient term 'instar' has been proposed by Fischer and
+advocated by Sharp (1895) for the form assumed by an insect during a
+stage of its life-story. Thus the creature as hatched from the egg is
+the _first instar_, after the first moult it has become the _second
+instar_, and so on, the number of moults being always one less than the
+number of instars.
+
+[Illustration: Fig. 5. Nymph of Locust (_Schistocera americana_) with
+distinct wing-rudiments. After Howard, _Insect Life_, vol. VII.]
+
+The changes through which these insects pass are therefore largely
+connected with the development of the wings. It is noteworthy that in an
+immature cockroach the entire dorsal cuticle is hard and firm. In the
+adult, however, while the cuticle of the prothorax remains firm, that of
+the two hinder thoracic and of all the abdominal segments is somewhat
+thin and delicate on the dorsal aspect. It needs not now to be
+resistant, because it is covered by the two firm forewings, which shield
+and protect it, except when the insect is flying. There are, indeed,
+slight changes in other structures not directly connected with the
+wings. In a young grasshopper, for example, the feelers are relatively
+stouter than in the adult, and the prothorax does not show the
+specifically distinctive shape with its definite keels and furrows.
+Changes in the secondary sexual characters may also be noticed. For
+instance, in an immature cockroach both male and female carry a pair of
+jointed tail-feelers or cercopods on the tenth abdominal segment, and a
+pair of unjointed limbs or stylets on the ninth. In the adult stage,
+both sexes possess cercopods, but the males only have stylets, those of
+the female disappearing at the final moult.
+
+Reviewing the main features of the life-story of a grasshopper or
+cockroach, we notice that there is no marked or sudden change of form.
+The newly-hatched insect resembles generally its parent, except that it
+has no wings. Wing-rudiments appear, however, in an early instar as
+visible outgrowths on the thoracic segments, and become larger after
+each moult. All through its various stages the immature insect--_nymph_
+as it is called--lives in the same kind of situations and on the same
+kind of food as its parent, and it is all along active and lively,
+undergoing no resting period like the pupal stage in the transformation
+of the butterfly.
+
+One interesting and suggestive fact remains to be mentioned. There are
+grasshoppers and cockroaches in which the changes are even less than
+those just sketched, because the wings remain, even in the adult, in a
+rudimentary state (as for example in the female of the common kitchen
+cockroach, _Blatta orientalis_, see fig. 4 _a_), or are never developed
+at all. Such exceptional winglessness in members of a winged family can
+only be explained by the recognition of a life-story, not merely in the
+individual but in the race. We cannot doubt that the ancestors of these
+wingless insects possessed wings, which in the course of time have been
+lost by the whole species or by the members of the female sex. It is
+generally assumed that this loss has been gradual, and so in many cases
+it probably may have been. But there are species of insects in which
+some generations are winged and others wingless; a winged mother gives
+birth to wingless offspring, and a wingless parent to young with
+well-developed wings. Such discontinuity in the life-story of a single
+generation forces us to recognise the possibility of similar sudden
+mutations in the course of that age-long process of evolution to which
+the facts of insect growth, and indeed of all animal development, bear
+striking testimony.
+
+
+
+
+CHAPTER III
+
+THE LIFE-STORIES OF SOME SUCKING INSECTS
+
+
+We may now turn our attention to some examples of the remarkable
+alternation of winged and wingless generations in the yearly life-cycle
+of the same species, mentioned at the end of the last chapter.
+Cockroaches and grasshoppers belong to an order of insects, the
+Orthoptera[5], characterised by firm forewings and biting jaws; in all
+of them the change of form during the life-history is comparatively
+slight. A great contrast to those insects in the structure of the
+mouth-parts is presented by the Hemiptera, an order including the bugs,
+pond-skaters, cicads, plant-lice, and scale-insects. These all have an
+elongated, grooved labium projecting from the head in form of a beak,
+within which work, to and fro, the slender needle-like mandibles and
+maxillae by means of which the insect pierces holes through the skin of
+a leaf or an animal, and is thus enabled to suck a meal of sap or blood,
+according to its mode of life. In many Hemiptera--the various families
+of bugs both aquatic and terrestrial, for example--the life-history is
+nearly as simple as that of a cockroach. It is the family of the
+plant-lice (Aphidae) that affords typical illustrations of that
+alternation of generations to which reference has been made.
+
+[5] See outline classification of insects, p. 122.
+
+The yearly cycle of the common Aphids of the apple tree has been lately
+worked out in detail by J.B. Smith (1900) and E.D. Sanderson (1902). In
+late autumn tiny wingless males and females are found in large numbers
+on the withered leaves. The sexes pair together, and the females lay
+their relatively large, smooth, hard-coated black eggs on the twigs;
+these resistant eggs carry the species safely over the winter. At
+springtide, when the leaves begin to sprout from the opening buds the
+aphid eggs are hatched, and the young insects after a series of moults,
+through which hardly any change of form is apparent, all grow into
+wingless 'stem-mothers' much larger than the egg-laying females of the
+autumn. The stem-mothers have the power, unusual among animals as a
+whole, but not very infrequent in the insects and their allies, of
+reproducing their kind without having paired[6] with a male. Eggs
+capable of parthenogenetic development, produced in large numbers in the
+ovaries of these females, give rise to young which, developing within
+the body of the mother, are born in an active state. Successive broods
+of these wingless virgin females (fig. 6 _a_) appear through the spring
+and summer months, and as the rate of their development is rapid, often
+the whole life-story is completed within a week. The aphid population
+increases very fast. Later a generation appears in which the thoracic
+segments of the nymphs are seen to bear wing-rudiments like those of the
+young cockroach, and a host of winged females (fig. 6_b_) are produced;
+these have the power of migrating to other plants. We understand that
+wings are not necessary to the earlier broods whose members have plenty
+of room and food on their native shoots, but that when the population
+becomes crowded, a winged brood capable of emigration is advantageous to
+the race.
+
+[6] Such virgin reproduction is termed 'parthenogenesis.'
+
+Many generations of virgin female aphids, some wingless, others winged
+when adult, succeed each other through the summer months. At the close
+of the year the latest brood of these bring forth young, which develop
+into males and egg-laying females; thus the yearly cycle is completed.
+Variations in points of detail may be noticed in different species of
+aphids. The autumn males and egg-laying females are, for example,
+frequently winged, and the same species may have constantly recurring
+generations of different forms adapted for different food-plants, or for
+different regions of the same food-plant. But taking a general view of
+the life-story of aphids for comparison with the life-story of other
+insects, three points are especially noteworthy. Virgin reproduction
+recurs regularly, parthenogenetic broods being succeeded by a single
+sexual brood. A winged parent brings forth young which remain always
+wingless, and wingless adults produce young which acquire wings. The
+wings are developed, as in the cockroach, from outward and visible
+wing-rudiments.
+
+[Illustration: Fig. 6. Apple Aphid (_Aphis pomi_), virgin females, _a_,
+wingless; _b_, winged. Magnified 20 times.]
+
+A family of Hemiptera, related to the Aphidae and equally obnoxious to
+the gardener, is that of the Coccidae or scale-insects. These furnish an
+excellent illustration of features noticeable in certain insect
+life-histories. In the first place, the newly-hatched young differs
+markedly from the parent in the details of its structure. A young coccid
+(fig. 7 _c_) is flattened oval in shape, has well-developed feelers
+(fig. 7 _d_) and legs, and runs actively about, usually on the leaves or
+bark of trees and shrubs, through which it pierces with its long jaws,
+so that it may suck sap from the soft tissues beneath. After a time it
+fixes itself by means of these jaws and the characteristic scale or
+protective covering, composed partly of a waxy secretion and partly of
+dried excrement, begins to grow over its body. The female loses legs and
+feelers, and never acquires wings, becoming little more than a sluggish
+egg-bag (fig. 7 _e_). The male on the other hand passes into a second
+larval stage in which there are no functional legs, but rudiments of
+legs and of wings are present on the epidermis beneath the cuticle, as
+shown by B.O. Schmidt for Aspidiotus (1885). The penultimate instar of
+this sex in which the wing-rudiments are visible externally lies
+passively beneath the scale, its behaviour resembling that of a
+butterfly pupa. The adult winged male (fig. 7 _a_) leads a short, but
+active life.
+
+[Illustration: Fig. 7. Mussel Scale-insect (_Mytilaspis pomorum_). _a_,
+male; _b_, foot of male; _c_, larva, ventral view; _d_, feeler of larva;
+_e_, female, ventral view. After Howard, _Yearbook U.S. Dept. Agric._
+1904. Magnified, _a, c, e_ x 20; _b, d_ x 120.]
+
+Another family allied to the Aphidae is that of the Cicads, hardly
+represented in our fauna but abundant in many of the warmer regions of
+the earth. Here also the young insect differs widely from its parent in
+form, living underground and being provided with strong fore-legs for
+digging in the soil. After a long subterranean existence, usually
+extending over several years, the insect attains the penultimate stage
+of its life-story, during which it rests passively within an earthen
+cell, awaiting the final moult, which will usher in its winged and
+perfect state.
+
+In the life-histories of cicads and coccids, then, there are some
+features which recall those of the caterpillar's transformation into the
+butterfly. The newly-hatched insect is externally so unlike its parent
+that it may be styled a larva. The penultimate instar is quiescent and
+does not feed. But while the caterpillar shows throughout its life no
+outward trace of wings, external wing-rudiments are evident in the young
+stages of the cicad. In the male coccid we find a late larval stage with
+hidden wing-rudiments, the importance of which, for comparison with the
+caterpillar, will be appreciated later.
+
+
+
+
+CHAPTER IV
+
+FROM WATER TO AIR
+
+
+Insects as a whole are preeminently creatures of the land and the air.
+This is shown not only by the possession of wings by a vast majority of
+the class, but by the mode of breathing to which reference has already
+been made (p. 2), a system of branching air-tubes carrying atmospheric
+air with its combustion-supporting oxygen to all the insect's tissues.
+The air gains access to these tubes through a number of paired air-holes
+or spiracles, arranged segmentally in series.
+
+It is of great interest to find that, nevertheless, a number of insects
+spend much of their time under water. This is true of not a few in the
+perfect winged state, as for example aquatic beetles and water-bugs
+('boatmen' and 'scorpions') which have some way of protecting their
+spiracles when submerged, and, possessing usually the power of flight,
+can pass on occasion from pond or stream to upper air. But it is
+advisable in connection with our present subject to dwell especially on
+some insects that remain continually under water till they are ready to
+undergo their final moult and attain the winged state, which they pass
+entirely in the air. The preparatory instars of such insects are
+aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
+caddis-flies, many beetles and two-winged flies, and a few moths thus
+divide their life-story between the water and the air. For the present
+we confine attention to the Stone-flies, the May-flies, and the
+Dragon-flies, three well-known orders of insects respectively called by
+systematists the Plecoptera, the Ephemeroptera and the Odonata.
+
+In the case of many insects that have aquatic larvae, the latter are
+provided with some arrangement for enabling them to reach atmospheric
+air through the surface-film of the water. But the larva of a stone-fly,
+a dragon-fly, or a may-fly is adapted more completely than these for
+aquatic life; it can, by means of gills of some kind, breathe the air
+dissolved in water.
+
+The aquatic young of a stone-fly does not differ sufficiently in form
+from its parent to warrant us in calling it a larva; the life-history is
+like that of a cockroach, all the instars however except the final
+one--the winged adult or _imago_--live in the water. The young of one of
+our large species, a Perla for example, has well-chitinised cuticle,
+broad head, powerful legs, long feelers and cerci like those of the
+imago; its wings arise from external rudiments, which are conspicuous in
+the later aquatic stages. But it lives completely submerged, usually
+clinging or walking beneath the stones that lie in the bed of a clear
+stream, and examination of the ventral aspect of the thorax reveals six
+pairs of tufted gills, by means of which it is able to breathe the air
+dissolved in the water wherein it lives. At the base of the tail-feelers
+or cerci also, there are little tufts of thread-like gills as J.A.
+Palmen (1877) has shown. An insect that is continually submerged and has
+no contact with the upper air cannot breathe through a series of paired
+spiracles, and during the aquatic life-period of the stone-fly these
+remain closed. Nevertheless, breathing is carried on by means of the
+ordinary system of branching air-tubes, the trunks of which are in
+connection with the tufted hollow gill-filaments, through whose delicate
+cuticle gaseous exchange can take place, though the method of this
+exchange is as yet very imperfectly understood. When the stone-fly nymph
+is fully grown, it comes out of the water and climbs to some convenient
+eminence. The cuticle splits open along the back, and the imago, clothed
+in its new cuticle, as yet soft and flexible, creeps out. The spiracles
+are now open, and the stone-fly breathes atmospheric air like other
+flying insects. But throughout its winged life, the stone-fly bears
+memorials of its aquatic past in the little withered vestiges of gills
+that can still be distinguished beneath the thorax.
+
+The adult dragon-fly (fig. 8 _d_) is specialised in such a way that it
+captures its prey--flies and other small insects--on the wing, swooping
+through the air like a hawk and feeding voraciously. The head is
+remarkable for its large globular compound eyes, its short bristle-like
+feelers, and its very strong mandibles which bite up the bodies of the
+victims. The thorax bears the two pairs of ample wings, firm and almost
+glassy in texture, and its segments are projected forward ventrally, so
+that all six legs, which are armed with rows of sharp, slender spines,
+can be held in front of the mouth, where they form an effective
+fly-trap. The abdomen is very long and usually narrow.
+
+A female dragon-fly after a remarkable mode of pairing, the details of
+which are beside our present subject, drops her eggs in the water, or
+lays them on water-weeds, perhaps cutting an incision where they can be
+the more safely lodged, or even goes down below the surface and deposits
+them in the mud at the bottom of a pond. From the eggs are hatched the
+aquatic larvae which differ in many respects from the imago. The
+dragon-fly larva has the same predaceous mode of life as its parent, but
+it is sluggish in habit, lurking for its prey at the bottom of the pond,
+among the mud or vegetation, which it resembles in colour. The thoracic
+segments have not the specialisation that they show in the imago; the
+abdomen is relatively shorter and broader. The larval head has, like
+that of the imago, short feelers, and the eyes are somewhat large,
+though far from attaining the size of the great globular eyes of the
+dragon-fly. But the third pair of jaws, forming the labium, are most
+remarkably modified into a 'mask,' the distal central portion (mentum)
+being hinged to the basal piece (sub-mentum) which is itself jointed
+below the head. The mentum carries at its extremity a pair of lobes with
+sharp fangs. Thus the mask can be folded under the head when the larva
+lurks in its hiding place, or be suddenly darted out so as to secure any
+unwary small insect that may pass close enough for capture. Dragon-fly
+larvae walk, and also swim by movements of the abdomen or by expelling a
+jet of water from the hind-gut. The walls of this terminal region of the
+intestine have areas lined with delicate cuticle and traversed by
+numerous air-tubes, so that gaseous exchange can take place between the
+air in the tubes and that dissolved in the water. The larvae of the
+larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
+mostly in this way. Those of the slender and delicate 'Demoiselles'
+(Agrionidae) are provided with three leaf-like gill-plates at the tail,
+between whose delicate surfaces numerous air-tubes ramify. These
+gill-plates are at times used for propulsion. Thus air supply is ensured
+during aquatic life. But occasionally, when the water in which the
+larva lives is foul and poor in oxygen, the tail is thrust out of the
+water so that air can be admitted directly into the intestinal chamber.
+The aquatic life of these insects lasts for more than a year, and F.
+Balfour-Browne (1909) has observed from ten to fourteen moults in
+Agrion. Outward wing-rudiments are early visible on the thoracic
+segments; when these have become conspicuous the insect, beginning in
+some respects to approach the adult condition, is often called a nymph.
+In an advanced dragon-fly nymph, H. Dewitz (1891) has shown that the
+thoracic spiracles are open, and, as the time for its final moult draws
+near, the insect may thrust the front part of its body out of the water,
+and breathe atmospheric air through these. Thus before the great change
+takes place the nymph has foretastes of the aerial mode of breathing
+which it will practise when the perfect stage shall have been attained.
+The emergence of the dragon-fly from its nymph-cuticle has been
+described by many naturalists from de Reaumur (1740) to L.C. Miall
+(1895) and O.H. Latter (1904). The nymph climbs out of the water by
+ascending some aquatic plant, and awaits the change so graphically
+sketched by Tennyson:
+
+    A hidden impulse rent the veil,
+    Of his old husk, from head to tail,
+    Came out clear plates of sapphire mail.
+
+'From head to tail,' for the nymph-cuticle splits lengthwise down the
+back, and the head and thorax of the imago are freed from it (fig. 8
+_a_), then the legs clasp the empty cuticle, and the abdomen is drawn
+out (fig. 8 _b, c_). After a short rest, the newly-emerged fly climbs
+yet higher up the water-weed, and remains for some hours with the
+abdomen bent concave dorsalwards (fig. 8 _d_), to allow space for the
+expansion and hardening of the wings. For some days after emergence the
+cuticle of the dragon-fly has a dull pale hue, as compared with the dark
+or brightly metallic aspect that characterises it when fully mature. The
+life of the imago endures but a short time compared with the long
+aquatic larval and nymphal stages. After some weeks, or at most a few
+months, the dragon-flies, having paired and laid their eggs, die before
+the approach of winter.
+
+[Illustration: Fig. 8 _a, b_. Dragon-fly (_Aeschna cyanea_). Two stages
+in emergence of fly from nymph-cuticle. From Latter's _Natural
+History_.]
+
+[Illustration: Fig. 8 _c_. Dragon-fly emerged, wings
+expanding. From Latter's _Natural History_.]
+
+[Illustration: Fig. 8 _d_. Dragon-fly (_Aeschna cyanea_) with
+expanded wings.]
+
+The life-story of a may-fly follows the same general course as that just
+described for the dragon-flies, but there are some suggestive
+differences. In the first place, we notice a wider divergence between
+the imago and the larva. An adult may-fly is one of the most delicate
+of insects; the head has elaborate compound eyes, but the feelers are
+very short, and the jaws are reduced to such tiny vestiges that the
+insect is unable to feed. Its aquatic larva is fairly robust, with a
+large head which is provided with well-developed jaws, as the larval and
+nymphal stages extend over one or two years, and the insects browse on
+water-weeds or devour creatures smaller and weaker than themselves. They
+breathe dissolved air by means of thread-like or plate-like gills
+traversed by branching air-tubes, somewhat resembling those of the
+demoiselle dragon-fly larva. But in the may-fly larva, there is a series
+of these gills (fig. 9_b_) arranged laterally in pairs on the abdominal
+segments, and C. Boerner (1909) has recently given reasons, from the
+position and muscular attachments of these organs, for believing that
+they show a true correspondence to (in technical phraseology are
+homologous with) the thoracic legs. One feature in which the larva often
+agrees with the imago is the possession on the terminal abdominal
+segment of a pair of long jointed cerci, and in many genera a median
+jointed tail-process (see fig. 9) is also present, in some cases both in
+the larva and the imago, in others in the larva during its later stages
+only. The prolonged larval life in may-flies often involves a large
+series of moults; Lubbock (1863) has enumerated twenty-one in the
+life-history of Chloeon. In the second year of aquatic life
+wing-rudiments (fig. 9 _a_) are visible, and the larva becomes a nymph.
+When the time for the winged condition approaches the nymphs leave the
+water in large swarms. The vivid accounts of these swarms given by
+Swammerdam (1675), de Reaumur (1742) and other old-time observers are
+available in summarised form for English readers in Miall's admirable
+book (1895). May-flies are eagerly sought as food by trout, and the rise
+of the fly on many lakes ushers in a welcome season to the angler.
+
+The nymph-cuticle opens and the winged insect emerges. But this is not
+the final instar; may-flies are exceptional among insects in undergoing
+yet another moult after they have acquired wings which they can use for
+flight. The instar that emerges from the nymph-cuticle is a sub-imago,
+dull in hue, with a curious immature aspect about it. A few hours later
+the final moult takes place, a very delicate cuticle being shed and
+revealing the true imago. Then follow the dancing flight over the calm
+waters, the mating and egg-laying, the rapid death. The whole winged
+existence prepared for by the long aquatic life may be over in a single
+evening; at most it lasts but for a few days.
+
+[Illustration: Fig. 9. Nymph of May-fly (_Chloeon dipterum_) showing on
+right side wing-rudiment (_a_), on left tracheal gills (_b_). Magnified
+4 times. [Feelers and legs are cut short.] From Miall and Denny after
+Vayssiere.]
+
+In the development of the may-flies, then, we notice not only a
+considerable divergence between larva and imago, both in habitat and
+structure; we see also what is to be observed often in more highly
+organised insects--a feeding stage prolonged through the years of larval
+and nymphal life, while the winged imago takes no food and devotes its
+energies through its short existence to the task of reproduction. Such
+division of the life-history into a long feeding, and a short breeding
+period has, as will be seen later, an important bearing on the question
+of insect transformation generally, and the dragon-flies and may-flies
+afford examples of two stages in its specialisation. The sub-imaginal
+instar of the may-fly furnishes also a noteworthy fact for comparison
+with other insect histories. In two points, however, the life-story of
+these flies with their aquatic larvae recalls that of the cockroach. All
+the larval and nymphal instars are active, and the wing-rudiments are
+outwardly visible long before the final moult.
+
+
+
+
+CHAPTER V
+
+TRANSFORMATIONS,--OUTWARD AND INWARD
+
+
+We are now in a position to study in some detail the transformation of
+those insects whose life-story corresponds more or less closely with
+that of the butterfly, sketched in the opening pages of this little
+book. In the case of some of the insects reviewed in the last three
+chapters, the may-flies and cicads for example, a marked difference
+between the larva and the imago has been noticed; in others, as the
+coccids, we find a resting instar before the winged condition is
+assumed, suggesting the pupal stage in the butterfly's life-story.
+
+The various insect orders whose members exhibit no marked divergence
+between larva and imago (the Orthoptera for example) are often said to
+undergo no transformation, to be 'Ametabola.' Those with life-stories
+such as the dragon-flies' are said to undergo partial transformation,
+and are termed 'Hemimetabola.' Moths, caddis-flies, beetles, two-winged
+flies, saw-flies, ants, wasps, bees, and the great majority of insects,
+having the same type of life-story as the butterfly, are said to undergo
+complete transformation and are classed as 'Metabola' or 'Holometabola.'
+Wherein lies the fundamental difference between these Holometabola on
+the one hand and the Hemimetabola and Ametabola on the other? It is not
+that the larva differs from the imago or that there is a passive stage
+in the life-history; these conditions are observable among insects with
+a 'partial' transformation as we have seen, though the resting instar
+that simulates the butterfly pupa is certainly exceptional. It has been
+pointed out by Sharp (1899) that the most important indication of the
+difference between the two modes of development is furnished by the
+position of the wing-rudiments. In all Ametabola and Hemimetabola these
+are visible externally long before the penultimate instar has been
+reached; in the Holometabola they are not seen until the pupal stage.
+
+Attention has already been drawn to the contrast in outward form between
+a butterfly and its caterpillar. As in the case of dragon-fly or
+may-fly, the larval period is essentially a time for feeding and growth,
+and during this period the larval cuticle is cast four or five, in some
+species even seven or eight times. After each moult some changes in
+detail may be observable, for example in the proportions of the
+body-segments or their outgrowths, in the colour or the closeness of the
+hairy or spiny armature. But in all main features the caterpillar
+retains throughout its life the characteristic form in which it left
+the egg. From the tiny, newly-hatched larva to the full-fed caterpillar,
+possibly several inches in length, there is all along the same crawling,
+somewhat worm-like body, destitute of any outward trace of wings. When
+however the last larval cuticle has split open lengthwise along the
+back, and has been worked off by vigorous wriggling motions of the
+insect, the pupa thus revealed shows the wing-rudiments conspicuous at
+the sides of the body, and lying neatly alongside these are to be seen
+the forms of feelers, legs, and maxillae of the imago prefigured in the
+cuticle of the pupa (fig. 1 _e_). The pupa thus resembles the imago much
+more closely than it resembles the larva; even in the proportions of the
+body a relative shortening is to be noticed, and the imago of any insect
+with complete transformation is reduced in length as compared with the
+full-fed larva. Now these wings and other structures characteristic of
+the imago, appear in the pupa which is revealed by the shedding of the
+last larval cuticle. From these facts we infer that the wing-rudiments
+must be present in the larva, hidden beneath the cuticle; and until the
+last larval instar, not beneath the cuticle only, but growing in
+such-wise that they are hidden by the epidermis. For if they were
+growing outwardly the new cuticle would be formed over them, so that
+they would be apparent after the next moult. But it is clear that only
+in the pupa, forming beneath the cuticle of the last larval instar, can
+they grow outwards.
+
+Anatomical study of the caterpillar at various stages verifies the
+conclusions just drawn from superficial observation. A hundred and fifty
+years ago P. Lyonet in his monumental work (1762) on the caterpillar of
+the Goat Moth (Cossus) detected, in the second and third thoracic
+segments, four little white masses buried in the fat-body, and, while
+doubtful as to their real meaning, he suggested that their number and
+position might well give rise to the suspicion that they were rudiments
+of the wings of the moth. But it was a century later that A. Weismann in
+his classical studies (1864) on the development of common flies, showed
+the presence in the maggot of definite rudiments of wings, and other
+organs of the adult--rudiments to which he gave the name of _imaginal
+discs_. We will recur later to these transformations of the Diptera. For
+the present, we pursue our survey of changes in the life-history of the
+Lepidoptera and can take to guide us the excellent researches of J.
+Gonin (1894).
+
+Careful study of the imaginal discs of the wings in a caterpillar (fig.
+10) made by examining microscopically sections cut through them, shows
+that the epidermis is pushed in to form a little pouch (_C, p_) and that
+into this grows the actual wing-rudiment. Consequently the whitish disk
+which seems to lie within the body-wall of the larva, is really a
+double fold of the epidermis, the outer fold forming the pouch, the
+inner the actual wing-bud. Into the cavity of the latter pass branches
+from the air-tube system. In its earliest stage, the wing-bud is simply
+an ingrowing mass of cells (fig. 10 _A_) which subsequently becomes an
+inpushed pouch (_B_). Until the last stage of larval life the wing-bud
+remains hidden in its pouch, and no cuticle is formed over it. When the
+pupal stage draws near the bud grows out of its sheath, and projecting
+from the general surface of the epidermis becomes covered with cuticle
+to be revealed, as we have seen, after the last larval moult, as the
+pupal wing. Thus all through the life of the humble, crawling
+caterpillar, 'it doth not yet appear what it shall be,' but there are
+being prepared, hidden and unseen, the wondrous organs of flight, which
+in due time will equip the insect for the glorious aerial existence that
+awaits it.
+
+[Illustration: Fig. 10. A, B, C, Sections through epidermis and cuticle,
+showing three stages in growth of the imaginal disc (_w_) of a wing in
+the caterpillar of a White Butterfly (_Pieris_). _ep_, epidermis; _cu_,
+cuticle; _t_, air-tube, whence branches pass into the developing wing.
+In C, _cu'_ represents the new cuticle forming beneath the old one, and
+(_p_) the pouch within which the wing-disc (_w_) lies. Highly magnified.
+After Gonin, _Bull. Soc. Vaud._ XXX.]
+
+As mentioned above, this hidden growth of the wing-rudiments, in
+butterflies, beetles, flies, bees, and the great majority of the winged
+insects, has been emphasised by Sharp (1899) as a character contrasting
+markedly with the outward and visible growth of the wing-rudiments in
+such insects as cockroaches, bugs, and dragon-flies. The divergence
+between the two modes of development is certainly very striking, and a
+conceivable method of transition from the one to the other is not easy
+to explain. Sharp has expressed the divergence by the terms
+_Endopterygota_, applied to all the orders of insects with hidden
+wing-rudiments (the 'Metabola' or 'Holometabola' of most
+classifications) and _Exopterygota_, including all those insects whose
+wing-rudiments are visible throughout growth ('Hemimetabola' and
+'Ametabola'). Those curious lowly insects, belonging to the two orders
+of the Collembola and Thysanura, none of whose members ever develop
+wings at all, form a third sub-class, the _Apterygota_ (see
+Classificatory Table, p. 122).
+
+Not the wings only, but other structures of the imago, varying in extent
+in different orders, are formed from the imaginal discs. For example, de
+Reaumur and G. Newport (1839) found that if the thoracic leg of a
+late-stage caterpillar were cut off, the corresponding leg of the
+resulting butterfly would still be developed, although in a truncated
+condition. Gonin has shown that in the Cabbage White butterfly (_Pieris
+brassicae_) the legs of the imago are represented, through the greater
+part of larval life, only by small groups of cells situated within the
+bases of the larval legs. After the third moult these imaginal discs
+grow rapidly and the proximal portion of each, destined to develop into
+the thigh and shin of the butterfly's leg, sinks into a depression at
+the side of the thorax, while the tip of the shin and the
+five-segmented foot project into the cavity of the larval leg. Hence we
+understand that the amputation of the latter by the old naturalists
+truncated only and did not destroy the imaginal limb. In the blow-fly
+maggot, Weismann, B.T. Lowne (1890) and J. Van Rees (1888) have shown
+that the imaginal discs of the legs (fig. 11--1, 2, 3) grow out from
+deep dermal inpushings. Simple at first, these outgrowths by partial
+splitting, become differentiated into thigh and shin.
+
+[Illustration: Fig. 11. Front region of Maggot of Blow-fly
+(_Calliphora_) showing diagrammatically the imaginal discs, which are
+shaded. _e_, eye; _f_, feeler; _W_, fore-wing; _w_, hind-wing; 1, 2, 3,
+legs. _H_ is the 'cephalic vesicle,' which becomes everted at the close
+of the metamorphosis, so as to bring the feelers and eyes to the front,
+the brain (_B_) moving forwards at the same time. After Van Rees, _Zool.
+Jahrb._ 1894, and Lowne's _Blow-fly_.]
+
+Similarly the feelers and jaws of the butterfly are developed from
+imaginal discs, and this fact explains how it comes to pass that they
+differ so widely from the corresponding structures in the caterpillar.
+The larval feelers (fig. 3 _At_) are short and stumpy, those of the
+butterfly long and many-jointed. The maxilla of the larva (fig. 3 _Mx_)
+consists of a base carrying two short jointed processes; in the
+butterfly a certain portion of the maxilla, the hood or galea, is
+modified into a long, flexible grooved process, capable of forming with
+its fellow the trunk through which the insect sucks its liquid food
+(fig. 2). Nothing but some such provision as that of the imaginal discs
+could render possible the wonderful replacement of the caterpillar's
+jaws, biting solid food, into those of the butterfly sipping nectar from
+flowers.
+
+A curious segmental displacement of the imaginal discs with regard to
+the larva is noticeable in some Diptera. In the larva of the
+harlequin-midge (Chironomus) as described by Miall and Hammond (1900)
+the brain is situated in the thorax, and the imaginal discs for the
+head, eyes, and feelers of the adult lie in close association with it,
+though they arise from inpushings of the larval head. These rudiments do
+not appear until the last larval stage has been reached. In the gnats
+Culex and Corethra, on the other hand, the imaginal discs for the
+head-appendages retain their normal position within the larval head, and
+appear in an early stage of larval life. Among the flies of the
+bluebottle group (Muscidae) the brain (fig. 11 _B_) is situated, as in
+Chironomus, in the thoracic region of the legless maggot, which is the
+larva of an insect of this family, and the imaginal discs for eyes and
+feelers (fig. 11 _e_, _f_) lie just in front of it. Here, the imaginal
+buds of the legs (fig. 11--1, 2, 3) and wings (fig. 11 _W_, _w_) are
+deeply inpushed, retaining their connection with the skin only by means
+of a thread of cells. As the larva is legless and headless its outer
+form is not affected by the discs and it is not surprising to learn that
+they appear early. It has indeed been suggested that the pharyngeal
+region of the larva, in connection with which the imaginal head-discs
+are developed, should be regarded, though it lies in the thorax, as an
+inpushed anterior section of the larval head. In any case this region is
+pushed out during the formation of the pupa within the final larval
+cuticle, so that the imaginal head with its contained brain, its
+compound eyes, and its complex feelers, takes its rightful place at the
+front end of the insect.
+
+The mention of the brain suggests a few brief remarks on the changes in
+the internal organs during insect transformation. There are no imaginal
+discs for the nervous system; the brain, nerve-cords and ganglia of the
+butterfly or bluebottle are the direct outcome of those of the
+caterpillar or maggot. More than seventy years ago, Newport (1839)
+traced the rapid but continuous changes, which, during the early pupal
+period, convert the elongate nerve-cord of the caterpillar with its
+relatively far-separated ganglia into the shortened, condensed
+nerve-cord of the Tortoise-shell butterfly (_Vanessa urticae_) with
+several of the ganglia coalesced. In many Diptera, on the other hand,
+the nervous system of the larva is more concentrated than that of the
+imago.
+
+The tubular heart also of a winged insect is the directly modified
+survival of the larval heart.
+
+Similarly the reproductive organs undergo a gradual, continuous
+development throughout an insect's life-story. Their rudiments appear in
+the embryo, often at a very early stage; they are recognisable in the
+larva, and the matured structures in the imago are the result of their
+slow process of growth, the details of which must be reckoned beyond the
+scope of this book. For a full summary of the subject the reader is
+referred to L.F. Henneguy's work (1904) containing references to much
+important modern literature, which cannot be mentioned here.
+
+On the other hand, the digestive system of insects that undergo a
+metamorphosis, passes through a profound crisis of dissolution and
+rebuilding. This is not surprising when we remember that there is often
+a great difference between larva and imago in the nature of the food.
+The digestive canal of a caterpillar runs a fairly straight course
+through the body and consists of a gullet, stomach (mid-gut),
+intestine, and rectum; it is adapted for the digestion of solid food. In
+the butterfly there is one outgrowth of the gullet in the head--a
+pharyngeal sac adapted for sucking liquids; and another outgrowth at the
+hinder end of the gullet (which is much longer than in the larva)--a
+crop or food-reservoir lying in the abdomen. The intestine of the
+butterfly also is longer than that of the larva, being coiled or
+twisted. Towards the end of the last larval stage, the cells of the
+inner coat (epithelium) lining the stomach begin to undergo
+degeneration, small replacing cells appearing between their bases and
+later giving rise to the more delicate epithelium that lines the mid-gut
+of the imago. The larval cells are shed into the cavity of the stomach
+and become completely broken down. J. Anglas (1902), describing these
+microscopic changes in the transformations of wasps and bees, has shown
+that the tiny replacing cells can be recognised in sections through the
+digestive canal of a very young larva; they may be regarded as
+representing imaginal buds of the adult gastric epithelium. In the
+transformations of two-winged flies of the bluebottle group, A.
+Kowalevsky (1887) has shown that these replacing cells are aggregated in
+little masses scattered at different points along the stomach and thus
+corresponding rather closely to the imaginal discs of the legs and
+wings.
+
+The gullet, crop, and gizzard of an insect, which lie in front of the
+stomach, are lined by cells derived from the outer skin (ectoderm) which
+is pushed in to form what is called the 'fore-gut.' Similarly the
+intestine and rectum, behind the stomach, are lined with ectodermal
+cells which arise from the inpushed 'hind-gut.' The larval fore- and
+hind-guts are broken down at the end of larval life and their lining is
+replaced by fresh tissue derived from two imaginal bands which surround
+the cavity of the digestive tube, one at the hinder end of the fore-gut,
+and the other at the front end of the hind-gut. The larval salivary
+glands in connection with the gullet are also broken down, and fresh
+glands are formed for the imago.
+
+A large part of the substance of an insect larva consists of muscular
+tissue, surrounding the digestive tube, and forming the great muscles
+that move the various parts of the body, and of fat, surrounding the
+organs and serving as a store of food-material. Very many of the
+muscle-fibres and the fat-cells also become disintegrated during the
+late larval and pupal stages, and the corresponding tissues of the adult
+are new formations derived from special groups of imaginal cells, though
+some muscles may persist from the larva to the adult. Similarly the
+complex air-tube or tracheal system of the larva is broken down and a
+fresh set of tubes is developed, adapted to the altered body-form of
+pupa and imago.
+
+The destruction of larval tissue and the development of replacing organs
+from special groups of cells, derived of course from the embryo, and
+carrying on the continuity of cell-lineage to the adult, are among the
+most remarkable facts connected with the life-story of insects. The
+process of tissue-destruction is known as 'histolysis'; the rebuilding
+process is called 'histogenesis.' Considerable difference of opinion has
+existed as to factors causing histolysis, and for a summary of the
+conflicting or complementary theories, the reader is referred to the
+work of L.F. Henneguy (1904, pp. 677-684). In the histolysis of the
+two-winged flies, wandering amoeboid cells--like the white corpuscles or
+leucocytes of vertebrate blood--have been observed destroying the larval
+tissues that need to be broken down, as they destroy invading
+micro-organisms in the body. But students of the internal changes that
+accompany transformation in insects of other orders have often been
+unable to observe such devouring activity of these 'phagocytes,' and
+attribute the dissolution of the larval tissues to internal chemical
+changes. The fact that in all insect transformation a part, and in many
+a large part, of the larval organs pass over to the pupa and imago,
+suggests that only those structures whose work is done are broken down
+through the action of internally formed destructive substances, and one
+function of the phagocytes is to act as scavengers by devouring what has
+become effete and useless.
+
+
+
+
+CHAPTER VI
+
+LARVAE AND THEIR ADAPTATIONS
+
+
+Among the insects that undergo a complete transformation, there is, as
+we have seen in the preceding chapter, an amount of inward change, of
+dissolution and rebuilding of tissues, that varies in its completeness
+in members of different orders. It is now advisable to consider the
+various outward forms assumed by the larvae of these insects, or rather
+by a few examples chosen from a vast array of well-nigh 'infinite
+variety.'
+
+In comparing the transformations of endopterygote insects of different
+orders, it is worthy of notice that in some cases all the members of an
+order have larvae remarkably constant in their main structural features,
+while in others there is great variety of larval form within the order.
+For example, the caterpillars of all Lepidoptera are fundamentally much
+alike, while the grubs of beetles of different families diverge widely
+from one another. A review of a selected series of beetle-larvae will
+therefore serve well to introduce this branch of the subject.
+
+[Illustration: Fig. 12. _a_, Carrion-beetle (_Silpha_) with its larva,
+_b_. Magnified, _a_ 3 times, and _b_ 4 times.]
+
+[Illustration: Fig. 13. Larva of a Ground-beetle (_Aepus_). Magnified
+6 times. After Westwood, _Modern Classification of Insects_.]
+
+Beetles are as a rule remarkable among insects for the firm consistency
+of their chitinous cuticle, the various pieces (_sclerites_) of which
+are fitted together with admirable precision. In some families of
+beetles the larva also is furnished with a complete chitinous armour,
+the sclerites, both dorsal and ventral, of the successive body-segments
+being hard and firm, while the relatively long legs possess well-defined
+segments and are often spiny. Such a larva is evidently far less unlike
+its parent beetle than a caterpillar is unlike a butterfly. Perhaps of
+all beetle larvae, the woodlouse-like grub (fig. 12 _b_) of a
+carrion-beetle (Silpha) or of a semi-aquatic dascillid such as Helodes
+shows the least amount of difference from the typical adult, on account
+of the conspicuous jointed feelers. The larval glow-worm, however, is of
+the same woodlouse-like aspect, and in this case, where the female never
+acquires wings, but becomes mature in a form which does not differ
+markedly from that of the larva, the exceptional resemblance is closer
+still. In all beetle-grubs the legs are simplified, there being only one
+segment (a combined shin and foot) below the knee-joint, whereas in the
+adult there is a shin followed by five, four, or at least three
+distinct tarsal segments. The foot of an adult beetle bears two claws
+at its tip, while the larval foot in the great majority of families has
+only one claw. In one section of the order, however, the Adephaga
+comprising the predaceous terrestrial and aquatic beetles, the larval
+foot has, like that of the adult, two claws. Some adephagous larvae,
+notably those of the large carnivorous water-beetles (Dyticus), often
+destructive to tadpoles and young fish, have completely armoured bodies
+as well as long jointed legs. More commonly, as with most of the
+well-known Ground-beetles (Carabidae), the cuticle is less consistently
+hard, firm sclerites segmentally arranged alternating with considerable
+tracts of cuticle which remain feebly chitinised and flexible. Most of
+the adephagous larvae (fig. 13) have a pair of stiff processes on the
+ninth abdominal segment, and the insect, from its general likeness to a
+bristle-tail of the genus Campodea, is often called a _campodeiform_
+larva (Brauer, 1869). From such as these, a series of forms can be
+traced among larvae of beetles, showing an increasing divergence from
+the imago. The well-known wireworms--grubs of the Click-beetles
+(Elateridae)--that eat the roots of farm crops, have well-armoured
+bodies, but their shape is elongate, cylindrical, worm-like; and their
+legs are relatively short, the build of the insect being adapted for
+rapid motion through the soil. The grubs of the Chafers (Scarabaeidae)
+are also root-eaters, but they are less active in their habits than the
+wireworms, and the cuticle of their somewhat stout bodies is, for the
+most part, pale and flexible; only the head and legs are hard and horny.
+Usually an evident correspondence can be traced between the outward form
+of any larva and its mode of life. For example, in the family of the
+Leaf-beetles (Chrysomelidae) some larvae feed openly on the foliage of
+trees or herbs, while others burrow into the plant tissues. The exposed
+larvae of the Willow-beetles (Phyllodecta, fig. 14) have their somewhat
+abbreviated body segments protected by numerous spine-bearing, firm
+tubercles. But the grub of the 'Turnip Fly' (Phyllotreta) which feeds
+between the upper and lower skins of a leaf, or of _Psylliodes
+chrysocephala_ (fig. 15), which burrows in stalks, has a pale, soft
+cuticle like that of a caterpillar.
+
+[Illustration: Fig. 14. (_a_) Willow-beetle (_Phyllodecta vulgatissima_)
+and its larva (_b_). Magnified 5 times. After Carpenter, _Econ. Proc. R.
+Dublin Soc_. vol. I.]
+
+[Illustration: Fig. 15. (_a_) Cabbage-beetle (_Psylliodes chrysocephala_)
+magnified 5 times, and its larva (_b_) magnified 12 times.]
+
+In the larvae of the little timber-beetles and their allies (Ptinidae),
+including the 'death-watches' whose tapping in old furniture is often
+heard, a marked shortening of the legs and reduction in the size of the
+head accompany the whitening and softening of the cuticle. This
+shortening of the legs is still more marked in the larvae of the
+Longhorn Beetles (Cerambycidae) burrowing in the wood of trees or felled
+trunks; here the legs are reduced to small vestiges.
+
+[Illustration: Fig. 16. _a_, Grain Weevil (_Calandra granaria_); _b_,
+larva; _c_, pupa. Magnified 7 times. After Chittenden, _Yearbook U.S.
+Dept. Agric._ 1894.]
+
+Finally in the large family of the Weevils (Curculionidae, fig. 16) and
+the Bark-beetles (Scolytidae), the grubs, eating underground root or
+stem structures, mining in leaves or seeds, or tunnelling beneath the
+bark of trees, have no legs at all, the place of these limbs being
+indicated only by tiny tubercles on the thoracic segments. Such larvae
+as these latter are examples of the type called _eruciform_ by A.S.
+Packard (1898) who as well as other writers has laid stress on the
+series of transitional steps from the campodeiform to the eruciform type
+afforded by the larvae of the Coleoptera.
+
+A fact of much importance in the transformations of beetles as pointed
+out by Brauer (1869) is that in a few families, the first larval instar
+is campodeiform, while the subsequent instars are eruciform. We may take
+as an example of such 'hypermetamorphosis' the life-story of the Oil or
+Blister-beetles (Meloidae) as first described by J.H. Fabre (1857), and
+later with more elaboration by H. Beauregard (1890). From the egg of one
+of these beetles is hatched a minute armoured larva, with long feelers,
+legs, and cerci, whose task is, for example, to seize hold of a bee in
+order that the latter may carry it, an uninvited guest, to her nest.
+Safely within the nest, the little 'triungulin' beetle-grub moults; the
+second instar has a soft cuticle and relatively shorter legs, which, as
+the larva, now living as a cuckoo-parasite, proceeds to gorge itself
+with honey, soon appear still further abbreviated. Later comes a stage
+during which legs are entirely wanting, the larva then resting and
+taking no food. The last larval instar again has short legs like the
+grub of the second period. In connection with this life-history we
+notice that the newly-hatched larva is not in the neighbourhood of its
+appropriate food. Hence the preliminary armoured and active instar is
+necessary in order to reach the feeding place; this journey
+accomplished, the eruciform condition is at once assumed.
+
+In all cases indeed we may say that the particular larval form is
+adapted to the special conditions of life. A few examples from other
+orders of endopterygote insects will illustrate this point. The
+campodeiform type is relatively unusual, but most of the Neuroptera have
+larvae of this kind, active, armoured creatures with long legs, though
+devoid of the tail-processes often associated with similar larvae among
+the Coleoptera. Such are the 'Ant-lions,' larvae of the exotic lacewing
+flies, which hunt small insects, digging a sandy pit for their unwary
+steps in the case of the best-known members of the group, some of which
+are found as far north as Paris. In our own islands the 'Aphis-lions,'
+larvae of Hemerobius and Chrysopa, prowl on plants infested with
+'green-fly' which they impale on their sharp grooved mandibles, sucking
+out the victims' juices, and then, in some cases, using the dried
+cuticle to furnish a clothing for their own bodies. Among these insects,
+while the mouth of the imago is of the normal mandibulate type adapted
+for eating solid food, the larval mouth is constricted and the slender
+mandibles are grooved for the transmission of liquid food.
+
+Turning to eruciform types of larva, we find the _caterpillar_ (fig. 1
+_b_, _c_, _d_) distinguished by its elongate, usually cylindrical body
+with feeble cuticle, short thoracic legs and a variable number of pairs
+of abdominal pro-legs, universal among the moths and butterflies forming
+the great order Lepidoptera, and usual among the saw-flies, which belong
+to the Hymenoptera. The vast majority of caterpillars feed on the leaves
+of plants and their long worm-like bodies with the series of paired
+pro-legs, are excellently adapted for their habit of clinging to twigs,
+and crawling along shoots or the edges of leaves as they go in search of
+food. Of great importance to a caterpillar is its power of spinning
+silk, consisting of fine threads solidified from the secretion of
+specially modified salivary glands whose ducts open in the insect's
+mouth at the tip of the tubular tongue which forms a spinneret.
+
+On the same bush caterpillars of moths and of saw-flies may often be
+seen feeding together. The lepidopterous caterpillar, in our countries
+at least, has never more than five pairs of pro-legs, situated on the
+third, fourth, fifth, sixth, and tenth abdominal segments; each of these
+pro-legs bears a number of minute hooklets, arranged in a circular or
+crescentic pattern, which assist the caterpillar in clinging to its
+food-plant. The saw-fly caterpillar, on the other hand, may have as many
+as eight pairs of pro-legs, the series beginning on the second abdominal
+segment; here, however, the pro-legs have no hooklets. Among the
+Lepidoptera, we notice a reduction in the number of pro-legs in the
+'looper' caterpillars of Geometrid moths. Here only two pairs are
+present, those on the sixth and tenth abdominal segments. Consequently,
+as the caterpillar can cling only by the thorax and by the hinder region
+of the abdomen, the middle region of the body is first straightened out
+and then bent into an arch-like form, as the insect makes its progress
+by alternate movements of stretching and 'looping.'
+
+[Illustration: Fig. 17. _c_, Ruby Tiger Moth (_Phragmatobia
+fuliginosa_); _a_, caterpillar; _b_, cocoon. After Lugger, _Insect
+Life_, vol. II.]
+
+Caterpillars, with their relatively soft bodies, feeding openly on the
+leaves of plants, are exposed to the attacks of many enemies, and the
+various ways in which they obtain protection are well worth studying. A
+clothing of hairs[7] or spines is often present, and it is interesting
+to find that many species of our native Tiger and Eggar Moths (Arctiadae
+and Lasiocampidae) which pass the winter in the larval stage, have
+caterpillars with an especially dense hairy covering (fig. 17).
+Experiments have shown that hairy and spiny insects are distasteful to
+birds and other creatures that prey readily on smooth-skinned species, a
+conclusion that might well have been expected. Certain smooth
+caterpillars however appear to be protected by producing some nauseous
+secretion, which renders them unpalatable. Many of these, as the
+familiar cream yellow and black larva of the Magpie Moth (_Abraxas
+grossulariata_), are very conspicuously adorned, and furnish examples of
+what is known as 'warning coloration,' on the supposition that the gaudy
+aspect of such insects serves as an advertisement that they are not fit
+to eat, and that birds and other possible devourers thus learn to leave
+them alone. On the other hand, smooth caterpillars which are readily
+eaten by birds are usually 'protectively' coloured, so as to resemble
+their surroundings and remain hidden except to careful seekers. Many
+such caterpillars are green, the upper surface, which is naturally
+exposed to the light, being darker than the lower which is in shadow.
+When the caterpillar is large, the green area is often broken up by pale
+lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or
+oblique, as on the great caterpillars of most Hawk Moths (Sphingidae).
+Such an arrangement tends to make the insect less easily seen than were
+it to display a continuous area of the same colour. The 'looper'
+caterpillars mentioned above afford remarkable examples of 'protective'
+resemblance, for many of them show a marvellous likeness to the twigs of
+their food-plant, tubercles on the insect's body resembling closely the
+little outgrowths of the plant's cortex. It has been shown by E.B.
+Poulton (1892) that many caterpillars are, in their early stages,
+directly responsive to their surroundings as regards colour. Usually
+green when hatched, they remain green if kept among leaves or young
+shoots of plants, while they turn red, brown, or blackish if placed
+among twigs of these respective hues. This effect appears to be due to a
+direct response of the subcutaneous tissue to the rays of light
+reflected from the surrounding objects. The sensitiveness dies away as
+the caterpillar grows older, since little or no change of hue in
+response to a change of environment could be induced after the
+penultimate moult.
+
+[7] The 'hairs' of an insect are not in the least comparable to the
+hairs of mammals, being in truth, modified portions of the cuticle,
+secreted by special cells.
+
+Among those families of the Lepidoptera which are usually regarded as
+low in the scale of organisation, caterpillars are very generally
+protected by the habit of feeding in some concealed situation. For
+example, the great larvae of the Goat Moth (Cossus) and the whitish
+caterpillars of the Clearwing Moths (Sesiidae) burrow through the wood
+of trees, eating the timber as they go. The little irritable
+caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the
+edges together with silk, and thus make for themselves a shelter; or
+they bore their way into seeds or fruits, like the larva of the Codling
+Moth that is the cause of 'worm-eaten' apples, too well-known to
+orchard-keepers. Very many small caterpillars mine between the two skins
+of a leaf, eating out the soft green tissue, and giving rise to a
+characteristic blister in form of a spreading patch or a narrow sinuous
+track through the leaf. The caterpillars of the Clothes-moths (Tineidae)
+make for themselves garments out of their own excrement, the particles
+fastened together by silk. In such curious cylindrical cases they wander
+over the wool or fur, feeding and indirectly supplying themselves with
+clothing at the same time.
+
+The case-forming habit of the Clothes-moth caterpillars leads us
+naturally to consider the similar habit adopted by their allies the
+Caddis-larvae which live in the waters of ponds and streams, for the
+Caddis-flies (Trichoptera) have much in common with the more primitive
+Lepidoptera. The caddis-larva is as a rule of the eruciform type, but
+with well-developed thoracic legs, and with hook-like tail-appendages;
+by means of the latter it anchors itself to the extremity of its curious
+'house.' It is of interest to note that in the earlier stages of some
+caddises lately described and figured by A.J. Siltala (1907), the legs
+are relatively very long, and the larva is quite campodeiform in aspect.
+Some of these caddis-grubs retain the campodeiform condition and do not
+shelter permanently in cases, as their relations do. Different genera of
+caddises differ in their mode of building. Some fasten together
+fragments of water-weeds and plant refuse, others take tiny particles of
+stone, of which they make firmly compacted walls, others again lay hold
+of water-snail shells, which may even contain live inhabitants, and bind
+these into a limy rampart behind which their bodies are in safe hiding.
+
+The silk with which the 'caddis-worms' fasten together the materials for
+their houses is produced from spinning-glands which like those of the
+Lepidoptera open into the mouth.
+
+The survey of the various types of beetle-larvae enumerated above (pp.
+50-56) concluded with a short description of the _legless grub_, which
+is the young form of a weevil or a bark-beetle. This is a larva in which
+the head alone has its cuticle firm and hard; the rest of the body is
+covered with a pale, flexible cuticle, so that the grub is often
+described as 'fleshy.' This type of larva is by no means confined to
+certain families of the beetles, it is frequently met with, in more or
+less modified form, in two other important orders of insects, the
+Hymenoptera and the Diptera. Among the Hymenoptera this is indeed the
+predominant larval type. We have just seen that a caterpillar is the
+usual form of larva among the saw-flies, but in all other families of
+the Hymenoptera we find the legless grub. A grub of this order may
+usually be distinguished from the larva of a weevil or other beetle, by
+its relatively smaller head and smoother, less wrinkled cuticle; it
+strikes the observer as a feebler, more helpless creature than a
+beetle-grub. And it is of interest to note that this somewhat degraded
+type of larva is remarkably constant through a great series of
+families--gall-flies, ichneumon-flies, wasps, bees (fig. 18), ants--that
+vary widely in the details of their structure and in their habits and
+mode of life. Almost without exception, however, they make in some way
+abundant provision for their young. The feeble, helpless, larva is in
+every case well sheltered and well fed; it has not to make its own way
+in the world, as the active armoured larva of a ground-beetle or the
+caterpillar of a butterfly is obliged to do.
+
+[Illustration: Fig. 18. Young Larva (_FL_), Full-grown Larva (_SL_) and
+Pupa (_N_) of Hive-bee (_Apis mellifica_). _co_, cocoon; _sp_,
+spiracles; _ce_, eye; _an_, feeler; _m_, mandible; _l_, labium.
+Magnified 4 times. After Cheshire, _Bees_.]
+
+Among those saw-flies whose larvae feed throughout life in a concealed
+situation, we find an interesting transition between the caterpillar
+and the legless grub. For example, the giant saw-flies (so called
+'Wood-wasps') have larvae that burrow in timber, and these larvae
+possess relatively large heads, somewhat flattened bodies with pointed
+tail-end, and very greatly reduced legs. The feeble legless grub,
+characteristic of the remaining families of the Hymenoptera, is provided
+for in a well-nigh endless variety of ways. The female imago among these
+insects is furnished with an elaborate and beautifully formed
+ovipositor, and the act of egg-laying is usually in itself a provision
+for the offspring. Gall-flies pierce plant-tissues within which their
+grubs find shelter and food, the plant responding to the irritation due
+to the presence of the larva by forming a characteristic growth, the
+_gall_, pathological but often regular and shapely, in whose hollow
+chamber the grub lives and eats. Ichneumon-flies and their allies pierce
+the skin of caterpillars and other insect-larvae, laying their eggs
+within the victims' bodies, which their grubs proceed to devour
+internally. Some very small members of these families are content to lay
+their eggs within the eggs of larger insects, thus obtaining rich
+food-supply and effective protection for their tiny larvae. In
+Platygaster and other genera of the family Proctotrypidae, M. Ganin
+(1869) showed the occurrence of hypermetamorphosis somewhat like that
+already described as occurring among the Oil-beetles (Meloidae). The
+larva of Platygaster is at first rather like a small Copepod crustacean,
+with prominent spiny tail-processes; after a moult this form changes
+into the legless grub characteristic of the Hymenoptera, among which
+larvae even approaching the campodeiform type are very exceptional. The
+species of Platygaster pass their larval stages within the larvae of
+gall-midges.
+
+Wasps, bees and ants, have the ovipositor of the female modified into a
+sting, which is often used for the purpose of providing food for the
+helpless grubs. Thus the digging wasps (Sphegidae and Pompilidae) hunt
+for caterpillars, spiders, and other creatures which they can paralyse
+with their stings, and bury them alongside their eggs to furnish a
+food-supply for the newly-hatched young. The social wasps and many ants
+sting and kill flies and other insects, which they break up so as to
+feed their grubs within the nest. It is well known that the labour of
+tending the larvae in these insect societies is performed for the most
+part not by the mother ('Queen') but by the modified infertile females
+or 'workers.' Other ants and the bees feed their grubs (fig. 18), also
+sheltered in well-constructed nests, on honey elaborated from nectar
+within their own digestive canals. In all cases we see that the
+helplessness of the grub is associated with some kind of parental care.
+
+[Illustration: Fig. 19. Larva of Gall-midge (_Contarinia nasturtii_),
+ventral view showing anchor process (_a_), and spiracles projecting at
+sides. Magnified 30 times. From Carpenter, _Journ. Econ. Biol_, vol.
+VI.]
+
+From the Hymenoptera we may pass on to the Diptera or Two-winged Flies,
+an order of which the vast number of species and in many cases the
+myriads of individuals force themselves on the observer's notice. F.
+Brauer (1863) divided the Diptera into two sub-orders[8]; of the first
+of these a Crane-fly or 'Daddy-long-legs' may be taken as typical, of
+the second an ordinary House-fly or Bluebottle. All the larvae of the
+Diptera are legless, those of the Crane-fly group have well-developed
+hard heads, with biting mandibles, but in the House-fly section the
+larva is of the degraded _vermiculiform_ type known as the _maggot_,
+not only legless, but without a definite head, the front end of the
+creature usually tapering to the mouth, where there are a pair of strong
+hooks, used for tearing up the food. A few examples of each of these
+types must suffice in the present brief survey. A few pages back (p. 66)
+reference was made to the production of galls on various plants, through
+the activity of larvae of the hymenopterous family Cynipidae. Many
+plant-galls are due, however, to the presence of grubs of tiny dipterous
+insects, the Cecidomyidae or Gall-midges. A cecid grub (fig. 19) has an
+elongate body with flexible, wrinkled cuticle, tapering somewhat at the
+two ends. The head, if rather narrow, is distinct, and beneath the
+prothorax is a characteristic sclerite known as the 'anchor process' or
+'breast bone.' Along either side of the body is a series of paired
+spiracles, each usually situated at the tip of a little tubular
+outgrowth of the cuticle; the hindmost spiracles are often larger than
+the others. These little grubs live in family communities, their
+presence leading to some deformation of the plant that serves to shelter
+them. A shrivelled fruit or an arrested and swollen shoot, such as may
+be due respectively to the Pear-midge (_Diplosis pyrivora_) or the
+Osier-midge (_Rhabdophaga heterobia_), is a frequent result of the
+irritation set up by these little grubs. In a larva of the crane-fly
+family (Tipulidae, fig. 20) living underground and eating plant-roots,
+like the well-known 'leather-jacket' grubs of the large
+'Daddy-long-legs' (Tipula) or burrowing into a rotting turnip or swollen
+fungus, like the more slender grub of a 'Winter Gnat' (Trichocera), the
+student notices a somewhat tough cuticle, a relatively small but
+distinct head, and frequently prominent finger-like processes on the
+tail-segment. Further examination shows a striking modification in the
+arrangement of the spiracles. Instead of a paired series on most of the
+body-segments, as in caterpillars and the vast majority of insects
+whether larval or adult, there are two large spiracles surrounded by the
+prominent tail-processes, and a pair of very small ones on the
+prothorax, the latter possibly closed up and useless. This restriction
+of the breathing-holes to a front and hind pair (amphipneustic
+condition) or to a hind pair only (metapneustic type) is highly
+characteristic of the larvae of Two-winged flies.
+
+[8] Known as the Orthorrhapha and the Cyclorrhapha; these terms are
+derived from the manner in which the larval or pupal cuticle splits, as
+will be explained in the next chapter (p. 88).
+
+[Illustration: Fig. 20. Crane-fly (_Tipula oleracea_), _a_, female; _b_,
+larva ('leather-jacket' grub). Magnified twice.]
+
+[Illustration: Fig. 21. Maggot of House-fly (_Musca domestica_), _a_,
+side-view, magnified 5 times; _b_, prothoracic spiracle; _c_, feeler;
+_d_, hind-region with posterior spiracles; _e_, _f_, head-region with
+mouth-hooks; _g_, head-region of young maggot; _h_, eggs. All magnified.
+After Howard, _Entom. Bull._ 4, _U.S. Dept. Agric._]
+
+Turning now to the _maggot_, characteristic of the House-fly section
+(fig. 21) of the Diptera, we see the greatest contrast between the larva
+and the imago that can be found throughout the whole class of the
+insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer
+time on exposed meat, not unnaturally arouse feelings of disgust, yet
+they are the prelude to one of the most marvellous of all insect
+life-stories. The fly--with its large globular head, bearing the
+extensive compound eyes, the highly modified feelers with their
+exquisitely feathered slender sensory tips, and the complex suctorial
+jaws; with its compact thorax bearing the glassy fore-wings alone used
+for flight, though the hind-wings modified into tiny drumstick-like
+'halters' are the organs of a fine equilibrating sense--is perhaps the
+most specialised, structurally the 'highest' of all insects. Yet in a
+week or two this swift, alert, winged creature is developed from the
+degraded maggot, white, legless, headless, that buries itself in putrid
+flesh, 'feeding on corruption.'
+
+The broad end of the maggot is the tail, while the narrow extremity
+marks the position of the mouth. Above this are a pair of very short
+feelers (fig. 21 _c_), while from the aperture project the tips of the
+mouth-hooks (fig. 21 _e_, _f_), formidable, black, claw-like structures,
+articulated to the strong pharyngeal sclerites and moved by powerful
+muscles, tearing up the fibres of the flesh. On either side of the
+prothorax is an anterior spiracle, a curious branching or fan-like
+outgrowth (fig. 21 _b_), with a variable number of tiny openings which
+are probably of little use for the admission of air to the tubes. In
+many maggots the mouth-hooks and the front spiracles become more and
+more complex in form in the successive instars. The cuticle, white and
+smooth to the unaided eye, is seen on microscopic study to be set with
+rows of tiny spines which assist the maggot's movements through its
+food-mass. At the tail-end the large hind spiracles are conspicuous on a
+flattened dorsal area of the ninth abdominal segment; each shows a hard
+brown plate, traversed by three slits. And as we watch this curious
+degraded larva thrusting its narrow head-end into the depths of its
+ofttimes loathsome food-supply, we understand the advantage of access to
+the air-tube system being mainly confined to the hinder end of the body.
+
+Maggots, differing from that of the Bluebottle only in minor details,
+are the larval forms of a vast multitude of allied species and display
+great variation in the nature of their food. Most, however, hide their
+soft defenceless bodies in some substance which affords shelter as well
+as food. The Bluebottle maggot burrows into flesh, that of the House-fly
+into horse-dung or vegetable refuse. The maggot of the Cabbage-fly eats
+its way into the roots of cruciferous plants, that of the Mangel-fly
+works out a broad blister between the two skins of a leaf, into which
+the newly-hatched larva crawls directly from the egg. A large number of
+species, forming an entire subfamily (the Tachininae) have larvae that
+feed as parasites within the bodies of other insects.
+
+The habit of parasitism by maggots in back-boned animals has led to some
+remarkable modifications of the larva and to curious adventures in the
+course of the life-story. The Bot-fly of the Horse (_Gastrophilus equi_)
+and the Warble-fly of the Ox (_Hypoderma bovis_, fig. 22) lay eggs
+attached to the hairs of grazing animals, which, at least in the case of
+Gastrophilus, lick the newly-hatched larvae into their mouths. The
+'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach;
+often a whole family attach themselves by their mouth-hooks to a small
+patch of the mucous coat of that organ. The maggot is relatively short
+and stout, with rows of strong spicules surrounding the segments, and
+with spiracles capable of withdrawal through a cup-like inpushing of the
+tail-region of the body, so that the parasite is preserved from drowning
+when the host drinks water. The young maggot of Hypoderma (fig. 22 _e_)
+is elongate and slender, spends its first two stages burrowing in the
+gullet wall and then wandering through the dorsal tissues of its host;
+ultimately it arrives beneath the skin of the back and assumes for its
+third and fourth instars a broad barrel-like form (fig. 22 _b_). The
+supply of free oxygen within the ox's tissues being now insufficient,
+the warble-maggot bores a circular hole through the skin and rests with
+the tail spiracles directed upwards towards the outer air. When fully
+grown the maggot works its way through the hole in the host's skin, and
+falling to the ground pupates in some sheltered spot, the life cycle
+occupying about a year. Similarly the Horse-bot escapes from the host's
+intestine with the excrement, and pupates on the ground.
+
+A curious modification of the maggot is noticeable in the larva of the
+Hover-flies (Syrphus). These, unlike most of their allies, live exposed
+on the foliage of plants, where they feed by preying on aphids.
+
+[Illustration: Fig. 22. Ox Warble-fly (_Hypoderma bovis_), _a_, female;
+_b_, full-grown maggot from back of ox, dorsal view; _c_, egg; _d_,
+empty puparium, ventral view; _e_, young maggot from gullet, ventral
+view. Magnified (lines show natural size). _a-d_, after Theobald, _2nd
+Report Econ. Zool._ (_Brit. Mus._).]
+
+In agreement with this manner of life, the cuticle is roughly
+granulated, often greenish or reddish in hue, and the maggot, despite
+its want of definite head and sense organs, moves actively and
+purposefully about, often rearing up on its broad tail-end with an aphid
+victim impaled on its mouth-hooks.
+
+In a previous chapter reference was made to the exopterygote insects,
+stone-flies, dragon-flies, and may-flies, whose preparatory stages live
+in the water. Among the endopterygote orders many Neuroptera and
+Coleoptera, all Trichoptera, a very few Lepidoptera and many Diptera,
+have aquatic larvae. One or two examples of the adaptations of dipteran
+larvae to life in the water may well bring the present chapter to a
+close. Many members of the hover-fly family (Syrphidae) have maggots
+with the tail-spiracles situated at the end of a prominent tubular
+process. Among the best-known of syrphid flies are the drone-flies
+(Eristalis), often seen hovering over flowers, and presenting a curious
+likeness to hairy bees. The larva of Eristalis is one of the most
+remarkable in the whole order, the 'Rat-tailed maggot' found in the
+stagnant water of ditches and pools. It has a cylindrical body with the
+hinder end drawn out into a long telescopic tube, a more slender
+terminal section being capable of withdrawal into, or protrusion from, a
+thicker basal portion. At the extremity of the slender tube is a crown
+of sharp processes, forming a stellate guard to the spiracles. These
+processes can pierce the surface-film of the water, and place the
+tracheal system of the maggot in touch with the pure upper air; while
+its mouth may be far down, feeding among the foul refuse of the ditch,
+it can still reach out to the medium in which the end of its life-story
+must be wrought out.
+
+Reverting to the first great division of the Diptera, we find varied
+adaptations to aquatic life among many grubs that possess a definite
+head. The larva of a Gnat (Culex[9]) has projecting from the hind region
+of the abdomen a long tubular outgrowth, at the end of which are the
+spiracles, guarded by three pointed flaps forming a valve. When closed
+these pierce the surface-film of the water in which the larva lives;
+when opened a little cup-like depression is formed in the surface-film,
+from which the larva hangs. Or having accumulated a supply of air, it
+can disengage itself from the surface-film and dive through the water,
+its tracheal system safely closed. Another mode of breathing is found in
+the 'Blood-worms' and allied larvae of the Harlequin-midges
+(Chironomidae) whose transformations are described in detail by Miall
+and Hammond (1900). These larvae have two pairs of cylindrical,
+spine-bearing pro-legs--one on the prothorax and the other on the
+hindmost abdominal segment; the latter structures serve to fix the
+larva in the muddy tube which it inhabits at the bottom of its native
+pond. The penultimate abdominal segment has four long hollow outgrowths,
+which contain blood, and have the function of gills, while the hindmost
+segment has four shorter outgrowths of the same nature. Enabled thus to
+breathe dissolved air, the Chironomus larva needs not, like the Culex or
+the Eristalis, to find contact with the atmosphere beyond the
+surface-film.
+
+[9] See _Frontispiece_, A.
+
+Most remarkable, in many respects, of all aquatic larvae are the grubs
+of the Sand-midges (Simulium). These live entirely submerged and, having
+no special gills, carry out an exchange of gases through the general
+surface of the cuticle between the dissolved air in the water and the
+cavities of the air-tube system. The body is shaped like a flask swollen
+slightly at the hinder end and possesses a median pro-leg just behind
+the head, also another at the tail, which serves to attach the larva to
+a stone or to the leaf of an aquatic plant. The head has, in addition to
+feelers and jaws, a pair of processes with wonderful fringes which by
+their motion set up currents in the water, and bring food particles
+within reach of the mouth. A number of the larvae usually live in a
+community. Their power of spinning silken threads by which they can work
+their way back when accidentally dislodged from their resting-place, has
+been vividly described by Miall (1895).
+
+Examples might be multiplied, but enough have been given to enforce the
+conclusion that the forms of insect-larvae are wondrously varied, and
+that frequently, within the limits of the same order or even family,
+modifications of type may be found which are suited to various modes of
+life adopted by different insects. A survey of the multitudes of insect
+larvae--grubs, caterpillars, maggots--living on land, on plants,
+underground, in the water; feeding on leaves, in stems, on roots, on
+carrion, on refuse; by hunting or by lurking after prey; as parasites or
+as scavengers, brings home to us most strongly the conclusion that each
+larva is fitted to some little niche in the vast temple of life, each is
+specially adapted to its part in the great drama of being.
+
+
+
+
+CHAPTER VII
+
+PUPAE AND THEIR MODIFICATIONS
+
+
+The pupal stage is characteristic of the life-story of those insects
+whose larvae have wing-rudiments in the form of inpushed imaginal discs,
+and in all these insects there is, as we have seen, considerable
+divergence in form between larva and imago. In the pupa the wings and
+other characteristically adult structures are, for the first time,
+visible outwardly; it is the instar which marks the great crisis in
+transformation. The pupa rests, as a rule, in a quiescent condition, and
+during the early period of this stage the needful internal changes, the
+breaking down of many larval tissues, and their replacement by imaginal
+organs, go on. Both outwardly and inwardly therefore, the insect
+undergoes, at the pupal stage, a reconstruction necessitated by the
+differences in form and often in habit, between the larva and the winged
+adult; and the greater these differences, the more profound must be the
+changes that mark the pupal stage.
+
+From the prominence of imaginal structures in the pupa, it is at once
+seen that the pupa of any insect must resemble the adult more nearly
+than it resembles the larva. But in different groups of insects we find
+different degrees of likeness between pupa and imago. In a beetle pupa
+(see fig. 16 _c_), the appendages--feelers, jaws, legs, wings--stand out
+from the body as do those of the perfect insect. This type is called a
+_free_ pupa. The pupal cuticle has to be shed for the emergence of the
+imago, but the pupa is already a somewhat reduced model of the final
+instar, with abbreviated wings and doubled-up legs. A free pupa is
+characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera
+and many Diptera. In some cases the pupa requires to be specially
+adapted for a peculiar mode of life; for example, a special arrangement
+of breathing organs may be necessary for life under water, and there
+must needs be temporary pupal structures, not represented in the imago.
+
+On the other hand, in the pupae of most Lepidoptera and of some Diptera,
+there is more or less coalescence between the cuticle of the appendages
+and the cuticle of the body generally, so that the appendages do not
+stand out, but being, as it were, glued down to the body, are somewhat
+masked (see fig. 1 _e_ and fig. 23). Consequently the _obtect_ pupa, as
+this type is called, does not resemble its imago as fully as a free pupa
+does. The outline of the wings for example in a butterfly's pupa can in
+some cases be traced only with difficulty. T.A. Chapman has shown (1893)
+that the completely obtect pupa characterises the more highly developed
+families of Lepidoptera, while in the more primitive families the pupa
+is incompletely obtect. If the pupa of a butterfly or moth be lifted and
+held in the hand, a bending or wriggling motion of the abdomen can be
+observed. In the incompletely obtect pupa, this motion is evident in a
+greater number of segments than in the completely obtect, the number
+concerned varying from five to two in different families. In the
+nymphalid butterflies, the pupa is often called a 'chrysalis' on
+account of the golden hue displayed by the cuticle, and the term
+'chrysalis' is sometimes bestowed indiscriminately on any kind of pupa.
+It has been shown by Poulton (1892) and others, that the colour of a
+butterfly pupa is to some extent affected by the surroundings of the
+caterpillar just before its last moult.
+
+Reference has been made (p. 58) to the power of spinning silk possessed
+by many larvae; often the principal use of this silk is to form some
+protection for the pupa, the larva before its last moult constructing a
+_cocoon_ within which the pupa may rest safely. Many larvae bury
+themselves in the earth, and the pupa lies in an earthen chamber, the
+lining particles of soil fastened together by fine silken threads.
+Larvae that feed in wood, like the caterpillar of the Goat-moth (Cossus)
+make a cocoon of splinters spun together, while hairy caterpillars, such
+as those of the Tiger-moths, work some of their hairs in with the silk
+to make a firm cocoon (fig. 17 _b_). On the other hand, those
+caterpillars known as 'silkworms' make a dense cocoon of pure silk,
+consisting of two layers, the outer of coarse and the inner of fine
+threads. Silken cocoons very similar in appearance are spun by the
+larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed
+of a few interlacing threads, as for example the conspicuous black and
+yellow banded pupa of the Magpie-moth (_Abraxas grossulariata_) and the
+pupae of various leaf-beetles. Others again spin together the edges of
+leaves with connecting silken threads. The grubs of bees and wasps which
+are reared in the comb-chambers of their nests seal up the opening of
+the chamber with a lid, partly silk (fig. 18 _co_) and partly excretion,
+when ready to pass into the pupal state. An additional external
+'capping' may be also supplied by the workers.
+
+The pupae of butterflies are especially interesting, as illustrating the
+extreme reduction of the silken cocoon. The pupa of a 'swallowtail'
+(Papilionid) or a 'white' (Pierid) butterfly (fig. 23) may be found
+attached to a twig of its food-plant or to a wall, in an upright
+position, its tail fastened to a pad of silk and a slender silken girdle
+encircling its thorax. The pupa of a 'Tortoiseshell' or 'Admiral'
+(Nymphalid) butterfly hangs head downwards from a twig, supported only
+by the tail-pad of silk, which, useless as a shelter, serves only for
+attachment. The pupa is fastened to this pad by a spiny hook or process,
+the _cremaster_ (fig. 23 _cr_), on the last abdominal segment. The
+cremaster is a characteristic structure in the pupa of a moth or
+butterfly. C.V. Riley (1880) and W. Hatchett-Jackson (1890) have shown
+that it corresponds with a spiny area, the suranal plate, which lies
+above the opening of the caterpillar's intestine. The means by which the
+suspended pupa of a nymphalid butterfly attaches its cremaster to the
+silken pad which the larva has spun in preparation for pupation, is
+worthy of brief attention. The caterpillar, hanging head downwards, is
+attached to the silken pad by its hindmost pair of pro-legs or claspers
+and by the suranal plate, and the cuticle is slowly worked off from
+before backwards, so as to expose the pupa. Were the process of moulting
+to be simply completed while the insect hangs by the claspers, the pupa
+would of course fall to the ground. But there is enough adhesion between
+the pupal and larval cuticles at the hinder end of the body, especially
+by means of the everted lining of the hind-gut, for the pupa to be
+supported while it jerks its cremaster out of the larval cuticle and
+works it into the meshes of the silken pad. The moult is thus completed
+and the pupa hangs securely all the time. In the numerous cases where
+the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa
+to the surrounding silk. Chapman (1893) has drawn attention to the fact
+that among the more highly organised moths the pupa remains in the
+cocoon, the emergence being entirely left to the imago, while the pupae
+of the more primitive moths work their way partly out of the cocoon
+before the final moult begins. In the latter case, the cremaster is
+anchored by a strand of silk which allows a certain degree of emergence,
+and the pupa has rows of spines on its abdominal segments, of which a
+greater number retain the power of mutual motion than in those pupae
+which do not come out of their cocoons.
+
+[Illustration: Fig. 23. Pupa of White Butterfly (_Pieris_), side view;
+_f_, feeler; _w_, wing; _sp_, spiracle; _p_, anal pro-leg; _cr_,
+cremaster. Magnified 8 times. In part after Hatchett-Jackson, _Trans.
+Linn. Soc._ 1900, and Tutt's _British Butterflies_.]
+
+While the pupa on the whole resembles the imago that is to emerge from
+it, there are not a few cases in which a special structure necessary for
+some contingency in pupal life is retained or adopted in this stage. A
+butterfly pupa, like the imago, has no mandibles, but in the case of the
+Caddis-flies (Trichoptera) and two families of small moths, the most
+primitive of all Lepidoptera, the pupa, like the larva, has
+well-developed mandibles. These enable the caddis pupa to bite its way
+out of the shortened larval case in which it has pupated, and then to
+swim upwards through the water ready for the caddis-fly's emergence into
+the air. Pupae that are submerged require special breathing-organs. In
+the previous chapter (p. 77) mention was made of the gnat's aquatic
+larva with its tail-spiracles adapted for procuring atmospheric air
+through the surface-film. The pupa of the gnat[10] also has 'respiratory
+trumpets' serving the same purpose, but these are a pair of processes on
+the prothorax, so that the pupa, which is fairly active, hangs from the
+surface-film with its abdomen pointing downwards through the water. This
+change of position is correlated with the necessity for the imago to
+emerge into the air; were the pupa to hang head downwards as the larva
+does, the gnat would perforce have to dive into the water. With the
+beautifully adapted transfer of the functional spiracles, their position
+is appropriately arranged for the gnat's emergence at the surface, and
+the empty pupal cuticle floats serving the insect as a raft. On this it
+rests securely and the crumpled wings have opportunity to expand and
+harden before the insect takes to flight.
+
+[10] See _Frontispiece_, B.
+
+The aquatic pupae of other Diptera, many species of the midges
+Chironomus and Simulium for example, breathe dissolved air by means of
+tufts of thread-like gills, which arise on either side of the prothorax.
+The pupae of Simulium rest in their curious little cup-like dwellings,
+attached to submerged stones or plants. The Chironomus pupa is usually
+found in an elongate gelatinous case adhering to a stone. From this case
+the pupa rises to the surface of the water, that the midge may emerge
+into the air. Miall and Hammond (1900) describe the arrangement by
+which, when the pupal stage ends, and these gills are no longer
+required, their connection with the air-tube system is severed 'without
+undue violence.' The walls of the fine air-tubes that pass into the
+gills are specially strengthened, but just below the pupal cuticle these
+walls are exceedingly thin and delicate. Thus when the pupal cuticle is
+cast, they are readily broken there, and the cuticle of the midge
+forming beneath has a spiracular opening into the main air-trunk, ready
+for use during the insect's aerial life.
+
+Among those Diptera whose larva is the headless maggot a most
+remarkable arrangement for protecting the pupa is to be found. The last
+larval cuticle, instead of being as usual worked off and cast, after
+separation from the underlying structures, becomes hard and firm,
+forming a protective case (_puparium_) within which by the processes of
+histolysis and histogenesis already described the organs of the pupa and
+imago are built up. This puparium (fig. 22 _d_) is usually dark in
+colour, often brown and barrel-shaped, and a subcircular lid splits off
+from it at the head-end to allow the emergence of the fly[11]. While the
+maggot breathes by its tail-spiracles, the functional spiracles of the
+puparium (connected with the tracheal system of the enclosed pupa) are
+far forward, and these may be situated at the tips of long sometimes
+branching processes, which recall the thoracic gills of the aquatic
+pupae mentioned a few pages above. Adaptations, various and beautiful,
+to special modes of life, are thus seen to characterise pupae as well as
+larvae.
+
+[11] The presence of this sub-circular lid characterises Brauer's
+suborder Cyclorrhapha. Those Diptera in which the pupal cuticle splits
+in the normal, longitudinal manner are included in the Orthorrhapha (see
+p. 67).
+
+
+
+
+CHAPTER VIII
+
+THE LIFE-STORY AND THE SEASONS
+
+
+A number of interesting questions are associated with the seasonal cycle
+of an insect's life-history. In a previous chapter (IV. pp. 30, 34)
+reference has been made to the contrast between the long aquatic life of
+the larval dragon-fly or may-fly, extending over several years, and the
+short aerial existence of the winged adult restricted in the case of the
+may-flies to a few hours. Here we see that the feeding activities of the
+insect are carried on during the larval stage only; the may-fly in its
+winged condition takes no food, pairing and egg-laying form the whole of
+its appointed task. A similar though less extreme shortening of the
+imaginal life may be noticed in many endopterygote insects. For example,
+the bot- and warble-flies have the jaws so far reduced that they are
+unable to feed, and the parasitic life of the maggot (see p. 74)
+extending over eight or nine months in the body of the horse or ox,
+prepares for a winged existence of probably but a few days. Again in
+many moths the jaws are reduced or vestigial so that no food can be
+taken in the winged state, as for example in the 'Eggars'
+(Lasiocampidae) and the 'Tussocks' (Lymantriidae). It is noteworthy
+that in these short-lived insects the male is often provided with
+elaborate sense-organs which, we may believe, assist him to find a mate
+with as little delay as possible; the male may-fly has especially
+complex eyes, while the feelers of the male silk-moth or eggar are
+comb-like or feathery, the branches bearing thousands of sensory hairs.
+A box with a captive living female of one of these moths, if taken into
+a wood haunted by the species becomes rapidly surrounded by a swarm of
+would-be suitors, attracted by the odour emitted from the prisoner's
+scent-glands.
+
+Very exceptionally the imaginal stage may be omitted from the life-story
+altogether. Nearly fifty years ago N. Wagner (1865) made the remarkable
+discovery that in the larvae of certain gall-midges (Cecidomyidae) the
+ovaries might become precociously mature and unfertilised eggs might be
+developed into small larvae observable within the body of the
+mother-larva; ultimately these abnormally reared young break their way
+out. In this case therefore there may be a series of larval generations,
+neither pupa nor imago being formed. Extended observations on the
+precocious reproductive processes of these midges have lately been
+published by W. Kahle (1908). A less extreme instance of an abbreviated
+life-story was made known by O. Grimm (1870) who saw pupae of
+Harlequin-midges (Chironomus) lay unfertilised eggs, which developed
+into larvae. Here the imaginal stage only is omitted from the
+life-history. Not always however is it the imaginal stage of the
+life-history which is shortened. Reference (p. 18) has already been made
+to the case of the virgin female aphids, whose eggs develop within the
+mother's body, so that active, formed young are brought forth. Among the
+Diptera it is not unusual to find similar cases, the female fly giving
+birth to young maggots instead of laying eggs. Such is the habit of the
+great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.)
+whose larvae live as parasites on other insects, and occasionally of the
+Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a
+shortened larval period, and Brace's investigations in 1895, summarised
+by E.E. Austen (1911), have shown that females of the dreaded African
+Tsetse flies (Glossinia) bring forth nearly mature larvae, which pupate
+soon after birth. In another group of Diptera, the blood-sucking
+parasites of the Hippoboscidae and allied families, the whole larval
+development is passed through within the mother's body, and a full-grown
+larva is born the cuticle of which hardens and darkens immediately to
+form a puparium; hence these flies are often called, though incorrectly,
+Pupipara. Still more astonishing is the mode of reproduction in the
+allied family of the Termitoxeniidae, curious, degraded, wingless
+'guests' of the termites, or 'white ants,' lately made known through the
+researches of E. Wasmann (1901). Here the individual is hermaphrodite--a
+most exceptional condition among insects--and lays a large egg, whence
+is usually hatched a fully-developed adult! Here then we find that all
+the early stages, usual in the higher insects, are omitted from the
+life-story.
+
+Interesting comparison may be made between the total duration of various
+insect life-stories. To some extent at least, the length of an insect's
+life is correlated with its size, its food, the season of the year when
+it breeds. Small insects have, as a rule, shorter lives than large ones;
+those whose larvae devour highly nutritive food generally develop more
+quickly than those which have to live on dry, poor, substances;
+life-cycles follow one another most rapidly in summer weather when
+temperature is high and food plentiful.
+
+In early chapters we have already noticed the long aquatic life of the
+larva and nymph of a dragon-fly, relatively a large insect, and the
+rapid multiplication of the repeated summer broods of virgin aphids (p.
+18). Within the one order of the Coleoptera it is instructive to compare
+the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose
+larvae mine in the green tissues, and complete their transformations so
+rapidly that several successive broods appear in the spring and early
+summer, with the larger click-beetles whose larvae, the equally
+notorious 'wireworms,' feed on roots for three or four years before they
+become fully grown. Among the Diptera, the 'leather-jacket' grub of the
+crane-fly, feeding like the wireworm on roots, has a larval life
+extending through the greater part of a year, while the maggot of the
+bluebottle, feeding on a rich meat diet, becomes mature in a few days.
+As examples of excessively long life-cycles the 'thirteen-year' and
+'seventeen-year' cicads of North America, described by C.L. Marlatt
+(1895), are noteworthy. Certain specially populous 'broods' of these
+insects are known and localised, so that the appearance of the imagos in
+future years can be accurately predicted. Here again we have to do with
+bulky insects whose subterranean larvae and nymphs feed on comparatively
+innutritious roots.
+
+In our own climate, it is of interest to notice the variation among
+insects as to the stage which carries the race over the winter. The
+click-beetles, mentioned just above, emerge from their buried pupae in
+summer, hibernate under stones or clods, and lay eggs among the herbage
+next spring. At the same time of course, owing to the extended term of
+the larval life, many more individuals of the species are wintering
+underground as 'wireworms' of various ages, and these, except in very
+severe frosts, can continue their occupation of feeding on roots. But in
+the case of the 'turnip-flies' the food-supply is cut off in winter, and
+all those beetles of the latest summer brood that survive hibernate in
+some sheltered spot, waiting for the return of spring, that they may lay
+their eggs, and start the life-cycle once again. Among the Diptera, most
+species pass the winter as pupae, the sheltering puparium being a good
+protection against most adverse conditions, or as flies. But where there
+is a prolonged parasitic larval life, as with the bot- and warble-flies,
+the maggot, warm and well-fed within the body of its mammalian host,
+affords an appropriate wintering stage.
+
+Among the Hymenoptera an especially interesting seasonal life-cycle is
+afforded by the alternation of summer and winter generations in many
+Gall-flies (Cynipidae) as H. Adler (1881, 1896) demonstrated for most of
+our common species. The well-known 'oak-apples' are tenanted in summer
+by grubs, which after pupation develop into winged males and wingless
+females. The latter, after pairing, burrow underground and lay their
+eggs in the roots, the larvae causing the presence there of globular
+swellings or root-galls within which they live, pass through their
+transformations and develop into wingless virgin females. These shelter
+until February or March in their underground chambers, then climb up
+the tree and lay on the shoots eggs, from which will be hatched the
+grubs destined to grow within the oak-apples into the summer sexual
+brood of flies.
+
+The Lepidoptera afford examples of hibernation in all stages of the
+life-history. In this order a few large moths with wood-boring
+caterpillars, the 'Goat' (Cossus) for example, undergo a development
+extending over several years, while at the other extreme a few small
+species may have three or more complete cycles within the twelve months.
+But in the vast majority of Lepidoptera we find either one or two
+generations, definitely seasonal, within the year; the insect is either
+'single-brooded' or 'double-brooded.'
+
+Almost every winter one or more letters may be read in some newspaper
+recording the writer's surprise at seeing on a sunny day during the cold
+season, one of our common gaily-coloured butterflies of the Vanessa
+group, a 'Tortoiseshell' or 'Red Admiral,' flitting about. Surprise
+might be greater did the observers realise that the imaginal is the
+normal hibernating stage for these species. Emerging from the pupa in
+late summer or autumn, they shelter during winter in hollow trees, under
+thatched eaves, in outbuildings or in similar situations, coming out in
+spring to lay their eggs on the leaves of their caterpillars'
+food-plants. The larvae feed and grow through the early summer months,
+in the case of the Small Tortoiseshell (_Vanessa urticae_) pupating
+before midsummer and developing into a July brood of butterflies whose
+offspring after a late summer life-cycle, hibernate; while for the
+larger species of the group there is, in our islands, only one complete
+life-cycle in the year, though the same insects in warmer countries may
+be double-brooded. C.G. Barrett records (1893, vol. I. pp. 153-4) how in
+the August of 1879 hundreds and thousands of 'Painted Ladies' (_Pyrameis
+cardui_) migrated into the south of England from the European continent
+where in many places great swarms had been observed early in the summer.
+'These August butterflies, the progeny of the June swarms, coming from a
+warmer climate, had no intention of hibernating, but paired and laid
+eggs. Some of the larvae were collected and reared indoors [butterflies]
+emerging in November and December, but out of doors all must have been
+destroyed by damp or frost, in either the larva or pupa state, for no
+freshly emerged specimens were noticed in the spring, and no trace of
+the great migration remained.'
+
+In September and October the pedestrian, even in a suburban square, may
+see moths with pretty brown, white-spotted wings flying around trees.
+These are males of the common 'Vapourer' (_Orgyia antiqua_), in search
+of the females which, wingless and helpless, rest on the cocoons
+surrounding the pupae whence they have just emerged, the cocoons being
+attached to the branches of the trees where the caterpillars have fed.
+After pairing, the female lays her eggs among the silk of the cocoon,
+partly covering them with hairs shed from her body, and then dies. The
+eggs thus protected remain through the winter, the larvae not being
+hatched till springtide, when the young leaves begin to sprout forth.
+The caterpillars, adorned and probably protected by their 'tussocks' of
+black or coloured bristles, feed vigorously. Their activity and habit of
+occasional migration from one tree to another, compensates, to some
+extent, as Miall (1908) has pointed out, for the females' enforced
+passivity; only in the larval state can moths with such wingless females
+extend their range. The caterpillars spin their cocoons towards the end
+of summer, and then pupate, the moths emerging in the autumn and the
+eggs, as we have seen, furnishing the winter stage.
+
+After midsummer, the conspicuous cream, black and yellow-spotted
+'Magpie' moth (_Abraxas grossulariata_) is common in gardens. The female
+lays her eggs on a variety of shrubby plants; gooseberry and currant
+bushes are often chosen. From the eggs caterpillars are hatched in
+autumn, but these, instead of beginning to feed, seek almost at once for
+rolled-up leaves, cracks in walls, crannies of bark, or similar places,
+which may afford winter shelters. Here they remain until the spring,
+when they come out to feed on the young foliage and grow rapidly into
+the conspicuous cream, yellow and black 'looper' caterpillars mentioned
+in a previous chapter (p. 60). These, when fully-grown, spin among the
+twigs of the food-plant a light cocoon, in which the black and
+yellow-banded wasp-like pupa spends its short summer term before the
+emergence of the moth.
+
+An equally familiar garden insect, the common 'Tiger' moth (_Arctia
+caia_) with its 'woolly bear' caterpillar, affords a life-cycle slightly
+differing from that of the 'Magpie.' The gaudy winged insects are seen
+in July and August, and lay their eggs on a great variety of plants. The
+larvae hatched from these eggs begin to feed at once, and having moulted
+once or twice and attained about half their full size, they rest through
+the winter, the dense hairy covering wherewith they are provided forming
+an effective protection against the cold. At the approach of spring they
+begin to feed again, and the fully-grown 'woolly bear' is a common
+object on garden paths in May and June. Before midsummer it has usually
+spun its yellow cocoon under some shelter on the ground and changed into
+a pupa.
+
+Another modification with respect to seasonal change is shown by the
+Turnip moth (_Agrotis segetum_) and other allied Noctuidae (Owl-moths).
+These are insects with brown-coloured wings, flying after dark in June.
+The dull greyish larvae feed on many kinds of low-growing plants,
+usually hiding in the earth by day and wandering along the surface of
+the ground by night, biting off the farmer's ripening corn, or burrowing
+into his turnips or potatoes. On account of the burrowing habits of this
+insect it can feed throughout the winter, except when a hard frost puts
+a temporary stop to its activity. By April it has become fully grown and
+pupates in an earthen chamber a few inches below the surface. The Turnip
+moth in our countries is partially double-brooded, a minority of the
+autumn caterpillars growing more rapidly than their comrades so that
+they pupate, and a second brood of moths appear in September. These pair
+and lay eggs, the resulting caterpillars going as Barrett suggests
+(1896, vol. III. p. 291) 'to reinforce the great army of wintering
+larvae.'
+
+Such underground caterpillars, to a great extent protected from cold,
+can continue to feed through the winter. With other species we find that
+the larva becomes fully grown in autumn, yet lives through the winter
+without further change. This is the case with the Codling moth
+(_Carpocapsa pomonella_), a well-known orchard pest, which in our
+countries is usually single-brooded. The moth is flying in May and lays
+her eggs on the shoots or leaves of apple-trees, more rarely on the
+fruitlets, into which however the caterpillar always bores by the upper
+(calyx) end. Here it feeds, growing with the growth of the fruit,
+feeding on the tissue around the cores, ultimately eating its way out
+through a lateral hole, and crawling upwards if its apple-habitation has
+fallen, downwards if it still remains on the bough, to shelter under a
+loose piece of bark where it spins its cocoon about midsummer and
+hibernates still in the larval condition. Not until spring is the pupal
+form assumed, and then it quickly passes into the imaginal state. In the
+south of England, as F.V. Theobald (1909) has lately shown, and also in
+southwestern Ireland, this species may be double-brooded, the usual
+condition on the European continent and in the United States of America.
+There the midsummer larvae pupate at once and the moths of an August
+brood lay eggs on the hanging or stored fruit; in this case, again,
+however, the full-grown larva, quickly fed-up within the developed
+apples, is the wintering stage.
+
+Several of the insects mentioned in this survey, like the last-named
+codling moth, are occasionally double-brooded. As an example of the many
+Lepidoptera, which in our islands have normally two complete life-cycles
+in the year, we may take the very familiar White butterflies (Pieris) of
+which three species are common everywhere. The appearance of the first
+brood of these butterflies on the wing in late April or May is hailed as
+a sign of advanced spring-time. They pair and lay their eggs on
+cabbages and other plants, and the green hairy caterpillars feed in June
+and July, after which the spotted pupae may be found on fences and
+walls, attached by the silken tail-pad and supported by the
+waist-girdle. In August and September butterflies of the second brood
+have emerged from these and are on the wing; their offspring are the
+autumn caterpillars which feed in some seasons as late as November,
+doing often serious damage to the late cruciferous crops before they
+pupate. The pupae may be seen during the winter months, waiting for the
+spring sunshine to call out the butterflies whose structures are being
+formed beneath the hard cuticle.
+
+Reviewing the small selection of life-stories of various Lepidoptera
+just sketched, we notice an interesting and suggestive variety in the
+wintering stage. The vanessid butterflies hibernate as imagos; the
+'vapourer' winters in the egg, the magpie as a young ungrown larva, the
+'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
+winter, growing all the time; the codling caterpillar completes its
+growth in the autumn, and winters as a full-size resting larva; lastly,
+the 'whites' hibernate in the pupal state. And in every case it is
+noteworthy that the form or habit of the wintering stage is well adapted
+for enduring cold.
+
+Our native 'whites' afford illustration of another interesting feature
+often to be noticed in the life-story of double-brooded Lepidoptera. The
+butterflies of the spring brood differ slightly but constantly from
+their summer offspring, affording examples of what is called _seasonal
+dimorphism_. All three species have whitish wings marked with black
+spots, larger and more numerous in the female than in the male. In the
+spring butterflies these spots tend towards reduction or replacement by
+grey, while in the summer insects they are more strongly defined, and
+the ground colour of the wings varies towards yellowish. In the
+'Green-veined' white (_Pieris napi_) the characteristic greenish-grey
+lines of scaling beneath the wings along the nervures, are much broader
+and more strongly marked in the spring than in the summer generation,
+whose members are distinguished by systematic entomologists under the
+varietal name _napaeae_. The two forms of this insect were discussed by
+A. Weismann in his classical work on the Seasonal Dimorphism of
+butterflies (1876). He tried the effect of artificially induced cold
+conditions on the summer pupae of _Pieris napi_, and by keeping a batch
+for three months at the temperature of freezing water, he succeeded in
+completely changing every individual of the summer generation into the
+winter form. The reverse of this experiment also was attempted by
+Weismann. He took a female of _bryoniae_, an alpine and arctic variety
+of _Pieris napi_, showing in an intensive degree the characters of the
+spring brood. This female laid eggs the caterpillars from which fed and
+pupated. The pupae although kept through the summer in a hothouse all
+produced typical _bryoniae_, and none of these with one exception
+appeared until the next year, for in the alpine and arctic regions this
+species is only single-brooded. Weismann experimented also with a small
+vanessid butterfly, _Araschnia levana_, common on the European
+continent, though unknown in our islands, which is double (or at times
+treble) brooded, its spring form (_levana_) alternating with a larger
+and more brightly coloured summer form (_prorsa_). Here again by
+refrigerating the summer pupae, butterflies were reared most of which
+approached the winter pattern, but it was impossible by heating the
+winter pupae to change _levana_ into _prorsa_. Experiments with North
+American dimorphic species have given similar results. Weismann argued
+from these experiments that the winter form of these seasonally
+dimorphic species is in all cases the older, and that the butterflies
+developing within the summer pupae can be made to revert to the
+ancestral condition by repeating the low-temperature stimulus which
+always prevailed during the geologically recent Ice Age. On the other
+hand, a high temperature stimulus applied to one generation of the
+winter pupae cannot induce the change into the summer pattern, which has
+been evolved still more recently by slow stages, as the continental
+climate has become more genial. In tropical countries where instead of
+an alternation of winter and summer, alternate dry and rainy seasons
+prevail, somewhat similar seasonal dimorphism has been observed among
+many butterflies. Not a few forms of Precis, an African and Indian genus
+allied to our Vanessa, that had long been considered distinct species
+are now known, thanks to the researches of G.A.K. Marshall (1898), to be
+alternating seasonal forms of the same insect. The offspring when adult
+does not closely resemble the parent; its appearance is modified by the
+climatic environment of the pupa. The experiments of Weismann just
+sketched in outline show at least that the same principle holds for our
+northern butterflies.
+
+We are thus led to see from the life-story of such insects, that the
+course of the story is not rigidly fixed; the creature in its various
+stages is plastic, open to influence from its surroundings, capable of
+marked change in the course of generations. And so the seasonal changes
+in the history of the individual from egg to imago point us to changes
+in the age-long history of the race.
+
+
+
+
+CHAPTER IX
+
+PAST AND PRESENT; THE MEANING OF THE STORY
+
+
+In the previous chapter we recognised how the seasonal changes in
+various species of butterflies as observable in two or three
+generations, indicate changes in the history of the race as it might be
+traced through innumerable generations. The endless variety in the form
+and habits of insect-larvae and their adaptations to various modes of
+life, which have been briefly sketched in this little book, suggest
+vaster changes in the class of insects, as a whole, through the long
+periods of geological time. Every student of life, influenced by the
+teaching of Charles Darwin (1859) and his successors, now regards all
+groups of animals from the evolutionary standpoint, and believes that
+comparisons of facts of structure and life-history of orders and classes
+evidently akin to each other, furnish at least some indications of the
+course of development in the greater systematic divisions, even as the
+facts of seasonal dimorphism, mentioned in the last chapter, give hints
+as to the course of development in those restricted groups that we call
+species or varieties. A brief discussion of the main outlines of the
+life-story of insects in the wide, evolutionary sense may thus fitly
+conclude this book.
+
+In the first place we turn to the 'records' of those rocks, in whose
+stratified layers[12] are entombed remains, often fragmentary and
+obscure, of the insects of past ages of the earth's history. Compared
+with the thousands of extinct types of hard-shelled marine animals, such
+as the Mollusca, fossil insects are few, as could only be expected,
+seeing that insects are terrestrial and aerial creatures with slight
+chance of preservation in sediments formed under water. Yet a number of
+insect remains are now known to naturalists, who are, in this
+connection, more particularly indebted to the researches of S.H. Scudder
+(1885), C. Brongniart (1894), and A. Handlirsch (1906).
+
+[12] See Table of Geological Systems, p. 123.
+
+We are now considering insects from the standpoint of their
+life-histories, and the individual life-story of an insect of which we
+possess but a few fragments of wings or body, entombed in a rock formed
+possibly before the period of the Coal Measures, can only be a matter of
+inference. Still it may safely be inferred that when the structure of
+these remains clearly indicates affinity to some existing order or
+family, the life-history of the extinct creature must have resembled, on
+the whole, that of its nearest living allies. And all the fossil
+insects known can be either referred to existing orders, or shown to
+indicate definite relationship to some existing group.
+
+Passing over some doubtful remains of Silurian age, we find in rocks
+usually regarded as Devonian[13] the most ancient fossils that can be
+certainly referred to the insects, while from beds of the succeeding
+Carboniferous period, a number of insect remains have been disinterred.
+These Palaeozoic insects were frequently of large size, and they show
+distinct affinities with our recent may-flies, dragon-flies,
+stone-flies, and cockroaches. In the Permian period, the latest of the
+divisions of the Palaeozoic, lived Eugereon, an insect with hemipteroid
+jaws and orthopteroid wings. All these insects must have been
+exopterygote in their life-history, if we may trust the indications of
+affinity furnished by their structure. In the Mesozoic period, however,
+insects with complete transformations must have been fairly abundant.
+Rocks of Triassic age have yielded beetles and lacewing-flies, while
+from among Jurassic fossils specimens have been described as
+representing most of our existing orders, including Lepidoptera,
+Hymenoptera and Diptera. In Cainozoic rocks fossil insects of nearly six
+thousand species have been found, which are easily referable to
+existing families and often to existing genera. We may conclude then,
+imperfect though our knowledge of extinct insects is, that some of the
+most complex of insect life-stories were being worked out before the
+dawn of the Cainozoic era. Some instructive hints as to differences in
+the rate of change among different insect groups may be drawn from the
+study of parasites. For example, V.L. Kellogg (1913) points out that an
+identical species of the Mallophaga (Bird-lice) infests an Australian
+Cassowary and two of the South American Rheas; while two species of the
+same genus (Lipeurus) are common to the African Ostrich and a third kind
+of South American Rhea. These parasites must have been inherited
+unchanged by the various members of these three families of flightless
+birds from their common ancestors, that is from early Cainozoic times at
+latest. On the other hand, the various kinds of such highly specialised
+parasites as the warble-flies of the oxen and deer, must have become
+differentiated during those later stages of the Cainozoic period which
+witnessed the evolution of their respective mammalian hosts.
+
+[13] The 'Little River' beds of St John, New Brunswick, Canada, by some
+modern geologists however considered as Carboniferous.
+
+The foregoing brief outline of our knowledge of the geological
+succession of insects shows that the exopterygote preceded, in time, the
+endopterygote type of life-history. We have already seen that those
+insects undergoing little change in the life-cycle, and with visible,
+external wing-rudiments, are on the whole less specialised in structure
+than those which pass through a complete transformation. These two
+considerations, taken together, suggest strongly that in the evolution
+of the insect class, the simpler life-history preceded the more complex.
+Such a conclusion seems reasonable and what might have been expected,
+but we are confronted with the difficulty that if the most highly
+organised insects pass through the most profound transformations, then
+insects present a remarkable and puzzling exception to the general rules
+of development among animals, as has already been pointed out in the
+first chapter of this volume (p. 7). A few students of insect
+transformation have indeed supposed that the crawling caterpillar or
+maggot must be regarded as a larval stage which recalls the worm-like
+nature of the supposed far-off ancestors of insects generally. Even in
+Poulton's classical memoir (1891, p. 190), this view finds some support,
+and it may be hard to give up the seductive idea that the worm-like
+insect-larva has some phylogenetic meaning. But the weight of evidence,
+when we take a comprehensive survey of the life-story of insects, must
+be pronounced to be strongly in favour of the view put forward by Brauer
+(1869), and since supported by the great majority of naturalists who
+have discussed the subject, that the caterpillar or the maggot is itself
+a specialised product of the evolutionary process, adapted to its own
+particular mode of larval life.
+
+The explanation of insect transformation is, in brief, to be found in an
+increasing amount of divergence between larva and imago. The most
+profound metamorphosis is but a special type of growth, accompanied by
+successive castings and renewings of the chitinous cuticle, which
+envelopes all arthropods. In the simplest type of insect life-story,
+there is no marked difference in form between the newly-hatched young
+and the adult, and in such cases we find that the young insect lives in
+the same way as the adult, has the same surroundings, eats the same
+food. This is the rule (see Chapters II and III) with the Apterygota,
+the Orthoptera, and most of the Hemiptera. In the last-named order,
+however, we find in certain families marked divergence between larva and
+imago, for example in the cicads, whose larvae live underground, while
+in the coccids, whose males are highly specialised and females degraded,
+there succeeds to the larva--very like the young stage in allied
+families--a resting instar, which in the case of the male, suggests
+comparison with the pupa of a moth or beetle.
+
+Turning to the stone-flies, dragon-flies and may-flies, whose
+life-stories have been sketched in Chapter IV, we find that the early
+stages are passed in water, whence before the final moult, the insects
+emerge to the upper air. Except for the possession of tufted gills,
+adapting them to an aquatic life, the stone-fly nymphs differ but
+slightly from the adults; the grubs of the dragon-flies and may-flies,
+however, are markedly different from their parents. In connection with
+these comparisons, it is to be noted that the dragon-flies and may-flies
+are more highly specialised insects than stone-flies, divergent
+specialisation of the adult and larva is therefore well illustrated in
+these groups, which nevertheless have, like the Hemiptera and
+Orthoptera, visible external wing-rudiments.
+
+From the vast array of insects that show internal wing-growth and a true
+pupal stage, a few larval types were chosen for description in Chapter
+VI, and a review of these suggests again the thought of increasing
+divergence between larva and imago. Reference has been made previously
+to the many instances in which the former has become pre-eminently the
+feeding, and the latter the breeding stage in the life-cycle. It seems
+impossible to avoid the conclusion that the active, armoured
+campodeiform grub differing less from its parent than an eruciform larva
+differs from its parent, is as a larval type more primitive than the
+caterpillar or maggot. A. Lameere has indeed, while admitting the
+adaptive character of insect larvae generally, argued (1899) with much
+ingenuity that the eruciform or vermiform type must have been primitive
+among the Endopterygota, believing that the original environment of the
+larvae of the ancestral stock of all these insects must have been the
+interior of plant tissues. He is thus forced to the necessity of
+suggesting that the campodeiform larvae of ground-beetles or lacewings
+must be regarded as due to secondarily acquired adaptations; 'they
+resemble Thysanura and the larvae of Heterometabola only as whales
+resemble fishes.' There are two considerations which render these
+theories untenable. The Neuroptera and Coleoptera among which
+campodeiform larvae are common, are less specialised than Lepidoptera,
+Hymenoptera, and Diptera, in which they are unknown. And among the
+Coleoptera which as we have seen (pp. 50 _f._) display a most
+interesting variety of larval structure, the legless, eruciform larva
+characterises families in which the imago shows the greatest
+specialisation, while in the same life-story, as in the case of the
+oil-beetles (pp. 56-7), the newly-hatched grub may be campodeiform,
+changing to the eruciform type as soon as it finds itself within reach
+of its host's rich store of food.
+
+A certain amount of difficulty may be felt with regard to the theory of
+divergent evolution between imago and larva, in the case of those
+insects with complete transformation whose grubs and adults live in much
+the same conditions. By turning over stones the naturalist may find
+ground-beetles in company with the larvae of their own species. On the
+leaves of a willow tree he may observe leaf-beetles (Phyllodecta and
+Galerucella) together with their grubs, all greedily eating the foliage;
+or lady-bird beetles (Coccinella) and their larvae hunting and devouring
+the 'greenfly.' All of these insects are, however, Coleoptera, and the
+adult insects of this order are much more disposed to walk and crawl and
+less disposed to fly than other endopterygote insects. Their heavily
+armoured bodies and their firm shield-like forewings render them less
+aerial than other insects; in many genera the power of flight has been
+altogether lost. It is not surprising, therefore, that many beetles,
+even when adult, should live as their larvae do; since the acquirement
+of complete metamorphosis they have become modified towards the larval
+condition, and an extreme case of such modification is afforded by the
+wingless grub-like female Glow-worm (Lampyris).
+
+With most insects, however, the larva must be regarded as the more
+specially modified, even if degraded, stage. Miall (1895) has pointed
+out that the insect grub is not a precociously hatched embryo, like the
+larvae of multitudes of marine animals, but that it exhibits in a
+modified form the essential characters of the adult. Comparison for
+example can be readily made between the parts of the caterpillar and the
+butterfly, whose story was sketched in the first chapter of this book,
+widely different though caterpillar and butterfly may appear at a
+superficial glance. And the survey of variety in form, food, and habit
+of insect larvae given in Chapter VI enforces surely the conclusion that
+the larva is eminently plastic, adaptable, capable of changing so as to
+suit the most diverse surroundings. In a most suggestive recent
+discussion on the transformation of insects P. Deegener (1909) has
+claimed that the larva must be regarded as the more modified stage,
+because while all the adult's structures are represented in the larva,
+even if only as imaginal buds, there are commonly present in the larva
+special adaptive organs not found in the imago, for example the pro-legs
+of caterpillars or the skin-gills of midge-grubs. The correspondence of
+parts in butterfly and caterpillar just referred to, may still be
+traced, though less easily, in bluebottle and maggot. The latter is an
+extreme example of degenerative evolution, and its contrast with the
+elaborately organised two-winged fly marks the greatest divergence
+observable between the larva and imago. With this divergence the resting
+pupal stage, during which more or less dissolution and reconstruction of
+organs goes on, becomes a necessity, and it has already been pointed out
+how the amount of this reconstruction is greatest where the divergence
+between the larval and perfect stages is most marked. Whatever
+differences of opinion may prevail on points of detail, the general
+explanation of insect metamorphosis as the result of divergent evolution
+in the two active stages of the life-story must assuredly be accepted.
+No other explanation accords with the increasing degree of divergence to
+be observed as we pass from the lower to the higher insect orders.
+
+The successive incidents of the life-story of most insects are largely
+connected with the acquisition of wings. Wings, and the power of flight
+wherewith they endow their possessors, are evidently beneficial to the
+race in giving power of extending the range during the breeding period
+and thus ensuring a wide distribution of the eggs. In no case are wings
+fully developed until the closing stage of the insect's life, they are
+always acquired after hatching or birth. We have already noticed (p. 40)
+how Sharp (1899) has laid stress on the essential difference between the
+exopterygote and endopterygote insects, the wing-rudiments of the former
+growing outwards throughout life while those of the latter remain hidden
+until the pupal instar. Sharp considers that there is some difficulty in
+bridging, in thought, the gap between these two methods of wing-growth,
+and has put forward an ingenious suggestion to meet it (1902). Reference
+has already been made to insects of various orders in which one sex is
+wingless, the Vapourer Moth (p. 96) for example, or all the individuals
+of both sexes are wingless, as the aberrant cockroaches mentioned in
+Chapter II (p. 15), or certain generations of virgin females are
+wingless, for example aphids (pp. 18-19) and gall-flies (pp. 94-5).
+Insects may thus become secondarily wingless, that is to say be
+manifestly the offspring of winged parents, and such wingless forms may
+on the other hand give rise to offspring or descendants with
+well-developed wings. Frequently, as in the case of the aphids, many
+wingless generations intervene between two winged generations. A
+striking illustration of this fact is afforded by an aquatic bug, _Velia
+currens_, commonly to be seen skating over the surface of running water.
+The adults of Velia are nearly always wingless, but now and then the
+naturalist meets with a specimen provided with functional wings, the
+possession of which enables the insect to make its way to a fresh
+stream. Moreover there are whole orders of parasitic insects, such as
+the lice and fleas, which, showing clear affinity to orders of winged
+insects, are believed to be secondarily wingless. These orders are
+designated by Sharp 'Anapterygota.' And from the analogy of the periodic
+loss and recovery of wings in various generations of the same species,
+he has concluded that the gap between the exopterygote and the
+endopterygote method of development may have been bridged by an
+anapterygote condition; that the ancestors of those insects with
+complete transformations were the wingless descendants of primitive
+insects which grew their wings from visible external rudiments, and
+that in later times re-acquiring wings, they developed these organs in a
+new way, from inwardly directed rudiments or imaginal buds.
+
+This theory of Sharp's is original, daring, and ingenious, but the loss
+and re-acquisition of wings which it presupposes is difficult to imagine
+in large groups during a prolonged evolutionary history, while the
+sudden appearance of a totally new mode of wing-growth in the offspring
+of wingless insects would be an extreme example of discontinuity in
+development.
+
+On the whole the most probable suggestion which can be made as to the
+origin of 'complete' transformation in insects is that the instar in
+which wings were first visible externally became later and later in the
+course of the evolution of the more highly organised groups. In this way
+a gradual transition from the exopterygote to the endopterygote type of
+life-story is at least conceivable. It will be remembered that a may-fly
+(p. 33) undergoes a moult after acquiring functional wings, emerging
+into the air as a 'sub-imago.' In not a few endopterygote insects, the
+pupa shows more or less activity, swimming through water intermittently
+(gnats) or just before the imago has to emerge (caddis-flies); working
+its way out of the ground (crane-flies) or coming half-way out of its
+cocoon (many moths). The pupa of the higher insects almost certainly
+corresponds with the may-fly's sub-imago, and the facts just recalled as
+to remnants of pupal activity suggest that in the ancestors of
+endopterygote insects what is now the pupal instar was represented by an
+active nymphal or sub-imaginal stage, possibly indeed by more than one
+stage, as Packard and other writers have stated that pupae of bees and
+wasps undergo two or three moults before the final exposure of the
+imago. Such an early pupal instar has been defined as a 'pro-nymph' or a
+'semi-pupa.' Examples have been given of the exceptional passive
+condition of the penultimate instar in Exopterygota. The instars
+preceding this presumably had originally outward wing-rudiments in all
+insect life-histories, and the endopterygote condition was attained by
+the postponement of the outward appearance of these to successively
+later stages. The leg and wing rudiments of the male coccid (pp. 20-1)
+beneath the cuticle of the second instar are strictly comparable to
+imaginal buds, and these are present in one instar of what is generally
+regarded as an exopterygote life-history. The first instar in all
+insects has no visible wing-rudiments, but when they grow outwardly from
+the body, they necessarily become covered with cuticle, so that they
+must be visible after the first moult. There is no supreme difficulty in
+supposing that the important change was for these early rudiments to
+become sunk into the body, so that the cuticle of the second, and,
+later, of the third and succeeding instars, showed no outward sign of
+their presence. This suggestion is confirmed by Heymons' (1896, 1907)
+observation of the occasional appearance of outward wing-rudiments on
+the thoracic segments of a mealworm, the larva of the beetle _Tenebrio
+molitor_, and by F. Silvestri's discovery (1905) of a 'pro-nymph' stage
+with short external wing-rudiments between the second larval and the
+pupal instars of the small ground-beetle _Lebia scapularis_. Whatever
+may be the exact explanation of these abnormalities, they show that in
+the life-story of the higher insects outward wing-rudiments may even yet
+appear before the pupal stage, confirming our belief that such
+appearance is an ancestral character. The inward growth of these
+wing-rudiments may well have been correlated with a difference in form
+between the newly-hatched insect and its parent. As this difference
+persisted until a constantly later stage, and the pre-imaginal instar
+became necessarily a stage for reconstruction, the present condition of
+complete metamorphosis in the more highly organised orders was finally
+attained.
+
+To explain satisfactorily these complex life-stories is however
+admittedly a difficult task. The acquisition of wings is, as we have
+seen, a dominating feature in them all, but if we try to go yet a step
+farther back and speculate on the origin of wings in the most primitive
+exopterygote insects, the task becomes still more difficult. Many years
+ago Gegenbaur (1878) was struck by the correspondence of insect wings to
+the tracheal gills of may-fly larvae, which are carried on the abdominal
+segments somewhat as wings are on the thoracic segments. But Boerner has
+recently (1909) brought forward evidence that these abdominal gills
+really correspond serially with legs. Moreover Gegenbaur's theory
+suggests that the ancestral insects were aquatic, whereas the presence
+of tubes for breathing atmospheric air in well-nigh all members of the
+class, and the fact that aquatic adaptations, respiratory and otherwise,
+in insect-larvae are secondary force the student to regard the ancestral
+insects as terrestrial. It is indeed highly probable that insects had a
+common origin with aquatic Crustacea, but all the evidence points to the
+ancestors of insects having become breathers of atmospheric air before
+they acquired wings. How the wings arose, what function their precursors
+performed before they became capable of supporting flight, we can hardly
+even guess.
+
+Our study of the life-story of insects, therefore, while it has taught
+us something of what is going on around us to-day, and has given us
+hints of the course of a few threads of that long life-story which runs
+through the ages, brings us face to face with the most instructive, if
+humbling fact that 'there are many more things of which we are
+ignorant.' The passage from creeping to flight, as the caterpillar
+becomes transformed into the butterfly, was a mystery to those who first
+observed it, and many of its aspects remain mysterious still. Perhaps
+the most striking result of the study of insect transformation is the
+appreciation of the divergent specialisation of larva and imago, and it
+is a suggestive thought that of the two the larva has in many cases
+diverged the more from the typical condition. The caterpillar crawling
+over the leaf, or the fly-grub swimming through the water, may thus be
+regarded as a creature preparing for a change to the true conditions of
+its life. It is a strange irony that the preparation is often far longer
+than the brief hours of achievement. But the light which research has
+thrown on the nature of these wonderful life-stories, the demonstration
+of the unseen presence and growth within the insect, during its time of
+preparation among strange surroundings, of the organs required for
+service in the coming life amid its native air, confirm surely the
+intuition of the old-time students, who saw in these changes, so
+familiar and yet so wonderful, a parable and a prophecy of the higher
+nature of man.
+
+
+
+
+OUTLINE CLASSIFICATION OF INSECTS
+
+
+Class INSECTA or HEXAPODA.
+
+Sub-class A, APTERYGOTA.
+
+Order 1. _Thysanura_ (Bristle-tails).
+      2. _Collembola_ (Spring-tails).
+
+Sub-class B, EXOPTERYGOTA.
+
+Order 1. _Dermaptera_ (Earwigs).
+      2. _Orthoptera_ (Cockroaches, Grasshoppers, Crickets).
+      3. _Plecoptera_ (Stone-flies).
+      4. _Isoptera_ (Termites or 'White Ants').
+      5. _Corrodentia_
+         (_a_) _Copeognatha_ (Book-lice).
+         (_b_) _Mallophaga_ (Biting-lice).
+      6. _Ephemeroptera_ (May-flies).
+      7. _Odonata_ (Dragon-flies).
+      8. _Thysanoptera_ (Thrips).
+      9. _Hemiptera_
+         (_a_) _Heteroptera_ (Bugs, Pond-skaters)
+         (_b_) _Homoptera_ (Cicads, 'Greenfly,' Scales).
+     10. _Anoplura_ (Lice).
+
+Sub-class C, ENDOPTERYGOTA.
+
+Order 1. _Neuroptera_ (Alder-flies, Ant-lions, Lacewings).
+      2. _Coleoptera_ (Beetles).
+      3. _Mecaptera_ (Scorpion-flies).
+      4. _Trichoptera_ (Caddis-flies).
+      5. _Lepidoptera_ (Moths and Butterflies).
+      6. _Diptera_ (Two-winged flies)
+         (_a_) _Orthorrhapha_ (Crane-flies, Midges, Gnats)
+         (_b_) _Cyclorrhapha_ (Hover-flies, House-flies, Bot-flies, &c).
+      7. _Siphonaptera_ (Fleas).
+      8. _Hymenoptera_
+         (_a_) _Symphyta_ (Saw-flies)
+         (_b_) _Apocrita_ (Gall-flies, Ichneumon-flies, Wasps, Bees, Ants).
+
+
+
+
+TABLE OF GEOLOGICAL SYSTEMS
+
+
+These names, given by geologists to the various divisions of rocks, as
+indicated by the fossils entombed in them, are arranged in 'descending'
+order, the more recent formations above, the more ancient below, as
+newer deposits necessarily lie over older beds.
+
+CALNOZOIC OR TERTIARY GROUP.
+
+Pleistocene.
+Pliocene.
+Miocene.
+Eocene.
+
+
+MESOZOIC OR SECONDARY GROUP.
+
+Cretaceous.
+Jurassic.
+Triassic.
+
+
+PALAEOZOIC OR PRIMARY GROUP.
+
+Permian.
+Carboniferous.
+Devonian.
+Silurian.
+Cambrian.
+
+
+
+
+BIBLIOGRAPHY
+
+
+The following list of some books and papers, referred to in this little
+volume or of especial service to the author in its preparation, is
+needless to say very far from exhaustive. To save space, titles are
+often abbreviated. Most of the works in the general list (A) contain
+extensive lists of literature on insects and their transformations,
+these should be consulted by the serious student.
+
+
+A. GENERAL WORKS.
+
+1909.     C. Boerner. Die Verwandlungen der Insekten. _Sitzb. d. Gesellsch.
+          naturforsch. Freunde, Berlin._
+
+1869.     F. Brauer. Betrachtung ueber die Verwandlung der Insekten.
+          _Verhandl. der K.K. zool.-bot. Gesellschaft in Wien._ XIX.
+
+1899.     G.H. Carpenter. Insects, their Structure and Life. London.
+
+1859.     C. Darwin. The Origin of Species. London.
+
+1909.     P. Deegener. Die Metamorphose der Insekten. Leipzig.
+
+1906.     J.W. Folsom. Entomology. London.
+
+1878.     C. Gegenbaur. Grundriss der Vergleichende Anatomie. Leipzig.
+
+1906.     A. Handlirsch. Die fossilen Insekten. Leipzig.
+
+1904.     L.F. Henneguy. Les Insectes. Paris.
+
+1907.     R. Heymons. Die verschiedenen Formen der Insectenmetamorphose.
+          _Ergebnisse der Zoologie._ I.
+
+1899.     A. Lameere. La raison d'etre des Metamorphoses chez les Insectes.
+          _Ann. Soc. Entom. Bruxelles._ XLIII.
+
+1874.     J. Lubbock. The Origin and Metamorphoses of Insects. London.
+
+1895.     L.C. Miall. (_a_) The Transformations of Insects. _Nature._ LIII.
+
+1895.     ---- (_b_) The Natural History of Aquatic Insects. London.
+
+1908.     ---- Injurious and Useful Insects. 2nd edition. London.
+
+1839.     G. Newport. Insects. _Todd Cyclopaedia._ II. London.
+
+1898.     A.S. Packard. Text book of Entomology. New York.
+
+1734-42.  R.A.F. de Reaumur. Memoires pour servir a l'Histoire naturelle
+          et a l'anatomie des Insectes. Paris.
+
+1895-8.   D. Sharp. The Cambridge Natural History, V, VI. London.
+
+1899.     ---- Some points in the Classification of Insects. IV. _Internat.
+          Zoolog. Congress._
+
+1902.     ---- Insects in _Encycl. Brit._ 10th Edition, XXIX. London.
+
+1910.     ---- and G.H. Carpenter. Hexapoda in _Encycl. Brit._ 11th
+          Edition. Cambridge.
+
+1737.     J. Swammerdam. Biblia Naturae. Leyden (incorporates works on
+          Insects published during the author's lifetime 1669-75).
+
+1909.     F.V. Theobald. Insect Pests of Fruit. Wye.
+
+
+B. SPECIAL WORKS.
+
+1881.     H. Adler. Ueber den Generationswechsel den Eichen-Gallwespen.
+          _Zeitsch. f. wissensch. Zoologie._ XXXV.
+
+1896.     ---- and C.R. Straton. Alternating Generations. Oxford.
+
+1902.     J. Anglas. Nouvelles Observations sur les Metamorphoses Internes.
+          _Arch. d'Anat. Microscop._ IV.
+
+1911.     E.E. Austen. Handbook of the Tsetse-Flies. London (Brit. Museum).
+
+1909.     F. Balfour-Browne. Life-History of Agrionid Dragonfly. _Proc.
+          Zool. Soc. Lond._
+
+1893, &c. C.G. Barrett. Lepidoptera of the British Islands. London.
+
+1890.     H. Beauregard. Les Insectes Vesicants. Paris.
+
+1909.     C. Boerner. Die Tracheenkiemen der Ephemeriden. _Zoolog. Anz._
+          xxxiii.
+
+1863.     F. Brauer. Monographie der Oestriden. Wien.
+
+1894.     C. Brongniart. Recherches pour servir a l'histoire des Insectes
+          fossiles des Temps Primaires. St Etienne.
+
+1893.     T.A. Chapman. Structure of Pupae of Heterocerous Lepidoptera.
+          _Trans. Entom. Soc. Lond._
+
+1891.     H. Dewitz. Das geschlossene Tracheensystem bei Insektenlarven.
+          _Zoolog. Anz._ xiii.
+
+1857-8.   J.H. Fabre. L'Hypermetamorphose et les Moeurs des Meloides.
+          _Ann. Sci. Nat._ (_Zool._), (4). VII. IX.
+
+1869.     M. Ganin. Die Entwicklungsgeschichte bei den Insekten. _Zeitsch.
+          f. wissensch. Zoolog._ xix.
+
+1894.     J. Gonin. La Metamorphose des Lepidopteres. _Bull. Soc. Vaud.
+          Sci. Nat._ xxx.
+
+1870.     O. Grimm. Die ungeschechtliche Fortpflanzung einer Chironomus.
+          _Mem. Acad. Imper. St Petersbourg_ (7). xv.
+
+1890.     W. Hatchett-Jackson. Morphology of the Lepidoptera. _Trans. Linn.
+          Soc. (Zool.) Lond._ (2). v.
+
+1896.     R. Heymons. Fluegelbildung bei der Larve von Tenebrio molitor.
+          _Sitzb. d, Gesellsch. Naturforsch. Freunde, Berlin._
+
+1906.     ---- Ueber die ersten Jugendformen von Machilis alternata. _Ib._
+
+1908.     W. Kahle. Die Paedogenesis der Cecidomyiden. _Zoologica._ IV.
+
+1913.     V.L. Kellogg. Distribution and Species-forming of Ectoparasites.
+          _Amer. Naturalist._ XLVII.
+
+1887.     A. Kowalevsky. Die nachembryonale Entwicklung der Musciden.
+          _Zeitsch. f. wissensch. Zool._ XLV.
+
+1904.     O.H. Latter. Natural History of Common Animals (chaps. III, IV,
+          V). Cambridge.
+
+1890-95.  B.T. Lowne. The Blowfly, 2 vols. London.
+
+1863.     J. Lubbock. Development of Chloeon. _Trans. Linn. Soc. Lond._
+          XXIII.
+
+1762.     P. Lyonet. Traite anatomique de la Chenille. Haag.
+
+1669.     M. Malpighi. De Bombyce. London.
+
+1898.     C.L. Marlatt. The periodical Cicada. _Entom. Bull._ 14, _U.S.
+          Dept. Agric._
+
+1898.     G.A.K. Marshall. Seasonal Dimorphism in Butterflies. _Ann. Mag.
+          Nat. Hist._ (7). II.
+
+1900.     L.C. Miall and A.B. Hammond. The Harlequin Fly. Oxford.
+
+1901-3.   R. Newstead. Coccidae of the British Isles. London.
+
+1877.     J.A. Palmen. Zur Morphologie des Tracheensystems. Leipzig.
+
+1891.     E.B. Poulton. External Morphology of the Lepidopterous Pupa.
+          _Trans. Linn. Soc. Zool._ (2). V.
+
+1892.     ---- Colour-relation between Lepidopterous Larvae &c. and their
+          surroundings. _Trans. Entom. Soc. Lond._
+
+1880.     C.V. Riley. Pupation of Butterflies. _Proc. Amer. Assoc._ XXVIII.
+
+1902.     E.D. Sanderson. Report of Entomologist. Delaware. U.S.A.
+
+1885.     E.O. Schmidt. Metamorphose und Anatomie des maennlichen
+          Aspidiotus. _Archiv f. Naturgeschichte._ LI.
+
+1885.     S.H. Scudder. Insekten in Zittel's Paleontologie. II.
+
+1907.     A.J. Siltala. Die postembryonale Entwicklung der
+          Trichopteren-Larven. _Zoolog. Jahrb. Suppl._ IX.
+
+1905.     F. Silvestri. Metamorfosi e Costumi della Lebia scapularis.
+          _Redia._ II.
+
+1900.     J.B. Smith. The Apple Plant-louse. _New Jersey Agric. Exp.
+          Station Bull._ 143.
+
+1888.     J. Van Rees. Die innere Metamorphose von Musca. _Zoolog. Jahrb.
+          Anat._ III.
+
+1911.     K.W. Verhoeff. Ueber Felsenspringer, Machiloidea. _Zoolog. Anz._
+          XXXVIII.
+
+1865.     N. Wagner. Die viviparen Gallmueckenlarven. _Zeitsch. f.
+          wissensch. Zoolog._ XV.
+
+1901.     E. Wasmann. Termitoxenia. _Zeitsch. f. wissensch. Zoolog._ LXX.
+
+1864.     A. Weismann. Die nachembryonale Entwicklung der Musciden.
+          _Zeitsch. f. wissensch. Zoolog._ XIV.
+
+1865.     ---- Die Metamorphose von Corethra. _Ib._ XVI.
+
+1876.     ---- Studien zur Descendenz-Theorie. Leipzig. (English
+          Translation by R. Meldola, London, 1882.)
+
+
+
+
+INDEX
+
+
+_Abraxas grossulariata_, 60, 83, 97-8
+
+Adaptation of larvae, 57, 79, 114
+
+Adephaga, 51
+
+Adler, H., 94
+
+Aeschnidae, 27, 29, 31
+
+Agrionidae, 27, 28
+
+_Agrotis segetum_, 98
+
+Air-tubes, 2, 11, 23, 47, 70, 77, 87, 120
+
+Alternation of generations, 17, 94
+
+Ametabola, 11, 35
+
+Anapterygota, 116
+
+Anglas, J., 46
+
+Ant-lions, 57
+
+Ants, 64, 66
+
+Aphidae, 17-20, 116
+
+_Aphis pomi_, 18-19
+
+Aphis-lion, 57
+
+Apterygota, 41, 110
+
+Aquatic insects, 23-34, 76-9, 120
+
+_Araschnia levana_ and var. _prorsa_, 103
+
+_Arctia caia_, 98
+
+Arctiadae, 59
+
+Arthropoda, 9
+
+Austen, E.E., 91
+
+Avebury, Lord, _see_ Lubbock, J.
+
+
+Balfour-Browne, F., 28
+
+Bark-beetles, 55
+
+Barrett, C.G., 96, 99
+
+Beauregard, H., 56
+
+Bees, 40, 46, 64, 83
+
+Beetles, 40, 50-7, 80, 107, 112-3, 119
+
+Bell Moths, 62
+
+Bird-lice, 108
+
+Birth, 18, 91
+
+_Blatta orientalis_, 15
+
+Blister-beetles, 56
+
+Blowfly or Bluebottle, 43, 44, 46, 67, 71-3, 93, 114
+
+Boerner, C., 32, 120
+
+Bot-flies, 73-4, 89, 91
+
+Brain, 44
+
+Brauer, F., 6, 52, 56, 67, 109
+
+Bristle-tails, 11
+
+Brongniart, C., 106
+
+Butterflies, 1, 83, 95-6, 114
+
+
+Cabbage-butterflies, 39, 41, 85, 100-1
+
+Cabbage-fly, 73
+
+Caddis-flies, 62-3, 86, 117
+
+Cainozoic insects, 107
+
+Calliphora, 43.
+  _See also_ Blowfly
+
+Campodeiform larvae, 52, 56, 111
+
+Carabidae, 52
+
+Carboniferous insects, 107
+
+_Carpocapsa pomonella_, 99-100
+
+Carrion-beetles, 50
+
+Caterpillar, 4, 36, 49, 58-62, 95-101, 109, 114
+
+Cecidomyidae, 68-70, 90
+
+Cerambycidae, 55
+
+Cercopods, 12, 15
+
+Chafers, 52
+
+Chapman, T.A., 81, 84
+
+Chironomus, 43, 77, 87, 91
+
+Chloeon, 33
+
+Chrysalis, 82.
+  _See also_ Pupa
+
+Chrysomelidae, 53.
+  _See also_ Leaf-beetles
+
+Chrysopa, 57
+
+Cicads, 22, 93, 110
+
+Classification, 122
+
+Clearwing Moths, 62
+
+Click-beetles, 52, 93
+
+Clothes-moths, 62
+
+Coccidae, 20, 110, 118
+
+Coccinella, 113
+
+Cockroaches, 11, 14, 15, 107, 115
+
+Cocoons, 82
+
+Codling Moth, 62, 99
+
+Coleoptera, 50-6, 80, 112, 119
+
+Collembola, 11
+
+Complete transformation, 35, 107, 119.
+  _See also_ Endopterygota
+
+Corethra, 43
+
+Cossus, 38, 62, 82, 95
+
+Crane-flies, 67, 70, 93, 117
+
+Cremaster, 83
+
+Crustacea, 7, 120
+
+Culex, 43, 77, 86
+
+Curculionidae, 55
+
+Cuticle, 2, 9, 29, 37, 40, 50, 81, 87, 110
+
+Cynipidae, 94.
+  _See also_ Gall-flies
+
+
+Daddy-long-legs, 69-70
+
+Darwin, C., 105
+
+Deegener, P., 6, 114
+
+Devonian insects, 107
+
+Dewitz, H., 28
+
+Digestive system, 10, 45-7
+
+_Diplosis pyrivora_, 70
+
+Diptera, 42, 64, 67-79, 81, 86-8, 91, 94, 107
+
+Divergence between larva and imago, 110, 114, 121
+
+Double-brooded Lepidoptera, 95, 100-4
+
+Dragon-flies, 26-31, 107, 110
+
+Drone-flies, 76
+
+Duration of life, 34, 89, 92-3, 95
+
+Dyticus, 51
+
+
+Ecdysis, 10.
+  _See also_ Moult
+
+Ectoderm, 9, 11, 47
+
+Eggar Moths, 59, 89
+
+Eggs, 6, 17-18, 26, 34, 65-7, 71, 90, 94-5, 97
+
+Elateridae, 52
+
+Endopterygota, 41, 49, 108, 112, 115-6
+
+Ephemeroptera, 24.
+  _See also_ May-flies
+
+Epidermis, 9, 40
+
+Eristalis, 76
+
+Eruciform larvae, 56, 58-70, 111
+
+Evolution, 16, 103, 105-21
+
+Exopterygota, 41, 108, 115-6, 118
+
+Exoskeleton, 9
+
+
+Fabre, J.H., 56
+
+Fat-body, 47
+
+Feeding-period, 27, 32, 36, 89, 111
+
+Feelers, 1, 4, 42, 71
+
+Fleas, 116
+
+Fore-gut, 47
+
+Free pupa, 80
+
+
+Gall-flies, 64-6, 94, 115
+
+Gall-midges, 68-70, 90
+
+Ganin, M., 66
+
+_Gastrophilus equi_, 73-4
+
+Gegenbaur, C., 120
+
+Geological history, 106-8, 123
+
+Geometridae, 59
+
+Gills, 24, 27, 32, 78, 87, 114, 120
+
+Glossinia, 91
+
+Glow-worm, 50, 113
+
+Gnats, 43, 77, 86
+
+Goat Moth, 38, 62, 82, 95
+
+Gonin, J., 38, 41
+
+Grasshoppers, 11, 14, 15
+
+Grimm, O., 90
+
+Ground-beetles, 52, 112
+
+Growth, 9
+
+Grub, 63-70.
+  _See also_ Caterpillar, Larva
+
+
+Hairs, 59, 82, 98
+
+Hammond, A.R., 43, 77, 87
+
+Handlirsch, A., 106
+
+Harvey, William, 7
+
+Hatchett-Jackson, W., 83
+
+Hawk Moths, 60
+
+Heart, 45
+
+Helodes, 50
+
+Hemerobius, 57
+
+Hemimetabola, 35
+
+Hemiptera, 17, 110
+
+Henneguy, L.F., 45, 48
+
+Heymons, R., 6, 11, 119
+
+Hibernation. _See_ Wintering stages
+
+Hind-gut, 47
+
+Hippoboscidae, 91
+
+Histogenesis and Histolysis, 48
+
+Holometabola, 35
+
+House-fly, 67, 71, 73
+
+Hover-flies, 74-6
+
+Hymenoptera, 58, 64, 94, 107
+
+Hypermetamorphosis, 56
+
+_Hypoderma bovis_, 73-5
+
+Hypodermis, 9
+
+
+Ichneumon-flies, 64, 66, 82
+
+Imaginal buds or discs, 34-48, 114, 117-8
+
+Imago, 24, 34, 114
+
+Instar, 13, 33, 56, 117-9
+
+
+Jaws of imago and larva, 2, 4, 5, 32, 42, 89
+
+Jurassic insects, 107
+
+
+Kahle, W., 90
+
+Kellogg, V.L., 108
+
+Kowalevsky, A., 46
+
+
+Labium, 2, 27
+
+Lacewing-flies, 57, 107
+
+Ladybirds, 113
+
+Lameere, A., 111
+
+Lampyris, 113
+
+Larva, 4, 22, 26-7, 32, 49-79, 110-15
+
+Larval reproduction, 90
+
+Lasiocampidae, 59, 89
+
+Latter, O.H., 28
+
+Leaf-beetles, 53, 83, 92-3, 113
+
+_Lebia scapularis_, 119
+
+Lepidoptera, 1, 36, 38, 49, 58, 81, 95-104, 107
+
+Libellulidae, 27
+
+Lice, 116
+
+Lipeurus, 108
+
+Longhorn Beetles, 55
+
+Looper caterpillars, 59, 61
+
+Lowne, B.T., 42
+
+Lubbock, J., 6, 32
+
+Lymantriidae, 90
+
+Lyonet, P., 38
+
+
+Machilis, 11
+
+Maggot, 44, 67, 71-6, 109, 114
+
+Magpie Moth, 60, 82, 97-8
+
+Mallophaga, 108
+
+Mandibles, 4, 17, 26, 58, 67, 86
+
+Mangel-fly, 73
+
+Marlatt, C.L., 93
+
+Marshall, G.A.K., 104
+
+Maxillae, 2, 17, 37, 42
+
+May-flies, 31-4, 107, 110, 117, 120
+
+Meloidae, 56
+
+Mesozoic insects, 107
+
+Metabola, 35
+
+Metamorphosis (in general), 6, 109;
+  (degrees of in insects) 8, 35, 109, 117-19
+
+Miall, L.C., 6, 28, 33, 43, 77, 78, 87, 97, 113
+
+Mosquito. _See_ Culex, Gnats
+
+Moths, 1, 58-62, 84, 95-100, 117
+
+Moult, 10, 32, 36, 41
+
+_Musca domestica_, 71
+
+Muscidae, 44
+
+Muscles, 47
+
+
+Nervous system, 44-5
+
+Neuroptera, 57, 80, 112
+
+Newport, G., 41, 44
+
+Noctuidae, 60, 98
+
+Nymph, 15, 28, 33
+
+
+Oak-apples, 94
+
+Obtect pupa, 81
+
+Odonata, 24.
+  _See also_ Dragon-flies
+
+_Oestrus ovis_, 91
+
+Oil-beetles, 56, 112
+
+_Orgyia antiqua_, 96-7
+
+Orthoptera, 17, 35, 110
+
+Owl Moths, 60, 98
+
+
+Packard, A.S., 56, 118
+
+Paedogenesis. _See_ Larval reproduction
+
+Painted Lady Butterfly, 96
+
+Palaeozoic insects, 107
+
+Palmen, J.A., 25
+
+Parasitic insects, 73-4, 108, 116
+
+Parental care, 64-6
+
+Parthenogenesis, 18
+
+Partial transformation, 35, 37
+
+Perla, 24
+
+Permian insects, 107
+
+Phagocytes, 48
+
+Phyllodecta, 53, 113
+
+Phyllotreta, 53
+
+_Pieris brassicae_, 39, 41, 85, 100
+
+_Pieris napi_ and var. _bryoniae_, 102-3
+
+Platygaster, 66
+
+Plecoptera, 24.
+  _See also_ Stone-flies
+
+Pompilidae, 66-7
+
+Poulton, E.B., 61, 82, 109
+
+Precis, 104
+
+Proctotrypidae, 66
+
+Pro-legs, 4, 58-9, 84, 114
+
+Pro-nymph, 118, 119
+
+Protective coloration, 60-1
+
+_Psylliodes chrysocephala_, 54
+
+Ptinidae, 54
+
+Pupa, 4, 37, 40, 79-88, 114, 117
+
+Puparium, 88
+
+Pupipara, 91
+
+_Pyrameis cardui_, 96
+
+
+Rat-tailed maggot, 76
+
+Reaumur, R.A.F. de, 8, 28, 33, 41
+
+Reproductive larvae, 90;
+  pupae, 91
+
+Reproductive organs, 45
+
+_Rhabdophaga heterobia_, 70
+
+Riley, C.V., 83
+
+
+Sanderson, E.D., 17
+
+Sand-midges, 78
+
+Sarcophaga, 91
+
+Saw-flies, 58-9
+
+Scale-insects, 20.
+  _See also_ Coccidae
+
+Scarabaeidae, 52
+
+Schmidt, E.O., 21
+
+Scolytidae, 55
+
+Scudder, S.H., 106
+
+Seasonal changes, 89-104
+
+Seasonal dimorphism, 102
+
+Semi-pupa, 118
+
+Sesiidae, 62
+
+Sexual differences, 15, 20-1, 90
+
+Sharp, D., 13, 36, 40, 115
+
+Silk-spinning, 58, 62-3, 82
+
+Silkworms, 82
+
+Silpha, 50
+
+Siltala, A.J., 63
+
+Silvestri, F., 119
+
+Simulium, 78, 87
+
+Smith, J.B., 17
+
+Sphegidae, 66-7
+
+Sphingidae, 60
+
+Spinneret, 58
+
+Spiracles, 2, 23, 70, 72, 77, 86, 87
+
+Spring-tails, 11
+
+Stone-flies, 24, 107, 110
+
+Sub-imago, 33, 117
+
+Sucking insects, 17
+
+Swammerdam, J., 33
+
+Syrphus, 74-6
+
+
+Tachininae, 73, 91
+
+_Tenebrio molitor_, 119
+
+Termitoxeniidae, 92
+
+Theobald, F.V., 100
+
+Thysanura, 11
+
+Tiger Moths, 59, 82, 98
+
+Timber-beetles, 54
+
+Tineidae, 62
+
+Tipulidae, 70
+
+Tortoiseshell Butterfly, 45, 95
+
+Tortricidae, 62
+
+Tracheal system. _See_ Air-tubes, Spiracles
+
+Transformation. _See_ Metamorphosis
+
+Triassic insects, 107
+
+Trichocera, 70
+
+Trichoptera, 62-3, 76, 80, 86
+
+Tsetse Flies, 91
+
+Turnip-fly, 53, 92, 94
+
+Turnip Moth, 98-9
+
+Tussock Moths, 90, 97
+
+
+_Vanessa urticae_, 45, 95
+
+Van Rees, J., 42
+
+Vapourer Moth, 96-7, 115
+
+_Velia currens_, 116
+
+Verhoeff, K.W., 11
+
+Vermiculiform larvae, 67, 71-6, 111
+
+Virgin stem-mothers, 18
+
+Viviparous reproduction. _See_ Birth
+
+
+Wagner, N., 90
+
+Warble-fly, 73-4, 89, 108
+
+Warning coloration, 60
+
+Wasmann, E., 92
+
+Wasps, 46, 64, 66-7, 83
+
+Water-insects. _See_ Aquatic insects
+
+Weevils, 55
+
+Weismann, A., 38, 42, 102
+
+White Butterflies, 41, 83, 85, 100-3
+
+Willow-beetles, 53
+
+Wingless insects, 15, 18, 20, 96, 115
+
+Wing-rudiments, 13, 18, 20, 22, 24, 28, 33, 36-8, 40, 111, 115, 117-19
+
+Wings, 1, 14, 115, 119-20
+
+Winter broods, 102-3
+
+Wintering stages, 93-101
+
+Wireworms, 52, 93
+
+Wood-wasps, 65
+
+
+
+
+CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS
+
+
+
+
+                                 THE
+                          CAMBRIDGE MANUALS
+                      OF SCIENCE AND LITERATURE
+
+             Published by the Cambridge University Press
+
+                           GENERAL EDITORS
+                          P. GILES, Litt.D.
+                      Master of Emmanuel College
+                                 and
+                      A.C. SEWARD, M.A., F.R.S.
+          Professor of Botany in the University of Cambridge
+
+                         70 VOLUMES NOW READY
+
+
+HISTORY AND ARCHAEOLOGY
+
+     Ancient Assyria. By Rev. C.H.W. Johns, Litt.D. Ancient Babylonia.
+     By Rev. C.H.W. Johns, Litt.D.
+
+     A History of Civilization in Palestine. By Prof. R.A.S. Macalister,
+     M.A., F.S.A.
+
+     China and the Manchus. By Prof. H.A. Giles, LL.D.
+
+     The Civilization of Ancient Mexico. By Lewis Spence.
+
+     The Vikings. By Prof. Allen Mawer, M.A.
+
+     New Zealand. By the Hon. Sir Robert Stout, K.C.M.G., LL.D., and J.
+     Logan Stout, LL.B. (N.Z.).
+
+     The Ground Plan of the English Parish Church. By A. Hamilton
+     Thompson, M.A., F.S.A.
+
+     The Historical Growth of the English Parish Church. By A. Hamilton
+     Thompson, M.A., F.S.A.
+
+     English Monasteries. By A.H. Thompson, M.A., F.S.A.
+
+     Brasses. By J.S.M. Ward, B.A., F.R.Hist.S.
+
+     Ancient Stained and Painted Glass. By F.S. Eden.
+
+
+ECONOMICS
+
+     Co-partnership in Industry. By C.R. Fay, M.A.
+
+     Cash and Credit. By D.A. Barker.
+
+     The Theory of Money. By D.A. Barker.
+
+
+LITERARY HISTORY
+
+     The Early Religious Poetry of the Hebrews. By the Rev. E.G. King,
+     D.D.
+
+     The Early Religious Poetry of Persia. By the Rev. Prof. J. Hope
+     Moulton, D.D., D.Theol. (Berlin).
+
+     The History of the English Bible. By John Brown, D.D.
+
+     English Dialects from the Eighth Century to the Present Day. By
+     W.W. Skeat, Litt.D., D.C.L., F.B.A.
+
+     King Arthur in History and Legend. By Prof. W. Lewis Jones, M.A.
+
+     The Icelandic Sagas. By W.A. Craigie, LL.D.
+
+     Greek Tragedy. By J.T. Sheppard, M.A.
+
+     The Ballad in Literature. By T.F. Henderson.
+
+     Goethe and the Twentieth Century. By Prof. J.G. Robertson, M.A.,
+     Ph.D.
+
+     The Troubadours. By the Rev. H.J. Chaytor, M.A.
+
+     Mysticism in English Literature. By Miss C.F.E. Spurgeon.
+
+
+PHILOSOPHY AND RELIGION
+
+     The Idea of God in Early Religions. By Dr F.B. Jevons.
+
+     Comparative Religion. By Dr F.B. Jevons.
+
+     Plato: Moral and Political Ideals. By Mrs A.M. Adam.
+
+     The Moral Life and Moral Worth. By Prof. Sorley, Litt.D.
+
+     The English Puritans. By John Brown, D.D.
+
+     An Historical Account of the Rise and Development of
+     Presbyterianism in Scotland. By the Rt Hon. the Lord Balfour of
+     Burleigh, K.T., G.C.M.G.
+
+     Methodism. By Rev. H.B. Workman, D.Lit.
+
+
+EDUCATION
+
+     Life in the Medieval University. By R.S. Rait, M.A.
+
+
+LAW
+
+     The Administration of Justice in Criminal Matters (in England and
+     Wales). By G. Glover Alexander, M.A., LL.M.
+
+
+BIOLOGY
+
+     The Coming of Evolution. By Prof. J.W. Judd, C.B., F.R.S.
+
+     Heredity in the Light of Recent Research. By L. Doncaster, M.A.
+
+     Primitive Animals. By Geoffrey Smith, M.A.
+
+     The Individual in the Animal Kingdom. By J.S. Huxley, B.A.
+
+     Life in the Sea. By James Johnstone, B.Sc.
+
+     The Migration of Birds. By T.A. Coward.
+
+     Spiders. By C. Warburton, M.A.
+
+     Bees and Wasps. By O.H. Latter, M.A.
+
+     House Flies. By C.G. Hewitt, D.Sc.
+
+     Earthworms and their Allies. By F.E. Beddard, F.R.S.
+
+     The Wanderings of Animals. By H.F. Gadow, F.R.S.
+
+
+ANTHROPOLOGY
+
+     The Wanderings of Peoples. By Dr A.C. Haddon, F.R.S.
+
+     Prehistoric Man. By Dr W.L.H. Duckworth.
+
+
+GEOLOGY
+
+     Rocks and their Origins. By Prof. Grenville A.J. Cole.
+
+     The Work of Rain and Rivers. By T.G. Bonney, Sc.D.
+
+     The Natural History of Coal. By Dr E.A. Newell Arber.
+
+     The Natural History of Clay. By Alfred B. Searle.
+
+     The Origin of Earthquakes. By C. Davison, Sc.D., F.G.S.
+
+     Submerged Forests. By Clement Reid, F.R.S.
+
+
+BOTANY
+
+     Plant-Animals: a Study in Symbiosis. By Prof. F.W. Keeble.
+
+     Plant-Life on Land. By Prof. F.O. Bower, Sc.D., F.R.S.
+
+     Links with the Past in the Plant-World. By Prof. A.C. Seward.
+
+
+PHYSICS
+
+     The Earth. By Prof. J.H. Poynting, F.R.S.
+
+     The Atmosphere. By A.J. Berry, M.A.
+
+     Beyond the Atom. By John Cox, M.A.
+
+     The Physical Basis of Music. By A. Wood, M.A.
+
+
+PSYCHOLOGY
+
+     An Introduction to Experimental Psychology. By Dr C.S. Myers.
+
+     The Psychology of Insanity. By Bernard Hart, M.D.
+
+
+INDUSTRIAL AND MECHANICAL SCIENCE
+
+     The Modern Locomotive. By C. Edgar Allen, A.M.I.Mech.E.
+
+     The Modern Warship. By E.L. Attwood.
+
+     Aerial Locomotion. By E.H. Harper, M.A., and Allan E. Ferguson,
+     B.Sc.
+
+     Electricity in Locomotion. By A.G. Whyte, B.Sc.
+
+     Wireless Telegraphy. By Prof. C.L. Fortescue, M.A.
+
+     The Story of a Loaf of Bread. By Prof. T.B. Wood, M.A.
+
+     Brewing. By A. Chaston Chapman, F.I.C.
+
+
+
+
+SOME VOLUMES IN PREPARATION
+
+
+HISTORY AND ARCHAEOLOGY
+
+     The Aryans. By Prof. M. Winternitz.
+
+     Ancient India. By Prof. E.J. Rapson, M.A.
+
+     The Peoples of India. By J.D. Anderson, M.A.
+
+     The Balkan Peoples. By J.D. Bourchier.
+
+     Canada of the present day. By C.G. Hewitt, D.Sc.
+
+     The Evolution of Japan. By Prof. J.H. Longford.
+
+     The West Indies. By Sir Daniel Morris, K.C.M.G.
+
+     The Royal Navy. By John Leyland.
+
+     Gypsies. By John Sampson.
+
+     A Grammar of Heraldry. By W.H. St John Hope, Litt.D.
+
+     Celtic Art. By Joseph Anderson, LL.D.
+
+
+ECONOMICS
+
+     Women's Work. By Miss Constance Smith.
+
+
+LITERARY HISTORY
+
+     Early Indian Poetry. By A.A. Macdonell.
+
+     The Book. By H.G. Aldis, M.A.
+
+     Pantomime. By D.L. Murray.
+
+     Folk Song and Dance. By Miss Neal and F. Kidson.
+
+
+PHYSICS
+
+     The Natural Sources of Energy. By Prof. A.H. Gibson, D.Sc.
+
+     The Sun. By Prof. R.A. Sampson.
+
+     Roentgen Rays. By Prof. W.H. Bragg, F.R.S.
+
+
+BIOLOGY
+
+     The Life-story of Insects. By Prof. G.H. Carpenter.
+
+     The Flea. By H. Russell.
+
+     Pearls. By Prof. W.J. Dakin.
+
+
+GEOLOGY
+
+     Soil Fertility. By E.J. Russell, D.Sc.
+
+     Coast Erosion. By Prof. T.J. Jehu.
+
+
+INDUSTRIAL AND MECHANICAL SCIENCE
+
+     Coal Mining. By T.C. Cantrill.
+
+     Leather. By Prof. H.R. Procter.
+
+
+
+
+                      Cambridge University Press
+                          C.F. Clay, Manager
+                      London: Fetter Lane, E.C.
+                    Edinburgh: 100, Princes Street
+
+
+
+
+
+End of Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter
+
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