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+The Project Gutenberg eBook, On the Origin and Metamorphoses of Insects,
+by Sir John Lubbock
+
+
+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: On the Origin and Metamorphoses of Insects
+
+
+Author: Sir John Lubbock
+
+
+
+Release Date: December 3, 2011  [eBook #38207]
+
+Language: English
+
+Character set encoding: UTF-8
+
+
+***START OF THE PROJECT GUTENBERG EBOOK ON THE ORIGIN AND METAMORPHOSES OF
+INSECTS***
+
+
+E-text prepared by Bryan Ness, Turgut Dincer, and the Online Distributed
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+      +--------------------------------------------------------------+
+      | Transcriber's note:                                          |
+      |                                                              |
+      | All non-italic genus names in the text have been italicized. |
+      +--------------------------------------------------------------+
+
+
+
+
+
+[Illustration: NATURE SERIES]
+
+
+
+
+ORIGIN AND METAMORPHOSES OF INSECTS.
+
+[Illustration]
+
+Nature Series
+
+ON THE ORIGIN AND METAMORPHOSES OF INSECTS
+
+BY
+
+Sir John Lubbock, Bart., M.P., F.R.S., D.C.L., LL.D.
+
+Principal of the London Working Men's College; President of the London
+Chamber of Commerce; and Vice-Chairman of the London County Council
+
+With Numerous Illustrations
+
+
+
+
+
+
+
+London Macmillan and Co. and New York 1890
+
+The Right of Translation and Reproduction is Reserved
+
+Richard Clay and Sons, Limited, London and Bungay.
+
+First Edition 1873. Reprinted 1874. New Edition 1890.
+
+
+
+
+PREFACE.
+
+
+For some years, much of my leisure time has been devoted to the study of
+the anatomy, development, and habits of the Annulosa, and especially of
+Insects, on which subjects I have published various memoirs, chiefly in
+the Transactions of the Royal, Linnæan, and Entomological Societies: of
+these papers I subjoin a list. Although the details, of which these
+memoirs necessarily for the most part consist, offer little interest,
+excepting to those persons who are specially devoted to Entomology,
+still there are portions which, having reference to the nature of
+metamorphoses and to the origin of insects, are of a more general
+character. I have also briefly referred to these questions in a
+Monograph of the Collembola and Thysanura, recently published by the Ray
+Society, and in the Opening Address to the Biological Section of the
+British Association at Brighton in 1872. Under these circumstances, it
+has been suggested to me that a small volume, containing, at somewhat
+greater length, in a more accessible form, and with the advantage of
+illustrations, the conclusions to which I have been led on this
+interesting subject, might not be altogether without interest to the
+general reader. The result, which has already appeared in the pages of
+_Nature_, is now submitted to the public, with some additions. I am well
+aware that it has no pretence to be in any sense a complete treatise;
+that the subject itself is one as to which our knowledge is still very
+incomplete, and on which the highest authorities are much divided in
+opinion. Whatever differences of opinion, however, there may be as to
+the views here put forward, the facts on which they are based will, I
+believe, be found correct. On this point I speak with the more
+confidence, on account of the valuable assistance I have received from
+many friends: to Mr. and Mrs. Busk and Dr. Hooker I am especially
+indebted.
+
+The papers above referred to are as follows:—
+
+   1. _On Labidocera._—Annals and Magazine of Natural History,
+        vol. xi., 1853.
+
+   2. On Two New Sub-genera of Calanidæ.—Annals and Magazine of
+        Natural History, vol. xii., 1853.
+
+   3. On Two New Species of Calanidæ.—Annals and Magazine of
+        Natural History, vol. xii., No. lxvii., 1853.
+
+   4. On Two New Species of Calanidæ.—Annals and Magazine of
+        Natural History, vol. xii., No. lxix., 1853.
+
+   5. On some Arctic Calanidæ.—Annals and Magazine of Natural
+        History, 1854.
+
+   6. On the Freshwater Entomostraca of South
+        America.—Transactions of the Entomological Society, vol.
+        iii., 1855.
+
+   7. On some New Entomostraca.—Transactions of the
+        Entomological Society, vol. iv., 1856.
+
+   8. On some Marine Entomostraca found at Weymouth.—Annals and
+        Magazine of Natural History, vol. xx., 1857.
+
+   9. On the Respiration of Insects.—Entomological Annual, 1857.
+
+  10. An Account of the Two Methods of Reproduction in
+         _Daphnia_.—Transactions of the Royal Society, 1857.
+
+  11. On the Ova and Pseudova of Insects.—Transactions of the
+        Royal Society, 1858.
+
+  12. On the Arrangement of the Cutaneous Muscles of _Pygæra
+        Bucephala_.—Linnean Society’s Transactions, vol. xxii., 1858.
+
+  13. On the Freshwater Entomostraca of South
+        America.—Entomological Society’s Transactions, 1858.
+
+  14. On _Coccus Hesperidum_.—Royal Society Proceedings, vol.
+        ix., 1858.
+
+  15. On the Distribution of Tracheæ in Insects.—Linnean
+        Society’s Transactions, vol. xxiii., 1860.
+
+  16. On the Generative Organs and on the Formation of the Egg
+        in Annulosa. Transactions of the Royal Society, 1861.
+
+  17. On _Sphærularia Bombi._—Natural History Review, 1861.
+
+  18. On some Oceanic Entomostraca.—Linnean Society’s
+        Transactions, vol. xxiii., 1860.
+
+  19. On the Thysanura. Part 1.—Linnean Society’s Transactions,
+        1862.
+
+  20. On the Development of Lonchoptera.—Entomological
+        Society’s Transactions, 1862.
+
+  21. On the Thysanura. Part 2.—Linnean Society’s Transactions,
+        1862.
+
+  22. On the Development of Chloëon. Part 1.—Linnean Society’s
+        Transactions, 1863.
+
+  23. On Two Aquatic Hymenoptera.—Linnean Society’s
+        Transactions, 1863.
+
+  24. On some little-known Species of Freshwater
+        Entomostraca.—Linnean Society’s Transactions, vol. xxiv.,
+        1863.
+
+  25. On _Sphærularia Bombi_.—Natural History Review, 1864.
+
+  26. On the Development of Chloëon. Part 2.—Linnean Society’s
+        Transactions, 1865.
+
+  27. Metamorphoses of Insects.—Journal of the Royal
+        Institution, 1866.
+
+  28. On _Pauropus_.—Linnean Society’s Transactions, 1866.
+
+  29. On the Thysanura. Part 3.—Linnean Society’s Transactions,
+        1867.
+
+  30. Address to the Entomological Society.—Entomological
+        Society’s Transactions, 1867.
+
+  31. On the Larva of Micropeplus Staphilinoides.—Entomological
+        Society’s Transactions, 1868.
+
+  32. On the Thysanura. Part 4.—Linnean Society’s Transactions,
+        1869.
+
+  33. Addresses to the Entomological Society.—Entomological
+        Society’s Transactions, 1867-1868.
+
+  34. On the Origin of Insects.—Journal of the Linnean Society,
+        vol. xi.
+
+  35. Opening Address to the Biological Section of the British
+        Association.—British Association Report, 1872.
+
+  36. Observations on Ants, Bees, and Wasps. Part 1.—Journal of
+        the Linnean Society, 1873.
+
+  37. On British Wild Flowers considered in relation to Insects,
+        1874.
+
+  38. Observations on Ants, Bees, and Wasps. Part 2.—Journal of
+        the Linnean Society, 1874.
+
+  39. Observations on Ants, Bees, and Wasps. Part 3.—Journal of
+        the Linnean Society, 1875.
+
+  40. Observations on Ants, Bees, and Wasps. Part 4.—Journal of
+        the Linnean Society, 1877.
+
+  41. On some Points in the Anatomy of Ants.—Quekett Lecture,
+        1877.—Microscopical Journal.
+
+  42. On the Colors of Caterpillars.—Entomological Society’s
+        Transactions, 1878.
+
+  43. Observations on Ants, Bees, and Wasps. Part 5.—Journal of
+        the Linnean Society, 1878.
+
+  44. Observations on Ants, Bees, and Wasps. Part 6.—Journal of
+        the Linnean Society, 1879.
+
+  45. On the Anatomy of Ants.—Linnean Society’s Transactions,
+        1880.
+
+  46. Observations on Ants, Bees, and Wasps. Part 7.—Journal of
+        the Linnean Society, 1880.
+
+  47. Observations on Ants, Bees, and Wasps. Part 8.—Journal of
+        the Linnean Society, 1881.
+
+  48. On Fruits and Seeds.—Journal of the Royal Institution,
+        1881.
+
+  49. Observations on Ants, Bees, and Wasps. Part 9.—Journal of
+        the Linnean Society, 1881.
+
+  50. On the Limits of Vision among some of the lower
+        Animals.—Journal of the Linnean Society, 1881.
+
+  51. Observations on Ants, Bees, and Wasps. Part 10.—Journal
+        of the Linnean Society, 1882.
+
+
+
+
+CONTENTS.
+
+
+  CHAPTER I.
+
+  THE CLASSIFICATION OF INSECTS.
+
+  Introduction.—Stages in the Life of an Insect.—Classification
+    of Insects.—Characters derived from the Wings; from the parts
+    of the Mouth; from the Metamorphoses.—The Classes of Insects:
+    Hymenoptera, Strepsiptera, Coleoptera, Euplexoptera,
+    Orthoptera, Thysanoptera, Neuroptera, Trichoptera, Diptera,
+    Aphaniptera, Heteroptera, Homoptera, Lepidoptera          _page_ 1-26
+
+
+  CHAPTER II.
+
+  THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE
+  OF LARVÆ.
+
+  Larvæ depend partly on the group to which they
+    belong.—Wood-eating Larvæ.—Larvæ of Lamellicorns.—Larvæ
+    depend also in part on mode of life.—Larvæ of Hymenoptera, of
+    _Sirex_; of _Tenthredo_; of Ichneumons; of Bees.—Rudimentary
+    legs of Bee Embryo.—Beetles, _Weevils_, _Scolytus_,
+    _Crioceris_, _Sitaris_, Metamorphoses of Pteromalidæ.
+    _Platygaster_, _Polynema_.—Influence of external
+    conditions.—Developmental and adaptive Metamorphoses    _page_ 27-41
+
+
+  CHAPTER III.
+
+  THE NATURE OF METAMORPHOSES.
+
+  The life history of an Insect must be considered as a
+    whole.—Vagueness of the term Larva.—Some larvæ much more
+    advanced than others.—Organs develope in different order, in
+    different groups.—Suppressed stages.—Apod condition of
+    _Phryganea_; of _Aphis_; of
+    _Chrysopa_.—Libellulidæ.—_Donacia_.—Spiders.—Myriapods.—Apod
+    stage of Homomorphous Insects once probably longer than
+    now.—Suppression of embryonic stages.—Metamorphoses of
+    Hydroida, Crustacea, Isopods, and
+    Amphipods.—Echinoderms.—Variations in development induced by
+    the influence of external conditions.                    _page_ 41-62
+
+
+  CHAPTER IV.
+
+  THE ORIGIN OF METAMORPHOSES.
+
+  Origin of Metamorphoses.—Views of Messrs. Kirby and
+    Spence.—Nature of the question.—Young animals often more
+    similar than mature.—Views of Darwin, Herbert Spencer,
+    Johannes Müller, Fritz Müller, and Agassiz.—Effect of size of
+    egg.—Insects leave the egg in a more or less developed
+    condition.—Consideration of pupal condition.—Quiescence of
+    pupa.—Period of quiescence at each moult.—Changes not so
+    abrupt as generally supposed.—Change in
+    mouth-parts.—Difficulty in reference to Darwinian
+    theory.—Mouth-parts of _Campodea_ and Collembola, as
+    intermediate between the mandibulate and haustellate
+    types.—Change in mouth-parts as connected with pupal
+    conditions.—Origin of wings.—Use of wings under
+    water.—Connection of metamorphoses with alternation of
+    generations.—Parthenogenetic larvæ of _Cecidomyia_.—In
+    alternation of generations one form always agamic.—Dimorphism
+    and Dieidism.—Summary and Conclusions                   _page_ 62-81
+
+
+  CHAPTER V.
+
+  THE ORIGIN OF INSECTS.
+
+  The Origin of Insects.—Mistaken views of Darwinian
+    theory.—Natural selection a _vera causa_.—Application of
+    Darwin’s views to Insects.—Similarity of young Crustacea as
+    compared with mature forms; ditto in Insects.—Type of
+    Insecta.—Two principal types of larvæ: Hexapod and
+    Apod.—Conclusions to be drawn from them.—_Campodea_ the modern
+    representative of the Insect-stock.—_Campodea_, perhaps
+    derived from Tardigrade.—Vermiform or Apod type of
+    larva.—Views of Fritz Müller, Brauer, and Packard.—Represents
+    a still earlier ancestor.—Modern representatives.—_Notommata_,
+    _Albertia_, _Lindia_.—Earlier forms difficult to trace.—Lowest
+    forms of animal life.—Yolk-segmentation.—Embryology and
+    Evolution.—Light thrown by the evolution of the individual on
+    that of the species                                     _page_ 82-108
+
+  DESCRIPTION OF THE PLATES.
+
+
+  PLATE I. p. 7.
+
+  FIG.
+
+  1. Cricket. Westwood, Intro. to the Modern Classification of
+       Insects, vol. i. p. 440.
+
+  2. Earwig. Westwood, loc. cit. vol. i. p. 399.
+
+  3. _Aphis_. Packard, Guide to the Study of Insects, pp. 521, 522.
+
+  4. _Scolytus_. Westwood, loc. cit. vol. i. p. 350.
+
+  5. _Anthrax_. Westwood, loc. cit. vol. ii. p. 538.
+
+  6. _Balaninus_.
+
+  7. _Cynips_. Westwood, loc. cit. vol. ii. p. 121.
+
+  8. Ant (_Formica_). Westwood, loc. cit. vol. ii. p. 218.
+
+  9. Wasp. Ormerod, Nat. Hist. of Wasps, pl. i. fig. 1.
+
+
+  PLATE II. p. 8.
+
+  FIG.
+
+  1. Larva of Cricket. Westwood, loc. cit. vol. i. p. 440.
+
+  2. Larva of _Aphis_. Packard, loc. cit. pp. 521, 522.
+
+  3. Larva of Earwig. Westwood, loc. cit. vol. i. p. 399.
+
+  4. Larva of _Scolytus_. Westwood, loc. cit. vol. i. p. 350.
+
+  5. Larva of _Anthrax_. Westwood, loc. cit. vol. ii. p. 546.
+
+  6. Larva of _Balaninus_.
+
+  7. Larva of _Cynips_. Westwood, loc. cit. vol. ii. p. 121.
+
+  8. Larva of Ant (_Formica_). Westwood, loc. cit. vol. ii. p. 226.
+
+  9. Larva of Wasp. Newport, Art. Insecta, Todd’s Cycl. Anat. and
+       Phys., p. 871.
+
+
+  PLATE III. p. 14.
+
+  FIG.
+
+  1. _Chloëon_. Linn. Trans. 1866.
+
+  2. _Meloë_. Spry and Shuckard, Coleoptera Delineated, pl. 56.
+
+  3. _Calepteryx_.
+
+  4. _Sitaris_. Spry and Shuckard, loc. cit. pl. 56.
+
+  5. _Campodea_. Suites à Buffon. Aptéres.
+
+  6. _Acilius_. Westwood, loc. cit. vol. i. p. 100.
+
+  7. _Termes_. Westwood, loc. cit. vol. ii. p. 12.
+
+  8. _Stylops_. Duncan, Met. of Insects, p. 387; Packard, p. 482.
+
+  9. _Thrips_. Westwood, loc. cit. vol. ii. p. 1.
+
+
+  PLATE IV. p. 15.
+
+  FIG.
+
+  1. Larva of _Chloëon_. Linn. Trans. 1863.
+
+  2. Larva of _Meloë_. Chapuis and Candèze, Mem. Soc. Roy. Liége,
+       1853, pp. 1, 7.
+
+  3. Larva of _Calepteryx_. Dufour, Ann. Sci. Nat. 1852.
+
+  4. Larva of _Sitaris_. Duncan, Met. of Insects, p. 309.
+
+  5. Larva of _Campodea_. Gervais’ Suites à Buffon. Aptéres.
+
+  6. Larva of _Acilius_. Westwood, loc. cit. vol. i. p. 100.
+
+  7. Larva of _Termes_. Duncan, loc. cit. p. 348.
+
+  8. Larva of _Stylops_. Westwood, Trans. Ent. Soc. 1839, vol. ii.
+       pl. xv. fig. 13a.
+
+  9. Larva of _Thrips_. Westwood, loc. cit. vol. ii. p. i.
+
+
+  PLATE V. p. 99.
+
+  FIG.
+
+  1-5. _Protamœba_.
+
+  6-9. _Protamyxa aurantiaca_. Haeckel Beit. zur. Monog. der
+         Moneren, pl. 1.
+
+  10-18. _Magosphœra planula_. Haeckel, loc. cit. pl. v.
+
+
+  PLATE VI. p. 105.
+
+  FIG.
+
+  1-4. Yolk-segmentation in _Laomedea_. After Allman. Mon. of
+         Tubularian Hydroids. Ray Society.
+
+  5-9. Yolk-segmentation in _Filaria_. After Van Beneden. Mem. sur
+         les Vers Intestinaux.
+
+  10-13. Yolk-segmentation in _Echinus_. After Derbès. Ann. des.
+           Sci. Nat. 1847.
+
+  14-17. Yolk-segmentation in _Lacinularia_. After Huxley. J. of
+           Mic. Sci. 1853.
+
+  18-21. Yolk-segmentation in _Purpura_. After Koren and
+           Danielssen. Ann. des. Sci. Nat. 1853.
+
+  22-24. Yolk-segmentation in _Amphioxus_. After Haeckel.
+           Naturliche Schöpfungsgeschichte, pl. x.
+
+  25-29. Yolk-segmentation in Vertebrate. After Allen Thompson.
+           Art. Ovum. Cyclop. of Anatomy and Physiology.
+
+
+  DESCRIPTION OF THE FIGURES.
+
+
+  FIG. 1. Larva of the Cockchafer (_Melolontha_)
+
+   2. Larva of _Cetonia_.
+
+   3. Larva of _Trox_.
+
+   4. Larva of _Oryctes_.
+
+   5. Larva of _Aphodius_.
+
+   6. Larva of _Lucanus_.
+
+   7. Larva of _Brachytarsus_.
+
+   8. Larva of _Crioceris_.
+
+   9. Larva of _Sitaris humeralis_.
+
+  10. Larva of _Sitaris humeralis_, in the second stage.
+
+  11. Larva of _Sitaris humeralis_, in the third stage.
+
+  12. Larva of _Sitaris humeralis_, in the fourth stage.
+
+  13. Pupa of _Sitaris_.
+
+  14. Larva of _Sirex_.
+
+  15. Egg of _Rhynchites_, showing the parasitic larva.
+
+  16. The parasitic larva, more magnified.
+
+  17. Egg of _Platygaster_.
+
+  18. Egg of _Platygaster_, showing the central cell.
+
+  19. Egg of _Platygaster_, after the division of the central cell.
+
+  20. Egg of _Platygaster_, more advanced.
+
+  21. Egg of _Platygaster_, more advanced.
+
+  22. Egg of _Platygaster_, showing the rudiment of the embryo.
+
+  23. Larva of _Platygaster_.—_mo_, mouth; _a_, antenna; _kf_,
+        hooked feet; _r_, toothed process; _lfg_, lateral
+        process; _f_, branches of the tail.
+
+  24. Larva of another species of _Platygaster_. (The letters
+        indicate the same parts as in the preceding figure.)
+
+  25. Larva of a third species of _Platygaster_. (The letters
+        indicate the same parts as in the preceding figure.)
+
+  26. Larva of _Platygaster_ in the second stage.—_mo_, mouth;
+        _slkf_, œsophagus; _gsae_, supra-œsophagal ganglion;
+        _lm_, muscles; _bsm_, nervous system; _gagh_, rudiments
+        of the reproductive glands.
+
+  27. Larva of _Platygaster_ in the third stage.—_mo_, mouth;
+        _ma_, mandibles; _gsae_, supra-œsophagal ganglion; _slk_,
+        œsophagus; _ag_, ducts of the salivary glands; _bnm_,
+        ventral nervous system; _sp_, salivary glands; _msl_,
+        stomach; _im_, imaginal discs; _tr_, tracheæ; _fk_, fatty
+        tissue; _ed_, intestine; _ga_, rudiments of reproductive
+        organs; _ew_, wider portion of intestine; _ao_, posterior
+        opening.
+
+  28. Embryo of _Polynema_.
+
+  29. Larva of _Polynema_.—_asch_, rudiments of the antennæ;
+        _flsch_, of the wings; _bsch_, of the legs; _vfg_,
+        lateral projections; _gsch_, rudiments of the ovipositor;
+        _fk_, fatty tissue.
+
+  30. Egg of _Phryganea_ (Mystacides).—_A_¹, mandibular segment;
+        _C_¹-_C_⁵, maxillary, labial, and three thoracic
+        segments; _D_, abdomen.
+
+  31. Egg of _Phryganea_ somewhat more advanced.—_b_, mandibles;
+        _c_, maxillæ; _cfs_, rudiments of the three pairs of
+        legs.
+
+  32. Egg of _Pholcus opilionides_, showing the Protozonites.
+
+  33. Embryo of _Julus_.
+
+  34. Colony of _Bougainvillea fruticosa_, natural size, attached
+        to the underside of a piece of floating timber.
+
+  35. Portion of the same, more magnified.
+
+  36. The Medusa from the same species.
+
+  37. Larva of Prawn, Nauplius stage.
+
+  38. Larva of Prawn, more advanced, Zoëa stage.
+
+  39. Larva of Echino-cidaris œquituberculata seen from above ×
+        6/10.
+
+  40. Larva of _Echinus_ × 100.—_A_, front arm; _F_, arms of the
+        mouth-process; _B_, posterior side arm; _E_¹, accessory
+        arm of the mouth-process; _a_, mouth; _a_¹, œsophagus;
+        _b_, stomach; _b_¹, intestine; _o_, posterior orifice;
+        _d_, ciliated bands; _f_, ciliated epaulets; _c_, disc of
+        future _Echinus_.
+
+  41. _Comatula rosacea_.
+
+  42. Larva of _Comatula rosacea_.
+
+  43. Larva of _Comatula rosacea_, more advanced.
+
+  44. Larva of _Comatula rosacea_, in the Pentacrinus state.
+
+  45. Larva of Starfish (Bipinnaria), × 100.
+
+  46. Larva of Starfish (Bipinnaria), × 100, seen from the
+  side.—_a_, mouth; _b_, œsophagus; _c_, stomach; _c_¹,
+  intestine.
+
+  47. Larva of another Bipinnaria, showing the commencement of
+  the Starfish.—_g_, canal of the ciliated sac; _i_, rudiments
+  of tentacles; _d_, ciliated band.
+
+  48. Larva of Moth (_Agrotis_).
+
+  49. Larva of Beetle (_Haltica_).
+
+  50. Larva of Saw-fly (_Cimbex_).
+
+  51. Larva of _Julus_.
+
+  52. _Agrotis suffusa_.
+
+  53. _Haltica_.
+
+  54. _Cimbex_.
+
+  55. _Julus_.
+
+  56. Tardigrade.
+
+  57. Larva of _Cecidomyia_.
+
+  58. _Lindia torulosa_.
+
+  59. _Prorhynchus stagnalis_.
+
+  60. Egg of Tardigrade.
+
+  61. Egg of Tardigrade, after the yolk has subdivided.
+
+  62. Egg of Tardigrade, in the next stage.
+
+  63. Egg of Tardigrade, more advanced.
+
+
+
+
+
+ON THE ORIGIN AND METAMORPHOSES OF INSECTS.
+
+
+
+
+CHAPTER I.
+
+  _THE CLASSIFICATION OF INSECTS._
+
+
+About forty years ago the civil and ecclesiastical authorities of St.
+Fernando in Chili arrested a certain M. Renous on a charge of
+witchcraft, because he kept some caterpillars which turned into
+butterflies.[1] This was no doubt an extreme case of ignorance; it is
+now almost universally known that the great majority of insects quit the
+egg in a state very different from that which they ultimately assume;
+and the general statement in works on entomology has been that the life
+of an insect may be divided into four periods.
+
+Thus, according to Kirby and Spence,[2] “The states through which
+insects pass are four: the _egg_, the _larva_, the _pupa_, and the
+_imago_.” Burmeister,[3] also, says that, excluding certain very rare
+anomalies, “we may observe four distinct periods of existence in every
+insect,—namely, those of the egg, the larva, the pupa, and the imago,
+or perfect insect.” In fact, however, the various groups of insects
+differ widely from one another in the metamorphoses they pass through:
+in some, as in the grasshoppers and crickets, the changes consist
+principally in a gradual increase of size, and in the acquisition of
+wings; while others, as for instance the common fly, acquire their full
+bulk in a form very different from that which they ultimately assume,
+and pass through a period of inaction in which not only is the whole
+form of the body altered, not only are legs and wings acquired, but even
+the internal organs themselves are almost entirely disintegrated and
+re-formed. It will be my object, after having briefly described these
+changes, to throw some light on the causes to which they are due, and on
+the indications they afford of the stages through which insects have
+been evolved.
+
+The following list gives the orders or principal groups into which the
+Class Insecta may be divided. I will not, indeed, here enter upon my own
+views, but will adopt the system given by Mr. Westwood in his excellent
+“Introduction to the Modern Classification of Insects,” from which also,
+as a standard authority, most of the figures on Plates I. to IV., when
+not otherwise acknowledged, have been taken. He divides insects into
+thirteen groups, and with reference to eight of them it may be said that
+there is little difference of opinion among entomologists. These orders
+are by far the most numerous, and I have placed them in capital
+letters. As regards the other five there is still much difference of
+opinion. It must also be observed that Prof. Westwood omits the
+parasitic Anoplura, as well as the Thysanura and Collembola.
+
+
+ORDERS OF INSECTS ACCORDING TO WESTWOOD.
+
+   1. HYMENOPTERA   Bees, Wasps, Ants, &c.
+   2. STREPSIPTERA  _Stylops_, _Zenos_, &c.
+   3. COLEOPTERA    Beetles.
+   4. EUPLEXOPTERA  Earwigs.
+   5. ORTHOPTERA    Grasshoppers, Crickets, Cockroaches, &c.
+   6. THYSANOPTERA  _Thrips_.
+   7. NEUROPTERA    _Ephemeras_, &c.
+   8. TRICHOPTERA   _Phryganea_.
+   9. DIPTERA       Flies and Gnats.
+  10. APHANIPTERA   Fleas.
+  11. HETEROPTERA   Bugs.
+  12. HOMOPTERA     _Aphis_, _Coccus_, &c.
+  13. LEPIDOPTERA   Butterflies and Moths.
+
+Of these thirteen orders, the eight which I have placed in capital
+letters—namely the first, third, fifth, seventh, ninth, eleventh,
+twelfth, and thirteenth, are much the most important in the number and
+variety of their species; the other five form comparatively small
+groups. The Strepsiptera are minute insects, parasitic on Hymenoptera:
+Rossi, by whom they were discovered, regarded them as Hymenopterous;
+Lamarck placed them among the Diptera; by others they have been
+considered to be most closely allied to the Coleoptera, but they are now
+generally treated as an independent order.
+
+The Euplexoptera or Earwigs are only too familiar to most of us. Linnæus
+classed them among the Coleoptera, from which, however, they differ in
+their transformations. Fabricius, Olivier, and Latreille regarded them
+as Orthoptera; but Dr. Leach, on account of the structure of their
+wings, considered them as forming the type of a distinct order, in which
+view he has been followed by Westwood, Kirby, and many other
+entomologists.
+
+The Thysanoptera, consisting of the Linnæan genus _Thrips_, are minute
+insects well known to gardeners, differing from the Coleoptera in the
+nature of their metamorphoses, in which they resemble the Orthoptera and
+Hemiptera. The structure of the wings and mouth-parts, however, are
+considered to exclude them from these two orders.
+
+The Trichoptera, or Caddis worms, offer many points of resemblance to
+the Neuroptera, while in others they approach more nearly to the
+Lepidoptera. According to Westwood, the genus _Phryganea_ “forms the
+connecting link between the Neuroptera and Lepidoptera.”
+
+The last of these small aberrant orders is that of the Aphaniptera,
+constituted for the family Pulicidæ. In their transformations, as in
+many other respects, they closely resemble the Diptera. Strauss
+Durckheim indeed said that “_la puce est un diptère sans ailes_.”
+Westwood, however, regards it as constituting a separate order.
+
+As indicated by the names of these orders, the structure of the wings
+affords extremely natural and convenient characters by which the various
+groups may be distinguished from one another. The mouth-parts also are
+very important; and, regarded from this point of view, the Insecta have
+been divided into two series—the Mandibulata and Haustellata, or
+mandibulate and suctorial groups, between which, as I have elsewhere
+shown,[4] the Collembola (_Podura_, _Smynthurus_, &c.) occupy an
+intermediate position. These two series are:—
+
+MANDIBULATA.
+
+  Hymenoptera.
+  Strepsiptera.
+  Coleoptera.
+  Euplexoptera.
+  Orthoptera.
+  Trichoptera?
+  Thysanoptera?
+
+HAUSTELLATA.
+
+  Lepidoptera.
+  Diptera.
+  Aphaniptera.
+  Hemiptera.
+  Homoptera.
+
+Again—and this is the most important from my present point of
+view—insects have sometimes been divided into two other series,
+according to the nature of their metamorphoses: “Heteromorpha,” to use
+the terminology of Prof. Westwood,[5] “or those in which there is no
+resemblance between the parent and the offspring; and Homomorpha, or
+those in which the larva resembles the imago, except in the absence of
+wings. In the former the larva is generally worm-like, of a soft and
+fleshy consistence, and furnished with a mouth, and often with six short
+legs attached in pairs to the three segments succeeding the head. In the
+Homomorpha, including the Orthoptera, Hemiptera, Homoptera, and certain
+Neuroptera, the body, legs, and antennæ are nearly similar in their form
+to those of the perfect insect, but the wings are wanting.”
+
+HETEROMORPHA.
+
+  Hymenoptera.
+  Strepsiptera.
+  Coleoptera.
+  Trichoptera.
+  Diptera.
+  Aphaniptera.
+  Lepidoptera.
+
+HOMOMORPHA.
+
+  Euplexoptera.
+  Orthoptera.
+  Hemiptera.
+  Homoptera.
+  Thysanoptera.
+  Neuroptera.
+
+But though the Homomorphic insects do not pass through such striking
+changes of form as the Heteromorphic, and are active throughout life,
+still it was until within the last few years generally (though
+erroneously) considered, that in them, as in the Heteromorpha, the life
+fell into four distinct periods; those of (1) the egg, (2) the larva,
+characterized by the absence of wings, (3) the pupa with imperfect
+wings, and (4) the imago, or perfect insect.
+
+I have, however, elsewhere[6] shown that there are not, as a matter of
+fact, four well-marked stages, and four only, but that in many cases the
+process is much more gradual.
+
+The species belonging to the order Hymenoptera are among the most
+interesting of insects. To this order belong the gallflies, the
+sawflies, the ichneumons, and, above all, the ants and bees. We are
+accustomed to class the Anthropoid apes next to man in the scale of
+creation, but if we were to judge animals by their works, the chimpanzee
+and the gorilla must certainly give place to the bee and the ant. The
+larvæ of the sawflies, which live on leaves, and of the Siricidæ or
+long-tailed wasps, which feed on wood, are very much like caterpillars,
+having three pairs of legs, and in the former case abdominal pro-legs
+as well: but in the great majority of Hymenoptera the larvæ are legless,
+fleshy grubs (Plate II., Figs. 7-9); and the various modes by which the
+females provide for, or secure to, them a sufficient supply of
+appropriate nourishment constitutes one of the most interesting pages of
+Natural History.
+
+The species of Hymenoptera are very numerous; in this country alone
+there are about 3,000 kinds, most of which are very small. In the pupa
+state they are inactive, and show distinctly all the limbs of the
+perfect insect, encased in distinct sheaths, and folded on the breast.
+In the perfect state they are highly organized and very active. The
+working ants and some few species are wingless, but the great majority
+have four strong membranous wings, a character distinguishing them at
+once from the true flies, which have only one pair of wings.
+
+The saw-flies are so called because they possess at the end of the body a
+curious organ, corresponding to the sting of a wasp, but which is in the
+form of a fine-toothed saw. With this instrument the female sawfly cuts
+a slit in the stem or leaf of a plant, into which she introduces her
+egg. The larva much resembles a caterpillar, both in form and habits. To
+this group belongs the nigger, or black caterpillar of the turnip, which
+is often in sufficient numbers to do much mischief. Some species make
+galls, but the greater number of galls are formed by insects of another
+family, the Cynipidæ.
+
+[Illustration: PLATE I.[7]—MATURE INSECTS.
+
+Fig. 1, Cricket; 2, Earwig; 3, _Aphis_; 4, _Scolytus_; 5, Anthrax;
+6, _Balaninus_; 7, _Cynips_; 8, Ant; 9, Wasp.]
+
+[Illustration: PLATE II.—LARVÆ OF THE INSECTS REPRESENTED ON PLATE I.
+
+Fig. 1, Larva of Cricket; 2, Larva of Aphis; 3, Larva of Earwig; 4,
+Larva of _Scolytus_ (Beetle); 5, Larva of _Anthrax_ (Fly); 6, Larva of
+_Balaninus_ (Nut Weevil); 7, Larva of _Cynips_; 8, Larva of Ant;
+9, Larva of Wasp.]
+
+In the Cynipidæ (Plate I., Fig. 7) the female is provided with an organ
+corresponding to the saw of the sawfly, but resembling a needle. With
+this she stings or punctures the surface of leaves, buds, stalks, or
+even roots of various plants. In the wound thus produced she lays one or
+more eggs. The effects of this proceeding, and particularly of the
+irritating fluid which she injects into the wound, is to produce a
+tumour or gall, within which the egg hatches, and on which the larva, a
+thick fleshy grub (Plate II., Fig. 7), feeds. In some species each gall
+contains a single larva; in others, several live together.
+
+The oak supports several kinds of gallflies: one produces the well-known
+oak-apple, one a small swelling on the leaf resembling a currant,
+another a gall somewhat like an acorn, another attacks the root; the
+species making the bullet-like galls, which are now so common, has only
+existed for a few years in this country; the beautiful little spangles
+so common in autumn on the under side of oak leaves are the work of
+another species, the _Cynus longipennis_. One curious point about this
+group is, that in some of the commonest species the females alone are
+known, no one yet having ever succeeded in finding a male.
+
+Another great family of the Hymenoptera is that of the ichneumons; the
+females lay their eggs either in or on other insects, within the bodies
+of which the larvæ live. These larvæ are thick, fleshy, legless grubs,
+and feed on the fatty tissues of their hosts, but do not attack the
+vital organs. When full-grown, the grubs eat their way through the skin
+of the insect, and turn into chrysalides. Almost every kind of insect
+is subject to the attacks of these little creatures, which are no doubt
+useful in preventing the too great multiplication of insects, and
+especially of caterpillars. Some species are so minute that they
+actually lay their eggs within those of other insects (Figs. 15, 16).
+These parasites assume very curious forms in their larval state.
+
+But of all the Hymenoptera, the group containing the ant, the bee, and
+the wasp is the most interesting. This is especially the case with the
+social species, though the solitary ones also are extremely remarkable.
+The solitary bee or wasp, for instance, forms a cell generally in the
+ground, places in it a sufficient amount of food, lays an egg, and
+closes the cell. In the case of bees, the food consists of honey; in
+that of wasps, the larva requires animal food, and the mother therefore
+places a certain number of insects in the cell, each species having its
+own special prey, some selecting small caterpillars, some beetles, some
+spiders. _Cerceris bupresticida_, as its name denotes, attacks beetles
+belonging to the genus _Buprestis_. Now if the Cerceris were to kill the
+beetle before placing it in the cell, it would decay, and the young
+larva, when hatched, would find only a mass of corruption. On the other
+hand, if the beetle were buried uninjured, in its struggles to escape it
+would be almost certain to destroy the egg. The wasp has, however, the
+instinct of stinging its prey in the centre of the nervous system, thus
+depriving it of motion, and let us hope of suffering, but not of life;
+consequently, when the young larva leaves the egg, it finds ready a
+sufficient store of wholesome food.
+
+Other wasps are social, and, like the bees and ants, dwell together in
+communities. They live for one season, dying in autumn, except some of
+the females, which hibernate, awake in the spring, and form new
+colonies. These, however, do not, under ordinary circumstances, live
+through a second winter. One specimen which I kept tame through last
+spring and summer, lived until the end of February, but then died. The
+larvæ of wasps (Plate II., Fig. 9) are fat, fleshy, legless grubs. When
+full-grown they spin for themselves a silken covering, within which they
+turn into chrysalides. The oval bodies which are so numerous in ants’
+nests, and which are generally called ants’ eggs, are really not eggs
+but cocoons. Ants are very fond of the honey-dew which is formed by the
+Aphides, and have been seen to tap the Aphides with their antennæ, as if
+to induce them to emit some of the sweet secretion. There is a species
+of _Aphis_ which lives on the roots of grass, and some ants collect these
+into their nests, keeping them, in fact, just as we do cows. Moreover
+they collect the eggs in the autumn and tend them through the winter
+(when they are of no use) with the same care as their own, so as to have
+a supply of young Aphides in the spring. This is one of the most
+remarkable facts I know in the whole history of animal life. One species
+of red ant does no work for itself, but makes slaves of a black kind,
+which then do everything for their masters. The slave makers will not
+even put food into their own mouths, but would starve in the midst of
+plenty, if they had not a slave to feed them. I found, however, that I
+could keep them in life and health for months if I gave them a slave for
+an hour or two in a week to clean and feed them.
+
+Ants also keep a variety of beetles and other insects in their nests.
+That they have some reason for this seems clear, because they readily
+attack any unwelcome intruder; but what that reason is, we do not yet
+know. If these insects are to be regarded as the domestic animals of the
+ants, then we must admit that the ants possess more domestic animals
+than we do.
+
+Some indeed of these beetles produce a secretion which is licked by the
+ants like the honey-dew; there are others, however, which have not yet
+been shown to be of any use to the ants, and yet are rarely, if ever,
+found, excepting in ants’ nests.
+
+M. Lespès, who regards these insects as true domestic animals, has
+recorded[8] some interesting observations on the relations between one
+of them (_Claviger Duvalii_) and the ants (_Lasius niger_) with which it
+lives. This species of _Claviger_ is never met with except in ants’
+nests, though on the other hand there are many communities of _Lasius_
+which possess none of these beetles; and M. Lespès found that when he
+placed _Clavigers_ in a nest of ants which had none of their own, the
+beetles were immediately killed and eaten, the ants themselves being on
+the other hand kindly received by other communities of the same species.
+He concludes from these observations that some communities of ants are
+more advanced in civilization than others; the suggestion is no doubt
+ingenious, and the fact curiously resembles the experience of navigators
+who have endeavoured to introduce domestic animals among barbarous
+tribes; but M. Lespès has not yet, so far as I am aware, published the
+details of his observations, without which it is impossible to form a
+decided opinion. I have sometimes wondered whether the ants have any
+feeling of reverence for these beetles; but the whole subject is as yet
+very obscure, and would well repay careful study.
+
+[Illustration: PLATE III.—MATURE INSECTS.
+
+Fig. 1, _Chloëon_; 2, _Meloë_ (after Shuckard); 3, _Calepteryx_;
+4, _Sitaris_ (after Shuckard); 5, _Campodea_ (after Gervais);
+6, _Acilius_; 7, _Termes_; 8, _Stylops_ (female); 9, _Thrips_.]
+
+[Illustration: PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ON PLATE
+III.—Fig. 1, Larva of _Chloëon_; 2, Larva of _Meloë_ (after Chapuis and
+Candèze); 3, Larva of _Calepteryx_ (after Léon Dufour); 4, Larva of
+_Sitaris_; 5, Larva of _Campodea_; 6, Larva of _Acilius_;
+7, Larva of Termes (after Blanchard); 8, Larva of _Stylops_; 9, Larva
+of _Thrips_.]
+
+The order Strepsiptera are a small, but very remarkable group of
+insects, parasitic on bees and wasps. The larva (Pl. IV., Fig. 8) is
+minute, six-legged, and very active; it passes through its
+transformations within the body of the bee or wasp. The male and female
+are very dissimilar. The males are minute, very active, short-lived, and
+excitable, with one pair of large membranous wings. The females (Pl.
+III., Fig. 8), on the contrary, are almost motionless, and shaped very
+much like a bottle; they never quit the body of the bee, but only thrust
+out the top of the bottle between the abdominal rings of the bee.
+
+In the order Coleoptera, the larvæ differ very much in form. The
+majority are elongated, active, hexapod, and more or less depressed; but
+those of the Weevils (Pl. II., Fig. 6), of _Scolytus_ (Pl. II., Fig. 4),
+&c., which are vegetable feeders, and live surrounded by their
+food,—as, for instance, in grain, nuts, &c.,—are apod, white, fleshy
+grubs, not unlike those of bees and ants. The larvæ of the Longicorns,
+which live inside trees, are long, soft, and fleshy, with six short
+legs. The Geodephaga, corresponding with the Linnæan genera _Cicindela_
+and _Carabus_, have six-legged, slender, carnivorous larvæ; those of
+_Cicindela_, which waylay their prey, being less active than the hunting
+larvæ of the Carabidæ. The Hydradephaga, or water-beetles (Dyticidæ and
+Gyrinidæ), have long and narrow larvæ (Pl. IV., Fig. 6), with strong
+sickle-shaped jaws, short antennæ, four palpi, and six small eyes on
+each side of the head; they are very voracious. The larvæ of the
+Staphylinidæ are by no means unlike the perfect insect, and are found in
+similar situations; their jaws are powerful, and their legs moderately
+strong. The larvæ of the Lamellicorn beetles (Figs. 1-6)—cockchafers,
+stag-beetles, &c.—feed on vegetable substances or on dead animal
+matter. They are long, soft, fleshy grubs, with the abdomen somewhat
+curved, and generally lie on their side. The larvæ of the Elateridæ,
+known as wireworms, are long and slender, with short legs. That of the
+glowworm (Lampyridæ) is not unlike the apterous female. The male
+glowworm, on the contrary, is very different. It has long, thin, brown
+wing-cases, and often flies into rooms at night, attracted by the light,
+which it probably mistakes for that of its mate.
+
+The metamorphoses of the Cantharidæ are very remarkable, and will be
+described subsequently. The larvæ are active and hexapod. The Phytophaga
+(_Crioceris_, _Galeruca_, _Haltica_, _Chrysomela_, &c.) are vegetable
+feeders, both as larvæ and in the perfect state. The larvæ are furnished
+with legs, and are not unlike the caterpillars of certain Lepidoptera.
+
+The larva of _Coccinella_ (the Ladybird) is somewhat depressed, of an
+elongated ovate form, with a small head, and moderately strong legs. It
+feeds on Aphides.
+
+Thus, then, we see that there are among the Coleoptera many different
+forms of larvæ. Macleay considered that there were five principal types.
+
+1. Carnivorous hexapod larvæ, with an elongated, more or less flattened
+body, six eyes on each side of the head, and sharp falciform mandibles
+(_Carabus_, _Dyticus_, &c.).
+
+2. Herbivorous hexapod larvæ, with fleshy, cylindrical bodies, somewhat
+curved, so that they lie on their side.
+
+3. Apod grub-like larvæ, with scarcely the rudiments of antennæ
+(_Curculio_).
+
+4. Hexapod antenniferous larvæ, with a subovate body, the second segment
+being somewhat larger than the others (_Chrysomela_, _Coccinella_).
+
+5. Hexapod antenniferous larvæ, of oblong form, somewhat resembling the
+former, but with caudal appendages (_Meloë_, _Sitaris_).
+
+The pupa of the Coleoptera is quiescent, and “the parts of the future
+beetle are plainly perceivable, being encased in distinct sheaths; the
+head is applied against the breast; the antennæ lie along the sides of
+the thorax; the elytra and wings are short and folded at the sides of
+the body, meeting on the under side of the abdomen; the two anterior
+pairs of legs are entirely exposed, but the hind pair are covered by
+wing-cases, the extremity of the thigh only appearing beyond the sides
+of the body.”[9]
+
+In the next three orders—namely, the Orthoptera (grasshoppers, locusts,
+crickets, walking-stick insects, cockroaches, &c.), Euplexoptera
+(earwigs), and Thysanoptera, a small group of insects well known to
+gardeners under the name of _Thrips_ (Pl. I. and II., Figs. 1 and 2)—the
+larvæ when they quit the egg already much resemble the mature form,
+differing, in fact, principally in the absence of wings, which are more
+or less gradually acquired, as the insect increases in size. They are
+active throughout life. Those specimens which have rudimentary wings
+are, however, usually called pupæ.
+
+The Neuroptera present, perhaps, more differences in the character of
+their metamorphoses than any other order of insects. Their larvæ are
+generally active, hexapod little creatures, and do not vary from one
+another in appearance so much, for instance, as those of the Coleoptera,
+but their pupæ differ essentially; some groups, namely, the Psocidæ,
+Termitidæ, Libellulidæ, Ephemeridæ, and Perlidæ, remaining active
+throughout life, like the Orthoptera; while a second division, including
+the Myrmeleonidæ, Hemerobiidæ, Sialidæ, Panorpidæ, Raphidiidæ, and
+Mantispidæ, have quiescent pupæ, which, however, in some cases, acquire
+more or less power of locomotion shortly before they assume the mature
+state; thus that of _Raphidia_, though motionless at first, at length
+acquires strength enough to walk, even while still enclosed in the pupa
+skin, which is very thin.[10]
+
+One of the most remarkable families belonging to this order is that of
+the Termites, or white ants. They abound in the tropics, where they are
+a perfect pest, and a serious impediment to human development. Their
+colonies are extremely numerous, and they attack woodwork and furniture
+of all kinds, generally working from within, so that their presence is
+often unsuspected, until it is suddenly found that they have completely
+eaten away the interior of some post or table, leaving nothing but a
+thin outer shell. Their nests, which are made of earth, are sometimes
+ten or twelve feet high, and strong enough to bear a man. One species,
+_Termes lucifugus_, is found in the South of France, where it has been
+carefully studied by Latreille. He found in these communities five kinds
+of individuals—(1) males; (2) females, which grow to a very large size,
+their bodies being distended with eggs, of which they sometimes lay as
+many as 80,000 in a day; (3) a form described by some observers as Pupæ,
+but by others as neuters. These differ very much from the others, having
+a long, soft body without wings, but with an immense head, and very
+large, strong jaws. These individuals act as soldiers, doing apparently
+no work, but keeping watch over the nest and attacking intruders with
+great boldness. (4) Apterous, eyeless individuals, somewhat resembling
+the winged ones, but with a larger and more rounded head; these
+constitute the greater part of the community, and, like the workers of
+ants and bees, perform all the labour, building the nest and collecting
+food. (5) Latreille mentions another kind of individual which he regards
+as the pupa, and which resembles the workers, but has four white
+tubercles on the back, where the wings afterwards make their appearance.
+There is still, however, much difference of opinion among entomologists,
+with reference to the true nature of these different classes of
+individuals. M. Lespès, who has recently studied the same species,
+describes a second kind of male and a second kind of female, and the
+subject, indeed, is one which offers a most promising field for future
+study.
+
+Another interesting family of Neuroptera is that of the Ephemeræ, or
+Mayflies (Pl. III., Fig. 1), so well known to fishermen. The larvæ (Pl.
+IV., Fig. 1) are semi-transparent, active, six-legged little creatures,
+which live in water; having at first no gills, they respire through the
+general surface of the body. They grow rapidly and change their skin
+every few days. After one or two moults they acquire seven pairs of
+branchiæ, or gills, which are generally in the form of leaves, one pair
+to the segment. When the larvæ are about half grown, the posterior
+angles of the two posterior thoracic segments begin to elongate. These
+elongations become more and more marked with every change of skin. One
+morning, in the month of June, some years ago, I observed a full-grown
+larva, which had a glistening appearance, owing to the presence of a
+film of air under the skin. I put it under the microscope, and, having
+added a drop of water with a pipette, looked through the glass. To my
+astonishment, the insect was gone, and an empty skin only remained. I
+then caught a second specimen, in a similar condition, and put it under
+the microscope, hoping to see it come out. Nor was I disappointed. Very
+few moments had elapsed, when I had the satisfaction of seeing the
+thorax open along the middle of the back; the two sides turned over; the
+insect literally walked out of itself, unfolded its wings, and in an
+instant flew up to the window. Several times since, I have had the
+pleasure of witnessing this marvellous change, and it is really
+wonderful how rapidly it takes place: from the moment when the skin
+first cracks, not ten seconds are over before the insect has flown away.
+
+Another family of Neuroptera, the Dragon-flies, or Horse-stingers, as
+they are sometimes called, from a mistaken idea that they sting severely
+enough to hurt a horse, though in fact they are quite harmless, also
+spend their early days in the water. The larvæ are brown, sluggish, ugly
+creatures, with six legs. They feed on small water-animals, for which
+they wait very patiently, either at the bottom of the water, or on some
+aquatic plant. The lower jaws are attached to a long folding rod; and
+when any unwary little creature approaches too near the larva, this
+apparatus is shot out with such velocity that the prey which comes
+within its reach seldom escapes. In their perfect condition, also,
+Dragon-flies feed on other insects, and may often be seen hawking round
+ponds. The so-called Ant-lions in many respects resemble the
+Dragon-flies, but the habits of the larvæ are very dissimilar. They do
+not live in the water, but prefer dry places, where they bury themselves
+in the loose sand, and seize with their long jaws any small insect which
+may pass. The true Ant-lion makes itself a round, shallow pit in loose
+ground or sand, and buries itself at the bottom. Any inattentive little
+insect which steps over the edge of this pit immediately falls to the
+bottom, and is instantaneously seized by the Ant-lion. Should the insect
+escape, and attempt to climb up the side of the pit, the Ant-lion is
+said to throw sand at it, knocking it down again.
+
+One other family of Neuroptera which I must mention, is the
+Hemerobiidæ. The perfect insect is a beautiful, lace-winged, very
+delicate, green creature, something like a tender Dragon-fly, and with
+bright, green, touching eyes. The female deposits her eggs on leaves,
+not directly on the plant itself, but attached to it by a long white
+slender footstalk. The larva has six legs and powerful jaws, and makes
+itself very useful in destroying the Hop-fly.
+
+The insects forming the order Trichoptera are well known in their larval
+condition, under the name of caddis worms. These larvæ are not
+altogether unlike caterpillars in form, but they live in water—which is
+the case with very few lepidopterous larvæ—and form for themselves
+cylindrical cases or tubes, built up of sand, little stones, bits of
+stick, leaves, or even shells. They generally feed on vegetable
+substances, but will also attack minute freshwater animals. When full
+grown, the larva fastens its case to a stone, the stem of a plant, or
+some other fixed substance, and closes the two ends with an open grating
+of silken threads, so as to admit the free access of water, while
+excluding enemies. It then turns into a pupa which bears some
+resemblance to the perfect insect, “except that the antennæ, palpi,
+wings, and legs are shorter, enclosed in separate sheaths, and arranged
+upon the breast.” The pupa remains quiet in the tube until nearly ready
+to emerge, when it comes to the surface, and in some cases creeps out of
+the water. It is not therefore so completely motionless as the pupæ of
+Lepidoptera.
+
+The Diptera, or Flies, comprise insects with two wings only, the hinder
+pair being represented by minute club-shaped organs called “haltères.”
+Flies quit the egg generally in the form of fat, fleshy, legless grubs.
+They feed principally on decaying animal or vegetable matter, and are no
+doubt useful as scavengers. Other species, as the gadflies, deposit
+their eggs on the bodies of animals, within which the grubs feed, when
+hatched. The mouth is generally furnished with two hooks which serve
+instead of jaws. The pupæ of Diptera are of two kinds. In the true
+flies, the outer skin of the full-grown larva is not shed, but contracts
+and hardens, thus assuming the appearance of an oval brownish shell or
+case, within which the insect changes into a chrysalis. The pupæ of the
+gnats, on the contrary, have the limbs distinct and enclosed in sheaths.
+They are generally inactive, but some of the aquatic species continue to
+swim about.
+
+One group of Flies, which is parasitic on horses, sheep, bats, and other
+animals, has been called the Pupipara, because it was supposed that they
+were not born until they had arrived at the condition of pupæ. They come
+into the world in the form of smooth, ovate bodies, much resembling
+ordinary dipterous pupæ, but as Leuckart has shown,[11] they are true,
+though abnormal, larvæ.
+
+The next order, that of the Aphaniptera, is very small in number,
+containing only the different species of Flea. The larva is long,
+cylindrical, and legless; the chrysalis is motionless, and the perfect
+insect is too well known, at least, as regards its habits, to need any
+description.
+
+The Heteroptera, unlike the preceding orders of insects, quit the egg in
+a form differing from that of the perfect insect principally in the
+absence of wings, which are gradually acquired. In their metamorphoses
+they resemble the Orthoptera, and are active through life. The majority
+are dull in colour, though some few are very beautiful. The species
+constituting this group, though very numerous, are generally small, and
+not so familiarly known to us as those of the other large orders, with
+indeed one exception, the well-known Bug. This is not, apparently, an
+indigenous insect, but seems to have been introduced. The word is indeed
+used by old writers, but either as meaning a bugbear, or in a general
+sense, and not with reference to this particular insect. In this country
+it never acquires wings, but is stated to do so sometimes in warmer
+climates. The Heteroptera cannot exactly be said either to sting or
+bite. The jaws, of which, as usual among insects, there are two pairs,
+are like needles, which are driven into the flesh, and the blood is then
+sucked up the lower lip, which has the form of a tube. This peculiar
+structure of the mouth prevails throughout the whole order; consequently
+their nutriment consists almost entirely of the juices of animals or
+plants. The Homoptera agree with the Heteroptera in the structure of the
+mouth, and in the metamorphoses. They differ principally in the front
+wings, which in Homoptera are membranous throughout, while in the
+Heteroptera, the front part is thickened and leathery. As in the
+Heteroptera, however, so also in the Homoptera, some species do not
+acquire wings. The Cicada, celebrated for its chirp, and the lanthorn
+fly, belong to this group. So also does the so-called Cuckoo-spit, so
+common in our gardens, which has the curious faculty of secreting round
+itself a quantity of frothy fluid which serves to protect it from its
+enemies. But the best known insects of this group are the Aphides or
+Plant-lice; while the most useful belong to the Coccidæ, or scale
+insects, from one species of which we obtain the substance called lac,
+so extensively used in the manufacture of sealing-wax and varnish.
+Several species also have been used in dyeing, especially the Cochineal
+insect of Mexico, a species which lives on the cactus. The male _Coccus_
+is a minute, active insect, with four large wings; while the female, on
+the contrary, never acquires wings, but is very sluggish, broad, more or
+less flattened, and in fact, when full grown, looks like a small brown,
+red, or white scale.
+
+The larva of the order Lepidoptera are familiar to us all, under the
+name of caterpillars. The insects of this order in their larval
+condition are almost all phytophagous, and are very uniform both in
+structure and in habits. The body is long and cylindrical, consisting of
+thirteen segments; the head is armed with powerful jaws; the three
+following segments, the future prothorax, mesothorax, and meta-thorax,
+each bears a pair of simple articulated legs. Of the posterior segments,
+five also bear false or pro-legs, which are short, unjointed, and
+provided with a number of hooklets. A caterpillar leads a dull and
+uneventful life; it eats ravenously, and grows rapidly, casting its skin
+several times during the process, which generally lasts only a few
+weeks; though in some cases, as for instance that of the goat-moth, it
+extends over a period of two or three years, after which the larva
+changes into a quiescent pupa or chrysalis.
+
+
+
+
+CHAPTER II.
+
+_THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE OF
+LARVÆ._
+
+
+The facts recapitulated briefly in the preceding chapter show, that the
+forms of insect larvæ depend greatly on the group to which they belong.
+Thus the same tree may harbour larvæ of Diptera, Hymenoptera,
+Coleoptera, and Lepidoptera; each presenting the form typical of the
+family to which it belongs.
+
+If, again, we take a group, such, for instance, as the Lamellicorn
+beetles, we shall find larvæ extremely similar in form, yet very
+different in habits. Those, for instance, of the common cockchafer (Fig.
+1) feed on the roots of grass; those of _Cetonia aurata_ (Fig. 2)
+inhabit ants’ nests; the larvæ of the genus _Trox_ (Fig. 3) are found on
+dry animal substances; of _Oryctes_ (Fig. 4) in tan-pits; of _Aphodius_
+(Fig. 5) in dung; of _Lucanus_ (the stag-beetle, Fig. 6) in wood.
+
+[Illustration: FIG. 1, Larva of the Cockchafer (_Melolontha_).
+(Westwood, Int. to the Modern Classification of Insects, vol. i. p.
+194.). 2, Larva of _Cetonia_. 3, Larva of _Trox_. 4, Larva of _Oryctes_.
+5, Larva of _Aphodius_ (Chapuis and Candèze, Mém. Soc. Roy. Liège,
+1853). 6, Larva of _Lucanus_. (Packard, Guide to the Study of Insects,
+Fig. 403).]
+
+On the other hand, in the present chapter it will be my object to show
+that the form of the larva depends very much on the conditions of its
+life. Thus, those larvæ which are internal parasites, whether in
+animals or plants, are vermiform, as are those which live in cells, and
+depend on their parents for food. On the other hand, larvæ which burrow
+in wood have strong jaws and generally somewhat weak thoracic legs;
+whilst those which feed on leaves have the thoracic legs more developed,
+but less so than the carnivorous species. Now, the Hymenoptera, as a
+general rule, belong to the first category: the larvæ of the Ichneumons,
+&c., which live in animals,—those of the Cynipidæ, inhabiting
+galls,—and those of ants, bees, wasps, &c., which are fed by their
+parents, are fleshy, apodal grubs; though the remarkable fact that the
+embryos of bees in one stage of their development possess rudiments of
+thoracic legs which subsequently disappear, seems to show, not indeed
+that the larvæ of bees were ever hexapod, but that bees are descended
+from ancestors which had hexapod larvæ, and that the present apod
+condition of these larvæ is not original, but results from their mode of
+life.
+
+On the other hand, the larvæ of _Sirex_ (Fig. 14) being wood-burrowers,
+possess well-developed thoracic legs. Again, the larvæ of the
+Tenthredinidæ, which feed upon leaves, closely resemble the caterpillars
+of Lepidoptera, even to the presence of abdominal pro-legs.
+
+[Illustration: FIG. 7, Larva of _Brachytarsus_ (Ratzeburg, Forst.
+Insecten). 8, Larva of _Crioceris_ (Westwood, loc. cit.).]
+
+The larvæ of most Coleoptera (Beetles) are active, hexapod, and more or
+less flattened: but those which live inside vegetable tissues, such as
+the weevils, are apod fleshy grubs, like those of Hymenoptera. Pl. II.,
+Fig. 6, represents the larva of the nut-weevil, _Balaninus_ (Pl. I., Fig.
+6), and it will be seen that it closely resembles Pl. II., Fig. 5, which
+represents that of a fly (_Anthrax_), Pl. I., Fig. 5, and Pl. II., Figs.
+7, 8, and 9, which represent respectively those of a _Cynips_ or gall-fly
+(Pl. I., Fig. 7), an ant (Pl. I., Fig. 8), and wasp (Pl. I., Fig. 9).
+Nor is _Balaninus_ the only genus of Coleoptera which affords us examples
+of this fact. Thus in the genus _Scolytus_ (Pl. I., Fig. 4), the larvæ
+(Pl. II., Fig. 4), which, as already mentioned, feed on the bark of the
+elm, closely resemble those just described, as also do those of
+_Brachytarsus_ (Fig. 7). On the other hand, the larvæ of certain beetles
+feed on leaves, like the caterpillars of Lepidoptera; thus that of
+_Crioceris Asparagi_ (Fig. 8)—which, as its name denotes, feeds on the
+asparagus—closely resembles the larvæ of certain Lepidoptera, as for
+instance of _Thecla spini_. From this point of view the transformations
+of the genus _Sitaris_ (Pl. III., Fig. 4), which have been very carefully
+investigated by M. Fabre, are peculiarly interesting.[12]
+
+[Illustration: FIG. 9, Larva of _Sitaris numeralis_ (Fabre, Ann. des Sci.
+Nat., sér. 4, tome vii.). 10, Larva of _Sitaris humeralis_, in the second
+stage. 11, Larva of _Sitaris humeralis_, in the third stage. 12, Larva of
+_Sitaris humeralis_, in the fourth stage. 13, Pupa of _Sitaris_.]
+
+The genus _Sitaris_ (a small beetle allied to Cantharis, the blister-fly,
+and to _Meloë_, the oil-beetle) is parasitic on a kind of Bee
+(Anthophora), which excavates subterranean galleries, each leading to a
+cell. The eggs of the _Sitaris_, which are deposited at the entrance of
+these galleries, are hatched at the end of September or beginning of
+October; and M. Fabre not unnaturally expected that the young larvæ,
+which are active little creatures with six serviceable legs (Fig. 9),
+would at once eat their way into the cells of the Anthophora. No such
+thing: till the month of April following they remain without leaving
+their birthplace, and consequently without food; nor do they in this
+long time change either in form or size. M. Fabre ascertained this, not
+only by examining the burrows of the _Anthophoras_, but also by direct
+observation of some young larvæ kept in captivity. In April, however,
+his captives at last awoke from their long lethargy, and hurried
+anxiously about their prisons. Naturally inferring that they were in
+search of food, M. Fabre supposed that this would consist either of the
+larvæ or pupæ of the Anthophora, or of the honey with which it stores
+its cell. All three were tried without success. The first two were
+neglected, and the larvæ, when placed on the latter, either hurried
+away, or perished in the attempt, being evidently unable to deal with
+the sticky substance. M. Fabre was in despair: “Jamais expérience,” he
+says, “n’a éprouvé pareille déconfiture. Larves, nymphes, cellules,
+miel, je vous ai tous offert; que voulez-vous donc, bestioles maudites?”
+The first ray of light came to him from our countryman, Newport, who
+ascertained that a small parasite found by Léon Dufour on one of the
+wild bees, and named by him Triungulinus, was, in fact, the larva of
+_Meloë_. The larvæ of _Sitaris_ much resembled Dufour’s Triungulinus;
+and acting on this hint, M. Fabre examined many specimens of Anthophora,
+and found on them at last the larvæ of his _Sitaris_. The males of
+Anthophora emerge from the pupæ sooner than the females, and M. Fabre
+ascertained that, as they come out of their galleries, the little
+_Sitaris_ larvæ fasten upon them. Not, however, for long: instinct
+teaches them that they are not yet in the straight path of development;
+and, watching their opportunity, they pass from the male to the female
+bee. Guided by these indications, M. Fabre examined several cells of the
+Anthophora: in some, the egg of the Anthophora floated by itself on the
+surface of the honey; in others, on the egg, as on a raft, sat the still
+more minute larva of the _Sitaris_. The mystery was solved. At the
+moment when the egg is laid the _Sitaris_ larva springs upon it. Even
+while the poor mother is carefully fastening up her cell, her mortal
+enemy is beginning to devour her offspring: for the egg of the
+Anthophora serves not only as a raft, but as a repast. The honey which
+is enough for either, would be too little for both; and the _Sitaris_,
+therefore, at its first meal, relieves itself from its only rival. After
+eight days the egg is consumed, and on the empty shell the _Sitaris_
+undergoes its first transformation, and makes its appearance in a very
+different form, as shown in Fig. 10.
+
+The honey which was fatal before is now necessary; the activity which
+before was necessary is now useless; consequently, with the change of
+skin, the active, slim larva changes into a white, fleshy grub, so
+organized as to float on the surface of the honey, with the mouth
+beneath, and the spiracles above the surface: “grâce à l’embonpoint du
+ventre,” says M. Fabre, “la larve est à l’abri de l’asphyxie.” In this
+state it remains until the honey is consumed; then the animal
+contracts, and detaches itself from its skin, within which the further
+transformations take place. In the next stage, which M. Fabre calls the
+pseudo-chrysalis (Fig. 11), the larva has a solid corneous envelope and
+an oval shape; and in its colour, consistency, and immobility reminds
+one of a Dipterous pupa. The time passed in this condition varies much.
+When it has elapsed, the animal moults again, again changes its form,
+and assumes that shown in Fig. 12; after this it becomes a pupa (Fig.
+13) without any remarkable peculiarities. Finally, after these wonderful
+changes and adventures, in the month of August the perfect _Sitaris_ (Pl.
+III., Fig. 4) makes its appearance.
+
+On the other hand, there are cases in which larvæ diverge remarkably
+from the ordinary type of the group to which they belong, without, as it
+seems in our present imperfect state of information, any sufficient
+reason.
+
+Thus the ordinary type of Hymenopterous larva, as we have already seen,
+is a fleshy apod grub; although those of the leaf-eating and wood-boring
+groups, Tenthredinidæ and Siricidæ (Fig. 14), are caterpillars, more or
+less closely resembling those of Lepidoptera. There is, however, a group
+of minute Hymenoptera, the larvæ of which reside within the eggs or
+larvæ of other insects. It is difficult to understand why these larvæ
+should differ from those of Ichneumons, which are also parasitic
+Hymenoptera, and should be, as will be seen by the accompanying figures,
+of such remarkable and grotesque forms. The first known of these curious
+larvæ was observed by De Filippi,[13] who, having collected some of the
+transparent eggs of a small Beetle (_Rhynchites betuleti_), to his great
+surprise found more than half of them attacked by a parasite, which
+proved to be the larva of a minute Hymenopterous insect belonging to the
+Pteromalidæ. Fig. 15 shows the egg of the Beetle, with the parasitic
+larva, which is represented on a larger scale in Fig. 16.
+
+[Illustration: FIG. 14, Larva of _Sirex_ (Westwood, loc. cit.). 15, Egg of
+_Rhynchites_, showing the parasitic Larva in the interior. 16, the
+parasitic Larva more magnified.]
+
+More recently this group has been studied by M. Ganin,[14] who thus
+describes the development of _Platygaster_. The egg, as in allied
+Hymenopterous families, for instance in _Cynips_, is elongated and
+club-shaped (Fig. 17). After a while a large nucleated cell appears in
+the centre (Fig. 18). This nucleated cell divides (Fig. 19) and
+subdivides. The outermost cells continue the same process, thus forming
+an outer investing layer. The central, on the contrary, enlarges
+considerably, and develops within itself a number of daughter cells
+(Figs. 20 and 21), which gradually form a mulberry-like mass, thus
+giving rise to the embryo (Fig. 22).
+
+[Illustration: FIG. 17, Egg of _Platygaster_ (after Ganin). 18, Egg of
+_Platygaster_ showing the central cell. 19, Egg of _Platygaster_ after the
+division of the central wall. 20, Egg of Platygaster more advanced. 21,
+Egg of _Platygaster_ more advanced. 22, Egg of Platygaster showing the
+rudiment of the embryo.]
+
+Ganin met with the larvæ of _Platygaster_ in those of a small gnat,
+_Cecidomyia_. Sometimes as many as fifteen parasites occurred in one gnat,
+but as a rule only one of these attained maturity. The three species of
+_Platygaster _differ considerably in form, as shown in Figs. 23-25. They
+creep about within the larva of _Cecidomyia_ by means of the strong hooked
+feet, _kf_, somewhat aided by movements of the tail. They possess a
+mouth, stomach, and muscles, but the nervous, vascular, and respiratory
+systems do not make their appearance until later. After some time the
+larva (Fig. 23) changes its skin, assuming the form represented in Fig.
+26. In this moult the last abdominal segment of the first larva is
+entirely thrown off: not merely the outer skin, as in the case of the
+other segments, but also the hypodermis and the muscles. This larva, as
+will be seen by the figure, resembles a barrel or egg in form, and is
+.870 mm. in length, the external appendages having disappeared, and the
+segments being indicated only by the arrangement of the muscles. _slkf_
+is the œsophagus leading into a wide stomach which occupies nearly
+the whole body, _gsae_ is the rudiment of the supra-œsophageal
+ganglia, _bsm_ the ventral nervous cords. The ventral nervous mass has
+the form of a broad band, with straight sides; it consists of embryonal
+cells, and remains in this undeveloped condition during the whole larval
+state.
+
+[Illustration: FIG. 23, Larva of _Platygaster_ (after Ganin)—_mo_, mouth;
+_a_, antenna; _kf_, hooked feet; _z_, toothed process; _lfg_, lateral
+process; _f_, branches of the tail. 24, Larva of another species of
+_Platygaster_. The letters indicate the same parts as in the preceding
+figure. 25, Larva of a third species of _Platygaster_. The letters
+indicate the same parts as in the preceding figures. 26, Larva of
+_Platygaster_ in the second stage—_mo_, mouth; _slkf_, œsophagus;
+_gsae_, supra-œsophageal ganglion; _lm_, muscles; _bsm_, nervous
+system; _ga_, _gh_, rudiments of the reproductive glands. 27, Larva of
+_Platygaster_ in the third stage—_mo_, mouth; _md_, mandibles; _gsae_,
+supra-œsophageal ganglion; _slk_, œsophagus; _ag_, ducts of the
+salivary glands; _bnm_, ventral nervous system; _sp_, salivary glands;
+_msl_, stomach; _im_, imaginal discs; _tr_, tracheæ; _fk_, fatty tissue;
+_ed_, intestine; _ga_, rudiments of reproductive organs; _ew_, wider
+portion of intestine; _ao_, posterior opening.]
+
+At the next moult the larva enters its third state, which, as far as the
+external form (Fig. 27) is concerned, differs from the second only in
+being somewhat more elongated. The internal organs, however, are much
+more complex and complete. The tracheæ have made their appearance, and
+the mouth is provided with a pair of mandibles. From this point the
+metamorphoses of _Platygaster_ do not appear to differ materially from
+those of other parasitic Hymenoptera.
+
+An allied genus, _Polynema_, has also very curious larvæ. The perfect
+insect is aquatic in its habits, swimming by means of its wings; flying,
+if we may say so, under water.[15] It lays its eggs inside those of
+Dragon-flies; and the embryo, as shown in Fig. 28, has the form of a
+bottle-shaped mass of undifferentiated embryonal cells, covered by a
+thin cuticle, but without any trace of further organization. Protected
+by the egg-shell of the Dragon-fly, and bathed in the nourishing fluid
+of the Dragon-fly’s egg, the young _Polynema_ imbibes nourishment through
+its whole surface, and increases rapidly in size. The digestive canal
+gradually makes its appearance; the cellular mass forms a new skin
+beneath the original cuticle, distinctly divided into segments, and
+provided with certain appendages. After a while the old cuticle is
+thrown off, and the larva gradually assumes the form shown in Fig. 29.
+The subsequent metamorphoses of _Polynema_ offer no special peculiarities.
+
+[Illustration: FIG. 28, Embryo of _Polynema_ (after Ganin). 29, Larva of
+_Polynema_—_asch_, rudiments of the antenna; _flsch_, rudiments of the
+wings; _bsch_, rudiments of the legs; _vfg_. lateral projections;
+_gsch_, rudiments of the ovipositor; _fk_, fatty tissue.]
+
+From these facts—and, if necessary, many more of the same nature might
+have been brought forward—it seems to me evident that while the form of
+any given larva depends to a certain extent on the group of insects to
+which it belongs, it is also greatly influenced by the external
+conditions to which it is subjected; that it is a function of the life
+which the larva leads and of the group to which it belongs.
+
+The larvæ of insects are generally regarded as being nothing more than
+immature states—as stages in the development of the egg into the
+imago; and this might more especially appear to be the case with those
+insects in which the larvæ offer a general resemblance in form and
+structure (excepting of course so far as relates to the wings) to the
+perfect insect. Nevertheless we see that this would be a very incomplete
+view of the case. The larva and pupa undergo changes which have no
+relation to the form which the insect will ultimately assume. With a
+general tendency to this goal, as regards size and the development of
+the wings, there are coincident other changes having reference only to
+existing wants and condition. Nor is there in this, I think, anything
+which need surprise us. External circumstances act on the insect in its
+preparatory states, as well as in its perfect condition. Those who
+believe that animals are susceptible of great, though gradual, change
+through the influence of external conditions, whether acting, as Mr.
+Darwin has suggested, through natural selection, or in any other manner,
+will see no reason why these changes should be confined to the mature
+animal. And it is evident that creatures which, like the majority of
+insects, live during the successive periods of their existence in very
+different circumstances, may undergo considerable changes in their
+larval organization, in consequence of forces acting on them while in
+that condition; not, indeed, without affecting, but certainly without
+affecting to any corresponding extent, their ultimate form.
+
+I conclude, therefore, that the form of the larva in insects, whenever
+it departs from the hexapod _Campodea_ type, has been modified by the
+conditions under which it lives. The external forces acting upon it are
+different from those which affect the mature form; and thus changes are
+produced in the young which have reference to its immediate wants,
+rather than to its final form.
+
+And, lastly, as a consequence, that metamorphoses may be divided into
+two kinds, developmental and adaptional or adaptive.
+
+
+
+
+CHAPTER III.
+
+_ON THE NATURE OF METAMORPHOSES._
+
+
+In the preceding chapters we have considered the life history of insects
+after they have quitted the egg; but it is obvious that to treat the
+subject in a satisfactory manner we must take the development as a
+whole, from the commencement of the changes in the egg, up to the
+maturity of the animal, and not suffer ourselves to be confused by the
+fact that insects leave the egg in very different stages of embryonal
+development. For though all young insects when they quit the egg are
+termed “larvæ,” whatever their form may be (the case of the so-called
+Pupipara not constituting a true exception), still it must be remembered
+that some of these larvæ are much more advanced than others. It is
+evident that the larva of a fly, as regards its stage of development,
+corresponds in reality neither with that of a moth nor with that of a
+grasshopper. The maggots of flies, in which the appendages of the head
+are rudimentary, belong to a lower grade than the grubs of bees, &c.,
+which have antennæ, mandibles, maxillæ, labrum, labium, and, in fact,
+all the mouth parts of a perfect insect.
+
+The caterpillars of Lepidoptera are generally classed with the vermiform
+larva of Diptera and Hymenoptera, and contrasted with those of
+Orthoptera, Hemiptera, &c.; but, in truth, the possession of thoracic
+legs places them, together with the similar larvæ of the Tenthredinidæ,
+on a decidedly higher level. Thus, then, the period of growth (that in
+which the animal eats and increases in size) occupies sometimes one
+stage in the development of an insect, sometimes another; sometimes, as
+for instance in the case of _Chloëon_, it continues through more than
+one; or, in other words, growth is accompanied by development. But, in
+fact, the question is even more complicated than this. It is not only
+that the larvæ of insects at their birth offer the most various grades
+of development, from the grub of a fly to the young of a grasshopper or
+a cricket; but that, if we were to classify larvæ according to their
+development, we should have to deal, not with a simple case of
+gradations only, but with a series of gradations, which would be
+different according to the organ which we took as our test.
+
+Apart, however, from the adaptive changes to which special reference was
+made in the previous chapter, the differences which larvæ present are
+those of gradation, not of direction. The development of a grasshopper
+does not pursue a different course from that of a butterfly, but the
+embryo attains a higher state before quitting the egg in the former than
+in the latter: while in most Hymenoptera, as for instance in Bees,
+Wasps, Ants, &c., the young are hatched without thoracic appendages; in
+the Orthoptera, on the contrary, the legs are fully developed before the
+young animal quits the egg.
+
+Prof. Owen,[16] indeed, goes so far as to say that the Orthoptera and
+other Homomorphous insects are, “at one stage of their development,
+apodal and acephalous larvæ, like the maggot of the fly; but instead of
+quitting the egg in this stage, they are quickly transformed into
+another, in which the head and rudimental thoracic feet are developed to
+the degree which characterizes the hexapod larvæ of the _Carabi_ and
+_Petalocera_.”
+
+[Illustration: FIG. 30, Egg of _Phryganea_ (Mystacides)—_A_¹,
+mandibular segment; _C_¹ to _C_⁵, maxillary, labial, and three
+thoracic segments; _D_, abdomen (after Zaddach). 31, Egg of _Phryganea_
+somewhat more advanced—_b_, mandibles; _c_, maxillæ; _cfs_, rudiments
+of the three pairs of legs.]
+
+I quite believe that this may have been true of such larvæ at an early
+geological period, but the fact now appears to be, so far at least as
+can be judged from the observations yet recorded, that the legs of those
+larvæ which leave the egg with these appendages generally make their
+appearance before the body-walls have closed, or the internal organs
+have approached to completion. Indeed, when the legs first appear, they
+are merely short projections, which it is not always easy to distinguish
+from the segments themselves. It must, however, be admitted, that the
+observations are neither so numerous, nor in most cases so full, as
+could be wished.
+
+Fig. 30 represents an egg of a May-fly (_Phryganea_), as represented by
+Zaddach in his excellent memoir,[17] just before the appearance of the
+appendages. It will be seen that a great part of the yolk is still
+undifferentiated, that the side walls are incomplete, the back quite
+open, and the segments merely indicated by undulations. This stage is
+rapidly passed through, and Zaddach only once met with an egg in this
+condition; in every other specimen which had indications of segments,
+the rudiments of the legs had also made their appearance, as in Fig. 31,
+which, however, as will be seen, does not in other respects show much
+advance on Fig. 30.
+
+Again in _Aphis_, the embryology of which has been so well worked out by
+Huxley,[18] the case is very similar, although the legs are somewhat
+later in making their appearance. When the young was 1/140th of an inch
+in length, he found the cephalic portion of the embryo beginning, he
+says, “to extend upwards again over the anterior face of the germ, so as
+to constitute its anterior and a small part of its superior wall. This
+portion is divided by a median fissure into two lobes, which play an
+important part in the development of the head, and will be termed the
+‘procephalic lobes.’ I have already made use of this term for the
+corresponding parts in the embryos of Crustacea. The rudimentary thorax
+presents traces of a division into three segments; and the dorso-lateral
+margins of the cephalic blastoderm, behind the procephalic lobes, have a
+sinuous margin. It is in embryos between this and 1/100th of an inch in
+length, that the rudiments of the appendages make their appearance; and
+by the growth of the cephalic, thoracic, and abdominal blastoderm,
+curious changes are effected in the relative position of those regions.”
+
+In _Chrysopa oculata_, one of the Hemerobiidæ, Packard has described[19]
+and figured a stage in which the body segments have made their
+appearance, but in which he says “there are no indications of limbs. The
+primitive band is fully formed, the protozorites being distinctly
+marked, the transverse impressed lines indicating the primitive segments
+being distinct, and the median furrow easily discerned.” Here also,
+again, the dorsal walls are incomplete, and the internal organs as yet
+unformed.
+
+In certain Dragon-flies (_Calepteryx_), and _Hemiptera_ (_Hydrometra_), the
+legs, according to Brandt,[20] appear at a still earlier stage.
+
+According to the observations of Kölliker,[21] it would appear that in
+the Coleopterous genus _Donacia_ the segments and appendages appear
+simultaneously.
+
+Kölliker himself, however, frankly admits that “meæ de hoc insecto
+observationes satis sunt manca,” and it is possible that he may never
+have met with an embryo in the state immediately preceding the
+appearance of the legs; especially as it appears from the observations
+of Kowalevski that in _Hydrophilus_ the appendages do not make their
+appearance until after the segments.[22]
+
+On the whole, as far as we can judge from the observations as yet
+recorded, it seems that in Homomorphous insects the ventral wall is
+developed and divided into segments, before the appearance of the legs;
+but that the latter are formed almost simultaneously with the cephalic
+appendages, and before either the dorsal walls of the body or the
+internal organs.
+
+[Illustration: FIG. 32.—Egg of _Pholcus opilionides_ (after Claparède).]
+
+As it is interesting, from this point of view, to compare the
+development of other Articulata with that of insects, I give a figure
+(Fig. 32), representing an early stage in the development of a spider
+(_Pholcus_) after Claparède,[23] who says, “C’est à ce moment qu’a lieu
+la formation des _protozonites_ ou segments primordiaux du corps de
+l’embryon. Le rudiment ventral s’épaissit suivant six zônes disposées
+transversalement entre le capuchon anal et le capuchon céphalique.”
+
+[Illustration: FIG. 33.—Embryo of _Julus_ (after Newport).]
+
+Among Centipedes the development of _Julus_ has been described by
+Newport.[24] The first period, from the deposition of the egg to the
+gradual bursting of the shell, and exposure of the embryo within it,
+which, however, remains for some time longer in connection with the
+shell, lasts for twenty-five days. The segments of the body, originally
+six in number, make their appearance on the twentieth day after the
+deposition of the egg, at which time there were no traces of legs. The
+larva, when it leaves the egg, is a soft, white, legless grub (Fig. 33),
+consisting of a head and seven segments, the head being somewhat firmer
+in texture than the rest of the body. It exhibits rudimentary antennæ,
+but the legs are still only represented by very slight papilliform
+processes on the undersides of the segments to which they belong.
+
+As already mentioned, it is possible that at one time the vermiform
+state of the Homomorphous insects—which, as we have seen, is now so
+short, and passed through at so early a stage of development—was more
+important, more prolonged, and accompanied by a more complete condition
+of the internal organs. The compression, and even disappearance of those
+embryonal stages which are no longer adapted to the mode of life—which
+do not benefit the animal—is a phenomenon not without a parallel in
+other parts of the animal or even of the vegetable kingdom. Just as in
+language long compound words have a tendency to concision, and single
+letters sometimes linger on, indicating the history of a word, like the
+“l” in “alms,” or the “b” in “debt,” long after they have ceased to
+influence the sound; so in embryology useless stages, interesting as
+illustrations of past history, but without direct advantage under
+present conditions, are rapidly passed through, and even, as it would
+appear, in some cases altogether omitted.
+
+[Illustration: FIG. 34.—Colony of _Bougainvillea fruticosa_, natural
+size, to the underside of a piece of floating timber (after Allman).]
+
+For instance, among the Hydroida, in the great majority of cases, the
+egg produces a body more or less resembling the common _Hydra_ of our
+ponds, and known technically as the “trophosome,” which develops into
+the well-known Medusæ or jelly-fishes. The group, however, for which
+Prof. Allman has proposed the term Monopsea,[25] and of which the genus
+_Ægina_ may be taken as the type, is, as he says, distinguished by the
+absence of a hydriform stage, “the ovum becoming developed through
+direct metamorphosis into a medusiform body, just as in the other orders
+it is developed into a hydriform body.” Fig. 34 represents, after
+Allman, a colony of _Bougainvillea fruticosa_ of the natural size. It is
+a British species, which is found growing on buoys, floating timber,
+&c., and, says Allman,[26] “when in health and vigour, offers a
+spectacle unsurpassed in interest by any other species—every branchlet
+crowned by its graceful hydranth and budding with Medusæ in all stages
+of development (Fig. 35), some still in the condition of minute buds, in
+which no trace of the definite Medusa-form can yet be detected; others,
+in which the outlines of the Medusa can be distinctly traced within the
+transparent _ectothèque_ (external layer); others, again, just casting
+off this thin outer pellicle, and others completely freed from it,
+struggling with convulsive efforts to break loose from the colony, and
+finally launched forth in the full enjoyment of their freedom into the
+surrounding water. I know of no form in which so many of the
+characteristic features of a typical hydroid are more finely expressed
+than in this beautiful species.”
+
+[Illustration: FIG. 35.—Portion of colony of _Bougainvillea fruticosa_,
+more magnified.]
+
+[Illustration: FIG. 36.—The Medusa form of the same species.]
+
+Fig. 36 represents the Medusa form of this species, and the development
+thus described may be regarded as typical of the Hydroida; yet, as
+already mentioned, the Æginidæ do not present us with any stage
+corresponding to the fixed condition of _Bougainvillea_, but, on the
+contrary, are developed into Medusæ direct from the egg.
+
+On the other hand, there are groups in which the Medusiform stage
+becomes less and less important.
+
+[Illustration: FIG. 37, Larva of Prawn, Nauplius stage (after F.
+Müller). 38, Larva of Prawn, more advanced, Zoëa stage.]
+
+The great majority of the higher Crustacea go through well-marked
+metamorphoses. Figs. 37 and 38 represent two stages in the development
+of the prawn. In the first (Fig. 37), representing the young animal as
+it quits the egg, the body is more or less oval and unsegmented; there
+is a median frontal eye, and three pairs of natatory feet, the first
+pair simple, while the two posterior are two-branched. Very similar
+larvæ occur in various other groups of Crustacea. They were at first
+regarded as mature forms, and O. F. Müller gave them the name of
+Nauplius. So also, the second or Zoëa form (Fig. 38) was at first
+supposed to be a mature animal, until its true nature was discovered by
+Vaughan Thompson.
+
+The Zoëa form of larva differs from the perfect prawn or crab in the
+absence of the middle portion of the body and its appendages. The
+mandibles have no palpi, the maxillipeds or foot-jaws are used as feet,
+whereas in the mature form they serve as jaws. Branchiæ are either
+wanting or rudimentary, respiration being principally effected through
+the walls of the carapace. The abdomen and tail are destitute of
+articulate appendages. The development of Zoëa into the perfect animal
+has been well described by Mr. Spence Bate[27] in the case of the common
+crab (_Carcinus mænas_).
+
+All crabs, as far as we know, with the exception of a species of land
+crab (_Gegarcinus_), described by Westwood, pass through a stage more or
+less resembling that shown in Fig. 38. On the other hand, the great
+group of Edriopthalma, comprising Amphipoda (shore-hoppers, &c.) and
+Isopoda (wood-lice, &c.) pass through no such metamorphosis; the
+development is direct, as in the Orthoptera. It is true that one
+species, _Tanais Dulongii_, though a typical Isopod in form and general
+character, is said to retain in some points, and especially in the mode
+of respiration, some peculiarities of the Zoëa type; but this is quite
+an exceptional case. In _Mysis_, says F. Müller,[28] “there is still a
+trace of the Nauplius stage; being transferred back to a period when it
+had not to provide for itself, the Nauplius has become degraded into a
+mere skin; in _Ligia_ this larva-skin has lost the traces of limbs, and
+in _Philoscia_ it is scarcely demonstrable.”
+
+The Echinodermata in most cases “go through a very well-marked
+metamorphosis, which often has more than one larval stage.... The mass
+of more or less differentiated sarcode, of which the larva, or
+pseud-embryo, as opposed to the Echinoderm within it, is made up, always
+carries upon its exterior certain bilaterally-arranged ciliated bands,
+by the action of which the whole organism is moved from place to place;
+and it may be strengthened by the super-addition to it of a framework
+of calcareous rods.”[29] Müller considered that the mouth and pharynx of
+the larva were either absorbed or cast off with the calcareous rods, but
+were never converted into the corresponding organs of the perfect
+Echinoderm. According to A. Agassiz, however, this is not the case, but
+on the contrary “the whole larva and all its appendages are gradually
+drawn into the body, and appropriated.”[30]
+
+Fig. 39 represents the larva of a sea-egg (_Echino cidaris_) after
+Müller.[31] The body is transparent, shaped somewhat like a double
+easel, but with two long horns in front, which, as well as the posterior
+processes, are supported by calcareous rods. This larva swims by means
+of minute vibratile hairs, or ciliæ. It has a mouth, stomach, and in
+fact a well-defined alimentary canal; but no nerves or other internal
+organs have yet been discovered in it. After swimming about in this
+condition for a while, it begins to show signs of change. An involution
+of the integument takes place on one side of the back, and continues to
+deepen till it reaches a mass or store of what is called blastema, or
+the raw material of the animal body. This blastema then begins to
+change, and gradually assumes the form of the perfect Echinoderm.[32]
+
+[Illustration: FIG. 39.—Larva of _Echino cidaris_, seen from above × 6/10
+(after Müller).]
+
+[Illustration: FIG. 40, Larva of _Echinus_, × 100. _A_, front arm; _F_,
+arms of the mouth process; _B_, posterior side arm; _E_₁, accessory
+arm of the mouth process; _a_, mouth; _a´_, œsophagus; _b_, stomach;
+_b´_, intestine; _o_, posterior orifice; _d_, ciliated bands; _f_,
+ciliated epaulets; _c_, disc of future _Echinus_ (after Müller).]
+
+Fig. 40 represents a larva, probably of another sea-egg (_Echinus
+lividus_), from the Mediterranean, and shows the commencement of the
+sea-egg within the body of the larva. The capital letters denote the
+different arms: _a_ is the mouth, _a´_ the œsophagus, _b_ the
+stomach, _b´_ the intestine, _f_ the ciliated lobes or epaulets, _c_ the
+young sea-egg.
+
+The development of the beautiful _Comatula rosacea_ (Fig. 41) has been
+described in the “Philosophical Transactions,” by Prof. Wyville Thomson
+and Dr. Carpenter.[33] The larva quits the egg, as shown in Fig. 42, in
+the form of an oval body about 1/30 inch in length, something like a
+barrel, surrounded by four bands or hoops of long vibratile hairs or
+ciliæ. There is also a tuft of still longer hairs at the narrower
+posterior end of the body. Gradually a number of minute calcareous
+spines and plates make their appearance (Fig. 43) in the body of this
+larva, and at length arrange themselves in a definite order, so as to
+form a bent calcareous club or rod with an enlarged head.
+
+[Illustration: FIG. 41.—_Comatula rosacea_ (after Forbes).]
+
+[Illustration: FIG. 42, Larva of Comatula rosacea (after Thomson). 43,
+Larva of _Comatula rosacea_, more advanced. 44, Larva of Comatula rosacea,
+in the Pentacrinus state.]
+
+As this process continues, the little creature gradually loses its power
+of swimming, and, sinking to the bottom, looses the bands of ciliæ, and
+attaches itself by its base to some stone or other solid substance, the
+knob of the club being free. The calcareous framework increases in size,
+and the expanded head forms itself into a cup, round which from five to
+fifteen delicate tentacles, as shown in Fig. 44, make their appearance.
+
+In this stage the young animal resembles one of the stalked Crinoids, a
+family of Echinoderms very abundant in earlier geological periods, but
+which has almost disappeared, being, as we see, now represented by the
+young states of existing more advanced, free, species. This attached,
+plant-like condition of _Comatula_ was indeed at first supposed to be a
+mature form, and was named Pentacrinus; but we now know that it is only
+a stage in the development of _Comatula_. The so-called Pentacrinus
+increases considerably in size, and after various gradual changes, which
+time does not now permit me to describe, quits the stalk, and becomes a
+free _Comatula_.
+
+The metamorphoses of the Starfishes are also very remarkable. Sars
+discovered, in the year 1835, a curious little creature about an inch in
+length, which he named _Bipinnaria asterigera_ (Figs. 45-47), and which
+he then supposed to be allied to the ciliograde Medusæ. Subsequent
+observations, however, made in 1844, suggested to him that it was the
+larva of a Starfish, and in 1847 MM. Koren and Danielssen satisfied
+themselves that this was the case.
+
+Figs. 45 and 46 represent the front and side view of a Bipinnaria found
+by Müller[34] near Marseilles. _a_ is the mouth, _b_ the œsophagus,
+_c_ the stomach, _c_´ the intestine. Fig. 47 represents a somewhat older
+specimen, in which the Starfish (_k_) is already beginning to make its
+appearance.
+
+[Illustration: FIG. 45, Larva of Starfish (Bipinnaria), × 100 (after
+Müller). 46, Larva of Starfish (Bipinnaria), × 100, seen from the
+side—_a_, mouth; _b_, œsophagus; _c_, stomach; _c´_, intestine. 47,
+Larva of another Bipinnaria, showing the commencement of the
+Starfish—_g_, canal of the ciliated sac; _i_, rudiments of tentacles;
+_d_, ciliated band.]
+
+But while certain Starfishes thus go through metamorphoses similar in
+character, and not less remarkable than those of sea-eggs, there are
+others—as, for instance, the genus _Asteracanthion_—in which development
+may be said to be direct—the organs and appendages special to the
+Pseud-embryo being in abeyance; while in another genus, _Pteraster_, they
+are reduced to a mere investing membrane.[35]
+
+Among the Ophiurans also we find two well-marked types of development.
+Some passing through metamorphoses, while others, as for instance
+_Ophiopholis bellis_, “is developed very much after the method of
+_Asteracanthion Mülleri_, without passing through the Plutean
+stage.”[36]
+
+Even in the same species of Echinoderm the degree of development
+attained by the larva differs to a certain extent according to the
+temperature, the supply of food, &c. Thus in _Comatula_, specimens which
+are liberally supplied with sea-water, and kept warm, hurry as it were
+through their early stages, and the free larva becomes distorted by the
+growing Pentacrinus (see Fig. 43), almost before it has attained its
+perfect form. On the other hand, under less favourable conditions, if
+the temperature is low and food less abundant, the early stages are
+prolonged, the larva is longer lived, and reaches a much higher degree
+of independent development. Similar differences occur in the development
+of other animals, as for instance, in the Hydroids,[37] and among the
+insects themselves, in Flies;[38] and it is obvious that these facts
+throw much light on the nature and origin of the metamorphoses of
+insects, which subject we shall now proceed to consider.
+
+
+
+
+CHAPTER IV.
+
+_ON THE ORIGIN OF METAMORPHOSES._
+
+
+The question still remains, Why do insects pass through metamorphoses?
+Messrs. Kirby and Spence tell us they “can only answer that such is the
+will of the Creator;”[39] this, however, is a general confession of
+faith, not an explanation of metamorphoses. So indeed they themselves
+appear to have felt; for they immediately proceed to make a suggestion.
+“Yet one reason,” they say, “for this conformation may be hazarded. A
+very important part assigned to insects in the economy of nature, as we
+shall hereafter show, is that of speedily removing superabundant and
+decaying animal and vegetable matter. For such agents an insatiable
+voracity is an indispensable qualification, and not less so unusual
+powers of multiplication. But these faculties are in a great degree
+incompatible; an insect occupied in the work of reproduction could not
+continue its voracious feeding. Its life, therefore, after leaving the
+egg, is divided into three stages.”
+
+But there are some insects—as, for instance, the Aphides—which
+certainly are not among the least voracious, and which grow and breed at
+the same time. There are also many scavengers among other groups of
+animals—such, for instance, as the dog, the pig, and the vulture—which
+undergo no metamorphosis.
+
+It is certainly true that, as a general rule, growth and reproduction do
+not occur together; and it follows, almost as a necessary consequence,
+that in such cases the first must precede the second. But this has no
+immediate connection with the occurrence of metamorphoses. The question
+is not, why an insect does not generally begin to breed until it has
+ceased to grow, but why, in attaining to its perfect form, it passes
+through such remarkable changes; why these changes are so sudden and
+apparently violent; and why they are so often closed by a state of
+immobility—that of the chrysalis or pupa; for undoubtedly the quiescent
+and death-like condition of the pupa is one of the most remarkable
+phenomena of insect-metamorphoses.
+
+In the first place, it must be observed that many animals which differ
+considerably in their mature state, resemble one another more nearly
+when young. Thus birds of the same genus, or of closely allied genera,
+which, when mature, differ much in colour, are often very similarly
+coloured when young. The young of the lion and the puma are often
+striped, and the fœtal Black whale has teeth, like its ally the Sperm
+whale.
+
+In fact, the great majority of animals do go through well-marked
+metamorphoses, though in many cases they are passed through within the
+egg, and thus do not come within the popular ken. “La larve,” says,
+Quatrefages, “n’est qu’un embryon à vie indépendante.”[40] Those
+naturalists who accept in any form the theory of evolution, consider
+that “the embryonal state of each species reproduces more or less
+completely the form and structure of its less modified progenitors.”[41]
+“Each organism,” says Herbert Spencer,[42] “exhibits within a short
+space of time a series of changes which, when supposed to occupy a
+period indefinitely great, and to go on in various ways instead of one
+way, give us a tolerably clear conception of organic evolution in
+general.”
+
+The naturalists of the older school do not, as Darwin and Fritz Müller
+have already pointed out, dispute these facts, though they explain them
+in a different manner—generally by the existence of a supposed tendency
+to diverge from an original type. Thus Johannes Müller says, “The idea
+of development is not that of mere increase of size, but that of
+progress from what is not yet distinguished, but which potentially
+contains the distinction in itself, to the actually distinct. It is
+clear that the less an organ is developed, so much the more does it
+approach the type, and that during its development it acquires more and
+more peculiarities. The types discovered by comparative anatomy and
+developmental history must therefore agree.” And again, “What is true in
+this idea is, that every embryo at first bears only the type of its
+section, from which the type of the class, order, &c., is only
+afterwards developed.” Agassiz also observes that “the embryos of
+different animals resemble each other the more the younger they are.”
+
+There are, no doubt, cases in which the earlier states are rapidly
+passed through, or but obscurely indicated; yet we may almost state it
+as a general proposition, that either before or after birth animals
+undergo metamorphoses. The state of development of the young animal at
+birth varies immensely. The kangaroo (_Macropus major_), which attains a
+height of seven feet ten inches, does not when born exceed one inch and
+two lines in length; the chick leaves the egg in a much more advanced
+condition than the thrush; and so, among insects, the young cricket is
+much more highly developed, when it leaves the egg, than the larva of
+the fly or of the bee; and, as I have already mentioned, differences
+occur even within the limit of one species, though not of course to
+anything like the same extent.
+
+In oviparous animals the condition of the young at birth depends much on
+the size of the egg: where the egg is large, the abundant supply of
+nourishment enables the embryo to attain a high stage of development;
+where the egg is small, and the yolk consequently scanty, the embryo
+requires an additional supply of food before it can do so. In the former
+case the embryo is more likely to survive; but when the eggs are large,
+they cannot be numerous, and a multiplicity of germs may be therefore in
+some circumstances a great advantage. Even in the same species the
+development of the egg presents certain differences.[43]
+
+The metamorphoses of insects depend then primarily on the fact that the
+young quit the egg at a more or less early stage of development; and
+that consequently the external forces, acting upon them in this state,
+are very different from those by which they are affected when they
+arrive at maturity.
+
+Hence it follows that, while in many instances mature forms, differing
+greatly from one another, arise from very similar larvæ, in other cases,
+as we have seen, among some the parasitic Hymenoptera, insects agreeing
+closely with one another, are produced from larvæ which are very unlike.
+The same phenomenon occurs in other groups. Thus, while in many cases
+very dissimilar jelly-fishes arise from almost identical Hydroids, we
+have also the reverse of the proposition in the fact that in some
+species, Hydroids of an entirely distinct character produce very similar
+Medusæ.[44]
+
+We may now pass to the second part of our subject: the apparent
+suddenness and abruptness of the changes which insects undergo during
+metamorphosis. But before doing so I must repeat that these changes are
+not always, even apparently, sudden and great. The development of an
+Orthopterous insect, say a grasshopper, from its leaving the egg to
+maturity, is so gradual that the ordinary nomenclature of entomological
+works (larva state and pupa state) does not apply to it; and even in the
+case of Lepidoptera, the change from the caterpillar to the chrysalis
+and from this to the butterfly is in reality less rapid than might at
+first sight be supposed; the internal organs are metamorphosed very
+gradually, and even the sudden and striking change in external form is
+very deceptive, consisting merely of a throwing off of the outer
+skin—the drawing aside, as it were of a curtain and the revelation of a
+form which, far from being new, has been in preparation for days;
+sometimes even for months.
+
+Swammerdam, indeed, supposed (and his view was adopted by Kirby and
+Spence) that the larva contained within itself “the germ of the future
+butterfly, enclosed in what will be the case of the pupa, which is
+itself included in three or more skins, one over the other, that will
+successively cover the larva.” This was a mistake; but it is true that,
+if a larva be examined shortly before it is full grown, the future pupa
+may be traced within it. In the same manner, if we examine a pupa which
+is about to disclose the butterfly, we find the future insect, soft
+indeed and imperfect, but still easily recognizable, lying more or less
+loosely within the pupa-skin.
+
+One important difference between an insect and a vertebrate animal is,
+that whereas in the latter—as, for instance, in ourselves—the muscles
+are attached to an internal bony skeleton, in insects no such skeleton
+exists. They have no bones, and their muscles are attached to the skin;
+whence the necessity for the hard and horny dermal investment of
+insects, so different from the softness and suppleness of our own skin.
+The chitine, or horny substance, of which the outside of an insect
+consists, is formed by a layer of cells lying beneath it, and, once
+secreted, cannot be altered. From this the result is, that without a
+change of skin, a change of form is impossible. In some cases, as for
+instance in _Chloëon_, each change of skin is accompanied by a change of
+form, and thus the perfect insect is gradually evolved. In others, as in
+caterpillars, several changes of skin take place without any material
+alteration of form, and the change, instead of being spread over many,
+is confined to the last two moults.
+
+One explanation of this difference between the larvæ which change their
+form with every change of skin, and those which do not, is, I believe,
+to be found in the structure of the mouth. That of the caterpillar is
+provided with a pair of strong jaws, fitted to eat leaves; and the
+digestive organs are adapted for this kind of food. On the contrary, the
+mouth of the butterfly is suctorial; it has a long proboscis,
+beautifully adapted to suck the nectar from flowers, but which would be
+quite useless, and indeed only an embarrassment to the larva. The
+digestive organs also of the butterfly are adapted for the assimilation,
+not of leaves, but of honey. Now it is evident that if the mouth-parts
+of the larva were slowly metamorphosed into those of the perfect insect,
+through a number of small changes, the insect would in the meantime be
+unable to feed, and liable to perish of starvation in the midst of
+plenty. In the Orthoptera, and among those insects in which the changes
+are gradual, the mouth of the so-called larva resembles that of the
+perfect insect, and the principal difference consists in the presence of
+wings.
+
+Similar considerations throw much light on the nature of the chrysalis
+or pupa state—that remarkable period of death-like quiescence which is
+one of the most striking characteristics of insect metamorphosis. The
+quiescence of the pupa is mainly owing to the rapidity of the changes
+going on in it. In that of a butterfly, not only (as has been already
+mentioned) are the mouth and the digestive organs undergoing change, but
+the muscles are in a similar state of transition. The powerful ones
+which move the wings are in process of formation; and even the nervous
+system, by which the movements are set on foot and regulated, is in a
+state of rapid change.[45]
+
+It must not be forgotten that all insects are inactive for a longer or
+shorter space of time after each moult. The slighter the change, as a
+general rule, the shorter is the period of inaction. Thus, after the
+ordinary moult of a caterpillar, the insect only requires a short rest
+until the new skin is hardened. When, however, the change is great, the
+period of inaction is correspondingly prolonged. Most pupæ indeed have
+some slight powers of motion; those which assume the chrysalis state in
+wood or beneath the ground usually come to the surface when about to
+assume the perfect state, and the aquatic pupæ of certain Diptera swim
+about with much activity. Among the Neuroptera, certain families have
+pupæ as quiescent as those of the Lepidoptera: others—as, for
+instance, _Raphidia_—are quiescent at first, but at length acquire
+sufficient strength to walk, though still enclosed within the pupa-skin:
+a power dependent partly on the fact that this skin is very thin. Others
+again—as, for instance, dragon-flies—are not quiescent on assuming the
+so-called pupa state for any longer time than at their other changes of
+skin. The inactivity of the pupa is therefore not a new condition
+peculiar to this stage, but a prolongation of the inaction which has
+accompanied every previous change of skin.
+
+Nevertheless the metamorphoses of insects have always seemed to me one
+of the greatest difficulties of the Darwinian theory. In most cases, the
+development of the individual reproduces to a certain extent that of the
+race; but the motionless, imbecile pupa cannot represent a mature form.
+No one, so far as I know, has yet attempted to explain, in accordance
+with Mr. Darwin’s views, a life-history in which the mouth is first
+mandibulate and then suctorial, as, for example, in a butterfly. A clue
+to the difficulty may, I think, be found in the distinction between
+developmental and adaptive changes; to which I have called attention in
+a previous chapter. The larva of an insect is by no means a mere stage
+in the development of the perfect animal. On the contrary, it is subject
+to the influence of natural selection, and undergoes changes which have
+reference entirely to its own requirements and condition. It is evident,
+then, that while the embryonic development of an animal in the egg may
+be an epitome of its specific history, this is by no means the case
+with species in which the immature forms have a separate and
+independent existence. If an animal which, when young, pursues one mode
+of life, and lives on one kind of food, subsequently, either from its
+own growth in size and strength, or from any change of season, alters
+its habits or food, however slightly, it immediately becomes subject to
+the action of new forces: natural selection affects it in two different,
+and, it may be, very distinct manners, gradually tending to changes
+which may become so great as to involve an intermediate period of change
+and quiescence.
+
+There are, however, peculiar difficulties in those cases in which, as
+among the Lepidoptera, the same species is mandibulate as a larva, and
+suctorial as an imago. From this point of view _Campodea_ and the
+Collembola (_Podura_, &c.) are peculiarly interesting. There are in
+insects three principal types of mouth:—
+
+First, the mandibulate;
+
+Secondly, the suctorial; and
+
+Thirdly, that of _Campodea_ and the Collembola generally,
+
+in which the mandibles and maxillæ are retracted, but have some freedom
+of motion, and can be used for biting and chewing soft substances. This
+type is, in some respects, intermediate between the other two. Assuming
+that certain representatives of such a type were placed under conditions
+which made a suctorial mouth advantageous, those individuals in which
+the mandibles and maxillæ were best calculated to pierce or prick would
+be favoured by natural selection, and their power of lateral motion
+would tend to fall into abeyance; while, on the other hand, if
+masticatory jaws were an advantage, the opposite process would take
+place.
+
+There is yet a third possibility—namely, that during the first portion
+of life, the power of mastication should be an advantage, and during the
+second that of suction, or _vice versâ_. A certain kind of food might
+abound at one season and fail at another; might be suitable for the
+animal at one age and not at another. Now in such cases we should have
+two forces acting successively on each individual, and tending to modify
+the organization of the mouth in different directions. It cannot be
+denied that the innumerable variations in the mouth-parts of insects
+have special reference to their mode of life, and are of some advantage
+to the species in which they occur. Hence, no believer in natural
+selection can doubt the possibility of the three cases above suggested,
+the last of which seems to throw some light on the possible origin of
+species which are mandibulate in one period of life and not in another.
+Granting then the transition from the one condition to the other, this
+would no doubt take place contemporaneously with a change of skin. At
+such times we know that, even when there is no change in form, the
+softness of the organs temporarily precludes the insect from feeding for
+a time, as, for instance, in the case of caterpillars. If, however, any
+considerable change were involved, this period of fasting must be
+prolonged, and would lead to the existence of a third condition, that of
+the pupa, intermediate between the other two. Since the acquisition of
+wings is a more conspicuous change than any relating to the mouth, we
+are apt to associate with it the existence of a pupa-state: but the case
+of the Orthoptera (grasshoppers, &c.) is sufficient proof that the
+development of wings is perfectly compatible with permanent activity;
+the necessity for prolonged rest is in reality much more intimately
+connected with the change in the constitution of the mouth, although in
+many cases, no doubt, this is accompanied by changes in the legs, and in
+the internal organization. An originally mandibulate mouth, however,
+like that of a beetle, could not, I think, have been directly modified
+into a suctorial organ like that of a butterfly or a gnat, because the
+intermediate stages would necessarily be injurious. Neither, on the
+other hand, for the same reasons, could the mouth of the Hemiptera be
+modified into a mandibulate type like that of the Coleoptera. But in
+_Campodea_ and the _Collembola_ we have a type of animal closely
+resembling certain larvæ which occur both in the mandibulate and
+suctorial series of insects, possessing a mouth neither distinctly
+mandibulate nor distinctly suctorial, but constituted on a peculiar
+type, capable of modification in either direction by gradual change,
+without loss of utility.
+
+In discussing this subject, it is necessary also to take into
+consideration the nature and origin of wings. Whence are they derived?
+why are there normally two pairs? and why are they attached to the
+meso-and meta-thorax? These questions are as difficult as they are
+interesting. It has been suggested, and I think with justice, that the
+wings of insects originally served for aquatic and respiratory
+purposes.
+
+In the larva of _Chloëon_ (Pl. IV., Fig. 1), for instance, which in
+other respects so singularly resembles _Campodea_ (Pl. III., Fig. 5),
+several of the segments are provided with foliaceous expansions which
+serve as respiratory organs. These so-called branchiæ are in constant
+agitation, and the muscles which move them in several points resemble
+those of true wings. It is true that in _Chloëon_ the vibration of the
+branchiæ is scarcely, if at all, utilized for the purpose of locomotion;
+the branchiæ are, in fact, placed too far back to act efficiently. The
+situation of these branchiæ differs in different groups; indeed, it
+seems probable that originally there were a pair on each segment. In
+such a case, those branchiæ situated near the centre of the body,
+neither too much in front nor too far back, would serve the most
+efficiently as propellers: the same causes which determined the position
+of the legs would also affect the wings. Thus a division of labour would
+be effected; the branchiæ on the thorax would be devoted to locomotion;
+those on the abdomen to respiration. This would tend to increase the
+development of the thoracic segments, already somewhat enlarged, in
+order to receive the muscles of the legs.
+
+That wings may be of use to insects under water is proved by the very
+interesting case of _Polynema natans_,[46] which uses its wings for
+swimming. This, however, is a rare case, and it is possible that the
+principal use of the wings was, primordially, to enable the mature forms
+to pass from pond to pond, thus securing fresh habitats and avoiding
+in-and-in breeding. If this were so, the development of wings would
+gradually have been relegated to a late period of life; and by the
+tendency to the inheritance of characters at corresponding ages, which
+Mr. Darwin has pointed out,[47] the development of wings would have thus
+become associated with the maturity of the insect. Thus the late
+acquisition of wings in the Insecta generally seems to be itself an
+indication of their descent from a stock which was at one period, if not
+originally, aquatic, and which probably resembled the present larvæ of
+_Chloëon_ in form, but had thoracic as well as abdominal branchiæ.
+
+Finally, from the subject of metamorphosis we pass naturally to that
+most remarkable phenomenon which is known as the “Alternation of
+Generations:” for the first systematic view of which we are indebted to
+my eminent friend Prof Steenstrup.[48]
+
+I have always felt it very difficult to understand why any species
+should have been created in this double character; nor, so far as I am
+aware, has any explanation of the fact yet been attempted. Nevertheless
+insects offer, in their metamorphoses, a phenomenon not altogether
+dissimilar, and give a clue to the manner in which alternation of
+generations may have originated.
+
+The caterpillar owes its difference from the butterfly to the
+undeveloped state in which it leaves the egg; but its actual form is
+mainly due to the influence of the conditions under which it lives. If
+the caterpillar, instead of changing into one butterfly, produced
+several, we should have an instance of alternation of generations. Until
+lately, however, we knew of no such case among insects; each larva
+produced one imago, and that not by generation, but by development. It
+has long been known, indeed, that there are species in which certain
+individuals remain always apterous, while others acquire wings. Many
+entomologists, however, regard these abnormal individuals as perfect,
+though wingless insects; and therefore I shall found no argument upon
+these cases, although they appear to me deserving of more attention than
+they have yet received.
+
+Recently, however, Prof. Wagner[49] has discovered that, among certain
+small gnats, the larvæ do not directly produce in all cases perfect
+insects, but give birth to other larvæ, which undergo metamorphoses of
+the usual character, and eventually become gnats. His observations have
+been confirmed, as regards this main fact, by other naturalists; and
+Grimm has met with a species of _Chironomus_ in which the pupæ lay
+eggs.[50]
+
+Here, then, we have a distinct case of alternation of generations, as
+characterized by Steenstrup. Probably other cases will be discovered in
+which insects undeniably in the larval state will be found fertile. Nay,
+it seems to me possible, if not probable, that some larvæ which do not
+now breed may, in the course of ages, acquire the power of doing so. If
+this idea is correct, it shows how the remarkable phenomenon, known as
+alternation of generations, may have originated.
+
+Summing up, then, the preceding argument, we find among insects various
+modes of development; from simple growth on the one hand, to well-marked
+instances of the so-called alternation of generation on the other. In
+the wingless species of Orthoptera there is little external difference,
+excepting in size, between the young larva and the perfect insect. The
+growth is gradual, and there is nothing which would, in ordinary
+language, be called a metamorphosis. In the majority of Orthoptera,
+though the presence of wings produces a marked difference between the
+larva and the imago, the habits are nearly the same throughout life, and
+consequently the action of external circumstances affects the larva in
+the same manner as it does the perfect insect.
+
+This is not the case with the Neuroptera. The larvæ do not live under
+the same conditions as the perfect insects: external forces accordingly
+affect them in a different manner; and we have seen that they pass
+through some changes which bear no reference to the form of the perfect
+insect: these changes, however, are for the most part very gradual. The
+caterpillars of Lepidoptera have even more extensive modifications to
+undergo; the mouth of the larva, for instance, being remarkably unlike
+that of the perfect insect. A change in this organ, however, could
+hardly take place while the insect was growing fast, and consequently
+feeding voraciously; nor, even if the change could be thus effected,
+would the mouth, in its intermediate stages, be in any way fitted for
+biting and chewing leaves. The same reasoning applies also to the
+digestive organs. Hence the caterpillar undergoes little, if any,
+change, except in size, and the metamorphosis is concentrated, so to
+say, into the last two moults. The changes then become so rapid and
+extensive, that the intermediate period is necessarily one of
+quiescence. In some exceptional cases, as in _Sitaris_ (_ante_, p. 30) we
+even find that, the conditions of life not being uniform throughout the
+larval period, the larva itself undergoes metamorphoses.
+
+Owing to the fact that the organs connected with the reproduction of the
+species come to maturity at a late period, larvæ are generally incapable
+of breeding. There are, however, some flies which have viviparous larvæ,
+and thus offer a typical case of alternation of generations.
+
+Thus, then, we find among insects every gradation, from simple growth to
+alternation of generations; and see how, from the single fact of the
+very early period of development at which certain animals quit the egg,
+we can throw some light on their metamorphoses, and for the still more
+remarkable phenomenon that, among many of the lower animals, the species
+is represented by two very different forms. We may even conclude, from
+the same considerations, that this phenomenon may in the course of ages
+become still more common than it is at present. As long, however, as the
+external organs arrive at their mature form before the internal
+generative organs are fully developed, we have metamorphosis; but if the
+reverse is the case, then alternation of generations often results.
+
+The same considerations throw much light on the remarkable circumstance,
+that in alternation of generations the reproduction is, as a general
+rule, agamic in one form. This results from the fact that reproduction
+by distinct sexes requires the perfection both of the external and
+internal organs; and if the phenomenon arise, as has just been
+suggested, from the fact that the internal organs arrive at maturity
+before the external ones, reproduction will result in those species only
+which have the power of agamic multiplication.
+
+Moreover, it is evident that we have in the animal kingdom two kinds of
+dimorphism.
+
+This term has usually been applied to those cases in which animals or
+plants present themselves at maturity under two forms. Ants and Bees
+afford us familiar instances among animals; and among plants the
+interesting case of the genus _Primula_ has recently been described by
+Mr. Darwin. Even more recently he has made known to us the still more
+remarkable phenomenon afforded by the genus _Lythrum_, in which there
+are three distinct forms, and which therefore offers an instance of
+polymorphism.[51]
+
+The other kind of dimorphism or polymorphism differs from the first in
+being the result of the differentiating action of external
+circumstances, not on the mature, but on the young individual. Such
+different forms, therefore, stand towards one another in the relation of
+succession. In the first kind the chain of being divides at the
+extremity; in the other it is composed of dissimilar links. Many
+instances of this second form of dimorphism have been described under
+the name of alternation of generations.
+
+The term, however, has met with much opposition, and is clearly
+inapplicable to the differences exhibited by insects in various periods
+of their life. Strictly speaking, the phenomena are frequently not
+alternate, and in the opinion of some eminent naturalists they are not,
+strictly speaking, cases of generation at all.[52]
+
+In order, then, to have some name for these remarkable phenomena, and to
+distinguish them from those cases in which the _mature_ animal or plant
+is represented by two or more different forms, I think it would be
+convenient to retain exclusively for these latter the terms dimorphism
+and polymorphism; and those cases in which animals or plants pass
+through a succession of different forms might be distinguished by the
+name of dieidism or polyeidism.
+
+The conclusions, then, which I think we may draw from the preceding
+considerations, are:—
+
+1. That the occurrence of metamorphoses arises from the immaturity of
+the condition in which some animals quit the egg.
+
+2. That the form of the insect larva depends in great measure on the
+conditions in which it lives. The external forces acting upon it are
+different from those which affect the mature form; and thus changes are
+produced in the young, having reference to its immediate wants, rather
+than to its final form.
+
+3. That metamorphoses may therefore be divided into two kinds,
+developmental and adaptional or adaptive.
+
+4. That the apparent abruptness of the changes which insects undergo,
+arises in great measure from the hardness of their skin, which admits of
+no gradual alteration of form, and which is itself necessary in order to
+afford sufficient support to the muscles.
+
+5. The immobility of the pupa or chrysalis depends on the rapidity of
+the changes going on in it.
+
+6. Although the majority of insects go through three well-marked stages
+after leaving the egg, still a large number arrive at maturity through a
+greater or smaller number of slight changes.
+
+7. When the external organs arrive at this final form before the organs
+of reproduction are matured, these changes are known as metamorphoses;
+when, on the contrary, the organs of reproduction are functionally
+perfect before the external organs, or when the creature has the power
+of budding, then the phenomenon is known as alternation of generations.
+
+
+
+
+CHAPTER V.
+
+_ON THE ORIGIN OF INSECTS._
+
+
+“Personne,” says Carl Vogt, “en Europe au moins, n’ose plus soutenir la
+Création indépendante et de toutes pièces des espèces,” and though this
+statement is perhaps not strictly correct, still it is no doubt true,
+that the Doctrine of Evolution, in some form or other, is accepted by
+most, if not by all, the greatest naturalists of Europe. Yet it is
+surprising how much, in spite of all that has been written, Mr. Darwin’s
+views are still misunderstood. Thus Browning, in one of his recent
+poems, says:—
+
+  “That mass man sprang from was a jelly lump
+   Once on a time; he kept an after course
+   Through fish and insect, reptile, bird, and beast,
+   Till he attained to be an ape at last,
+   Or last but one.”[53]
+
+This theory, though it would be regarded by many as a fair statement of
+his views, is one which Mr. Darwin would entirely repudiate. Whether
+fish and insect, reptile, bird and beast, are derived from one original
+stock or not, they are certainly not links in one sequence. I do not,
+however, propose to discuss the question of Natural Selection, but may
+observe that it is one thing to acknowledge that in Natural Selection,
+or the survival of the fittest, Mr. Darwin has called attention to a
+_vera causa_, has pointed out the true explanation of certain phenomena;
+but it is quite another thing to maintain that all animals are descended
+from some primordial source.
+
+For my own part, I am satisfied that Natural Selection is a true cause,
+and, whatever may be the final result of our present inquiries—whether
+animated nature be derived from one ancestral source, or from many—the
+publication of the Origin of Species will none the less have constituted
+an epoch in the History of Biology. But, how far the present condition
+of living beings is due to that cause; how far, on the other hand, the
+action of Natural Selection has been modified and checked by other
+natural laws—by the unalterability of types, by atavism, &c.; how many
+types of life originally came into being; and whether they arose
+simultaneously or successively,—these and many other similar questions
+remain unsolved, even admitting the theory of Natural Selection. All
+this has indeed been clearly pointed out by Mr. Darwin himself, and
+would not need repetition but for the careless criticism by which in too
+many cases the true question has been obscured. Without, however,
+discussing the argument for and against Mr. Darwin’s conclusions, so
+often do we meet with travesties of it like that which I have just
+quoted, that it is well worth while to consider the stages through which
+some group, say for instance that of insects, have probably come to be
+what they are, assuming them to have developed under natural laws from
+simpler organisms. The question is one of great difficulty. It is hardly
+necessary to say that insects cannot have passed through all the lower
+forms of animal life, and naturalists do not at present agree as to the
+actual line of their development.
+
+In the case of insects, the gradual course of evolution through which
+the present condition of the group has probably been reached, has been
+discussed by Mr. Darwin, by Fritz Müller, Haeckel, Brauer, myself and
+others.
+
+In other instances Palæontology throws much light on this question.
+Leidy has shown that the milk-teeth of the genus _Equus_ resemble the
+permanent teeth of the ancient _Anchitherium_, while the milk-teeth of
+_Anchitherium_ again approximate to the dental system of the still
+earlier _Merychippus_. Rütimeyer, while calling attention to this
+interesting observation, adds that the milk-teeth of _Equus caballus_ in
+the same way, and still more those of _E. fossilis_, resemble the
+permanent teeth of _Hipparion_.
+
+“If we were not acquainted with the horse,” says Flower,[54] “we could
+scarcely conceive of an animal whose only support was the tip of a
+single toe on each extremity, to say nothing of the singular
+conformation of its teeth and other organs. So striking have these
+characters appeared to many zoologists, that the animals possessing them
+have been reckoned as an order apart, called Solidungula; but
+palæontology has revealed that in the structure of its skull, its teeth,
+its limbs, the horse is nothing more than a modified _Palæotherium_; and
+though still with gaps in certain places, many of the intermediate
+stages of these modifications are already known to us, being the
+_Palæotherium_, _Anchitherium_, _Merychippus_, and _Hipparion_.”
+
+“All Echinoids,” says A. Agassiz,[55] “pass, in their early stages,
+through a condition which recalls to us the first Echinoids which made
+their appearance in geological ages.” On embryological grounds, he
+observes, we should “place true Echini lowest, then the Clypeastroids,
+next the Echinolamps, and finally the Spatangoids.” Now among the
+Echinoids of the Trias there are no Clypeastroids, Echinolamps, or
+Spatangoids. The Clypeastroids make their appearance in the Lias, the
+Echinolamps in the Jurassic, while the Spatangoids commence in the
+Cretaceous period.
+
+Again[56] “in the Radiates, the Acalephs in their first stages of
+growth, that is, in their Hydroid condition, remind us of the adult
+forms among Polyps, showing the structural rank of the Acalephs to be
+the highest, since they pass beyond a stage which is permanent with the
+Polyps; while the Adult forms of the Acalephs have in their turn a
+certain resemblance to the embryonic phases of the class next above
+them, the Echinoderms; within the limits of the classes, the same
+correspondence exists as between the different orders; the embryonic
+forms of the highest Polyps recall the adult forms of the lower ones,
+and the same is true of the Acalephs as far as these phenomena have been
+followed and compared among them.” Indeed, the accomplished authors from
+whom I have taken the above quotation, do not hesitate to say[57] that
+“whenever such comparisons have been successfully carried out, the
+result is always the same; the present representatives of the fossil
+types recall in their embryonic condition the ancient forms, and often
+explain their true position in the animal kingdom.”
+
+Fossil insects are unfortunately rare, there being but few strata in
+which the remains of this group are well preserved. Moreover,
+well-characterized Orthoptera and Neuroptera occur as early as the
+Devonian strata; Coleoptera and Hemiptera in the Coal-measures;
+Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the contrary,
+not until the Tertiary. But although it appears from these facts that,
+as far as our present information goes, the Orthoptera and Neuroptera
+are the most ancient orders, it is not, I think, conceivable that the
+latter should have been derived from any known species of the former; on
+the other hand, the earliest known Neuroptera and Orthoptera, though in
+some respects less specialized than existing forms, are as truly, and as
+well characterized, Insects, as any now existing; nor are we acquainted
+with any earlier forms, which in any way tend to bridge over the gap
+between them and lower groups, though, as we shall see, there are types
+yet existing which throw much light on the subject.
+
+In the consideration then of this question, we must rely principally on
+Embryology and Development. I have already referred to the cases in
+which species, very unlike in their mature condition, are very
+similar one to another when young. Haeckel, in his “Naturliche
+Schöpfungsgeschichte,” gives a diagram which illustrates this very well
+as regards Crustacea. Pls. 1-4 show the same to be the case with
+Insects.
+
+The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat,
+the Grasshopper,—these and other less familiar types seem at first to
+have little in common. They differ in size, in form, in colour, in
+habits, and modes of life. Yet the researches of entomologists,
+following the clue supplied by the illustrious Savigny, have proved, not
+only that while differing greatly in details, they are constructed on
+one common plan; but also that other groups, as for instance, Crustacea
+(Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown
+to be fundamentally similar. In Pl. 4 I have figured the larvæ of an
+_Ephemera_ (Fig. 1), of a _Meloë_ (Fig. 2), of a Dragon-fly (Fig. 3), of a
+Sitaris (Fig. 4), of a _Campodea_ (Fig. 5), of a _Dyticus_ (Fig. 6), of a
+Termite (Fig. 7), of a _Stylops_ (Fig. 8), and of a _Thrips_ (Fig. 9). All
+these larvæ possess many characters in common. The mature forms are
+represented in the corresponding figures of Plate 3, and it will at once
+be seen how considerably they differ from one another. The same fact is
+also illustrated in Figs. 48-55, where Figs. 48-51 represent the larval
+states of the mature forms represented in Figs. 52-55. Fig. 48 is the
+larva of a moth, _Agrotis suffusa_ (Fig. 52); Fig. 49 of a beetle,
+_Haltica_ (Fig. 53); Fig. 50 of a Saw-fly, _Cimbex_ (Fig. 54); and Fig.
+51 of a Centipede, _Julus_ (Fig. 55).
+
+[Illustration: FIG. 48, Larva of Moth (_Agrotis suffusa_), after
+Packard. 49, Larva of Beetle (_Haltica_), after Westwood. 50, Larva of
+Saw-fly (_Cimbex_), Brischke and Zaddach. Beob. ub d. arten. der Blatt
+und Holzwespen, Fig. 8. 51, Larva of _Julus_. Newport, Philos.
+Transactions, 1841.]
+
+Thus, then, although it can be demonstrated that perfect insects,
+however much they differ in appearance, are yet reducible to one type,
+the fact becomes much more evident if we compare the larvæ. M.
+Brauer[58] and I[59] have pointed out that two types of larvæ, which I
+have proposed to call _Campodea_-form and _Lindia_-form, and which Packard
+has named Leptiform and Eruciform, run through the principal groups of
+insects. This is obviously a fact of great importance: as all individual
+_Meloës_ are derived from a form resembling Pl. 2, Fig. 2, it is surely no
+rash hypothesis to suggest that the genus itself may have been so.
+
+[Illustration: FIG. 52, _Agrotis suffusa_ (after Packard). 53, _Haltica_
+(after Westwood).]
+
+[Illustration: FIG. 54, _Cimbex_, Brischae and Zaddach. l.c. T. 2, Fig.
+9.]
+
+[Illustration: FIG. 55. _Julus_ (after Gervais).]
+
+Firstly, however, let me say a word as to the general Insect type. It
+may be described shortly as consisting of animals possessing a head,
+with mouth parts, eyes and antennæ; a many segmented body, with three
+pairs of legs on the segments immediately following the head; with, when
+mature, either one or two pairs of wings, generally with caudal
+appendages I will not now enter into a description of their internal
+anatomy. It will be seen that, except as regards the wings, Pl. 4, Fig.
+4, representing the larva of a small beetle named _Sitaris_, answers very
+well to this description. Many other Beetles are developed from larvæ
+closely resembling those of _Meloë_ (Pl. 4, Fig. 2), and Sitaris (Pl. 4,
+Fig. 4); in fact—except those species the larvæ of which, as, for
+instance of the Weevils (Pl. 2, Fig. 6), are internal feeders, and do
+not require legs—we may say that the Coleoptera generally are derived
+from larvæ of this type.
+
+I will now pass to a second order, the Neuroptera. Pl. 4, Fig. 1,
+represents the larva of _Chloëon_, a species the metamorphoses of which I
+described some years ago in the Linnean Transactions,[60] and it is
+obvious that in essential points it closely resembles the form to which
+I have just alluded.
+
+The Orthoptera, again, the order to which Grasshoppers, Crickets,
+Locusts, &c. belong, commence life in a similar condition; and the same
+may also be said of the Trichoptera.
+
+The larvæ of Bees when they quit the egg are entirely legless, but in an
+earlier stage they possess well-marked rudiments of thoracic legs,
+showing, as it seems to me, that their apodal condition is an adaptation
+to their circumstances. Other Hymenopterous larvæ, those for example of
+_Sirex_ (Fig. 9), and of the Saw-flies (Fig. 50) have well-developed
+thoracic legs.
+
+From the difference in external form, and especially from the large
+comparative size of the abdomen, these larvæ, as well as those of
+Lepidoptera (Fig. 48), have generally been classed with the maggots of
+Flies, Weevils, &c., rather than with the more active form of larva just
+adverted to. This seems to me, as I have already pointed out,[61] to be
+a mistake. The caterpillar type differs, no doubt, in its general
+appearance, owing to its greater clumsiness, but still essentially
+agrees with that already described.
+
+No Dipterous larva, so far as I know, belongs truly to this type; in
+fact, the early stages of the pupa in the Diptera seem in some respects
+to correspond to the larvæ of other Insect orders. The Development of
+the Diptera is, however, as Weissman[62] has shown, very abnormal in
+other respects.
+
+Thus, then, we find in many of the principal groups of insects that,
+greatly as they differ from one another in their mature condition, when
+they leave the egg they more nearly resemble the typical insect type;
+consisting of a head; a three-segmented thorax, with three pairs of
+legs; and a many-jointed abdomen, often with anal appendages. Now, is
+there any mature animal which answers to this description? We need not
+have been surprised if this type, through which it would appear that
+insects must have passed so many ages since (for winged Neuroptera have
+been found in the carboniferous strata) had long ago become extinct. Yet
+it is not so. The interesting genus _Campodea_ (Pl. 3, Fig. 5) still
+lives; it inhabits damp earth, and closely resembles the larva of
+_Chloëon_ (Pl. 2, Fig. 1), constituting, indeed, a type which, as shown in
+Pl. 4, occurs in many orders of insects. It is true that the
+mouth-parts of _Campodea_ do not resemble either the strongly mandibulate
+form which prevails among the larvæ of Coleoptera, Orthoptera,
+Neuroptera, Hymenoptera, Lepidoptera; or the suctorial type of the
+Homoptera and Heteroptera. It is, however, not the less interesting or
+significant on that account, since, as I have elsewhere[63] pointed out,
+its mouth-parts are intermediate between the mandibulate and haustellate
+types; a fact which seems to me most suggestive.
+
+It appears, then, that there are good grounds for considering that the
+various types of insects are descended from ancestors more or less
+resembling the genus _Campodea_, with a body divided into head, thorax,
+and abdomen: the head provided with mouth-parts, eyes, and one pair of
+antennæ; the thorax with three pairs of legs; and the abdomen, in all
+probability, with caudal appendages.
+
+If these views are correct, the genus _Campodea_ must be regarded as a
+form of remarkable interest since it is the living representative of a
+primæval type, from which not only the Collembola and Thysanura, but the
+other great orders of insects have derived their origin.
+
+From what lower group the _Campodea_ type was itself derived is a question
+of great difficulty. Fritz Müller indeed says,[64] “if all the classes
+of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed
+all branches of a common stem (and of this there can scarcely be a
+doubt), it is evident that the water-inhabiting and water-breathing
+Crustacea must be regarded as the original stem from which the other
+terrestrial classes, with their tracheal respiration, have branched
+off.” Haeckel, moreover, is of the opinion that the Tracheata are
+developed from the Crustacea, and probably from the Zoëpoda. For my own
+part, though I feel very great diffidence in expressing an opinion at
+variance with that of such high authorities, I am rather disposed to
+suggest that the _Campodea_ type may possibly have been derived from a
+less highly developed one, resembling the modern Tardigrade,[65] a (Fig.
+56) smaller and much less highly organized being than _Campodea_. It
+possesses two eyes, three anterior pairs of legs, and one at the
+posterior end of the body, giving it a curious resemblance to some
+Lepidopterous larvæ.
+
+[Illustration: FIG. 56, Tardigrade (after Dujardin).]
+
+These legs, however, as will be seen, are reduced to mere projections.
+But for them, the Tardigrada would closely resemble the vermiform larva
+so common among insects. Among Trichoptera the larva early acquires
+three pairs of legs, but as Zaddach has shown,[66] there is a stage,
+though it is quickly passed through, in which the divisions of the body
+are indicated, but no trace of legs is yet present. Indeed, there appear
+to be reasons for considering that while among Crustacea the appendages
+appear before the segments, in Insects the segments precede the
+appendages, although this stage of development is very transitory, and
+apparently, in some cases, altogether suppressed. I say “apparently,”
+because, as I have already mentioned, I am not yet satisfied that it
+will not eventually be found to be so in all cases. Zaddach, in his
+careful observations of the embryology of _Phryganea_, only once found a
+specimen in this stage, which also, according to the researches of
+Huxley,[67] seems to be little more than indicated in _Aphis_. It is
+therefore possible that in other cases, when no such stage has been
+observed, it not really may be absent, but, from its transitoriness, may
+have hitherto escaped attention.
+
+Fritz Müller has expressed the opinion[68] that this vermiform type is
+of comparatively recent origin. He says: “The ancient insects approached
+more nearly to the existing Orthoptera, and perhaps to the wingless
+Blattidæ, than to any other order, and the complete metamorphosis of the
+Beetles, Lepidoptera, &c., is of later origin.” “There were,” he adds,
+“perfect insects before larvæ and pupæ.” This opinion has been adopted
+by Mr. Packard[69] in his “Embryological Studies on Hexapodous Insects.”
+
+M. Brauer[70] also considers that the vermiform larva is a more recent
+type than the Hexapod form, and is to be regarded not as a developmental
+form, but as an adaptational modification of the earlier active hexapod
+type. In proof of this he quotes the case of _Sitaris_.
+
+Considering, however, the peculiar habits of this genus, to which I have
+already referred, and also that the vermiform type is altogether lower
+in organization and less differentiated than the _Campodea_ form, I cannot
+but regard this case as exceptional; one in which the development has
+been, as it were, to use an expression of Fritz Müller’s, “falsified” by
+the struggle for existence, and which therefore does not truly indicate
+the successive stages of evolution. On the whole, the facts seem to me
+to point to the conclusion that, though the grub-like larvæ of Coleoptera
+and some other insects, owe their present form mainly to the influence
+of external circumstances, and partially also to atavism, still the
+_Campodea_ type is itself derived from earlier vermiform ancestors.
+Nicolas Wagner has shown in the case of a small gnat, allied to
+_Cecidomyia_, that even now, in some instances, the vermiform larvæ
+possess the power of reproduction. Such a larva (as, for instance, Fig.
+57) very closely resembles some of the Rotatoria, such for instance as
+_Albertia_ or _Notommata_, which however possess vibratile cilia. There is,
+indeed, one genus—_Lindia_ (Fig. 58)—in which these ciliæ are altogether
+absent, and which, though resembling _Macrobiotus_ in many respects,
+differs from that genus in being entirely destitute of legs. I have
+never met with it myself, but it is described by Dujardin, who found it
+in a ditch near Paris, as being oblong, vermiform, divided into rings,
+and terminating posteriorly in two short conical appendages. The jaws
+are not unlike those of the larvæ of Flies, and indeed many naturalists
+meeting with such a creature would, I am sure, regard it as a small
+Dipterous larva; yet Dujardin figures a specimen containing an egg, and
+seems to have no doubt that it is a mature form.[71]
+
+For the next descending stage we must, I think, look among the
+Infusoria, through such genera as _Chætonotus_ or _Ichthydium_. Other
+forms of the Rotatoria, such for instance as _Rattulus_, and still more
+the very remarkable species discovered in 1871 by Mr. Hudson,[72] and
+described under the name of _Pedalion mira_, seem to lead to the
+Crustacea through the Nauplius form. Dr. Cobbold tells me that he
+regards the _Gordii_ as the lowest of the Scolecida; Mr. E. Ray
+Lankester considers some of the Turbellaria, such genera as
+_Mesostomum_, _Vortex_, &c., to be the lowest of existing worms;
+excluding the parasitic groups. Haeckel[73] also regards the Turbellaria
+as forming the nearest approach to the Infusoria. The true worms seem,
+however, to constitute a separate branch of the animal kingdom.
+
+We may take, as an illustration of the lower worms, the genus
+Prorhynchus (Fig. 59), which consists of a hollow cylindrical body,
+containing a straight simple tube, the digestive organ.
+
+But however simple such a creature as this may be, there are others
+which are far less complex, far less differentiated; which therefore, on
+Mr. Darwin’s principles, may be considered still more closely to
+represent the primæval ancestor from which these more highly-developed
+types have been derived, and which, in spite of their great
+antiquity—in spite of, or perhaps in consequence of, their simplicity,
+still maintain themselves almost unaltered.
+
+Thus the form which Haeckel has described[74] under the name
+_Protamœba primitiva_, Pl. 5, Fig. 1-5, consists of a homogeneous and
+structureless substance, which continually alters its form; putting out
+and drawing in again more or less elongated processes, and creeping
+about like a true _Amœba_, from which, however, _Protamœba_ differs,
+in the absence of a nucleus. It seems difficult to imagine anything
+simpler; indeed, as described, it appears to be an illustration of
+properties without structure. It takes into itself any suitable particle
+with which it comes in contact, absorbs that which is nutritious, and
+rejects the rest. From time to time a constriction appears at the centre
+(Pl. 5, Fig. 2), its form approximates more and more to that of an
+hour-glass (Pl. 5, Fig. 3), and at length the two halves separate, and
+each commences an independent existence (Pl. 5, Fig. 5).
+
+[Illustration: FIG. 59, _Prorhynchus stagnaus_.[75]]
+
+[Illustration: PLATE V.
+
+FIGS. 1-5, _Protamœba_; 6-9, _Protamyxa Aurantiaca_,
+Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18, _Magosphæra planula_,
+Haeckel, loc. cit. pl. 5.]
+
+In the true _Amœbas_, on the contrary, we find a differentiation
+between the exterior and the interior: the body being more or less
+distinctly divisible into an outer layer and an inner parenchyme. In the
+_Amœbas_, as in _Protamœba_, multiplication takes place by
+self-division, and nothing corresponding to sexual reproduction has yet
+been discovered.
+
+Somewhat more advanced, but still of great simplicity, is the _Protomyxa
+aurantiaca_ (Pl. 5, Fig. 8), discovered by Haeckel[76] on dead shells of
+_Spirula_, where it appears as a minute orange speck, which shows well
+against the clear white of the _Spirula_. Examined with a microscope, the
+speck is seen to be a spherical mass of orange-coloured, homogeneous,
+albuminous matter, surrounded by a delicate, structureless membrane. It
+is obvious from this description that these bodies closely resemble
+eggs, for which indeed Haeckel at first mistook them. Gradually,
+however, the yellow sphere broke itself up into smaller spherules (Pl.
+5, Fig. 9), after which the containing membrane burst, and the separate
+spherules, losing their globular form, crept out as small _Amœbæ_ (Pl.
+5, Fig. 6), or amœboid bodies. These little bodies moved about,
+assimilated the minute particles of organic matter, with which they came
+in contact, and gradually increased in size (Pl. 5, Fig. 7) with more or
+less rapidity according to the amount of nourishment they were able to
+obtain. They threw out arms in various directions, and if divided each
+section maintained its individual existence. After a while their
+movements ceased, they contracted into a ball, and again secreted round
+themselves a clear structureless envelope.
+
+This completes their life history as observed by Haeckel, who found it
+easy to retain them in his glasses in perfect health, and who watched
+them closely.
+
+As another illustration I may take the _Magosphæra planula_, discovered
+by Haeckel on the coast of Norway.
+
+In one stage of its existence (Pl. 5, Fig. 10) it is a minute mass of
+gelatinous matter, which continually alters its form, moves about,
+feeds, and in fact behaves altogether like the _Amœba_ just described.
+It does not, however, remain always in this condition. After a while it
+contracts into a spherical form (Pl. 5, Fig. ii), and secretes round
+itself a structureless envelope, which, with the nucleus, gives it a
+very close resemblance to a minute egg.
+
+Gradually the nucleus divides, and the protoplasm also separates into
+two spherules (Pl. 5, Fig. 12); these two subdivide into four (Pl. 5,
+Fig. 13), and so on (Pl. 5, Fig 14), until at length thirty-two are
+present, compressed into a more or less polygonal form (Pl. 5, Fig. 15).
+Here this process ends. The separate spherules now begin to lose their
+smooth outline, to throw out processes, and to show amœboid movements
+like those of the creatures just described. The processes or pseudopods
+grow gradually longer, thinner, and more pointed. Their movements become
+more active, until at length they take the form of ciliæ. The spherical
+_Magosphæra_, the upper surface of which has thus become covered with
+ciliæ, now begins to rotate within the cyst or envelope, which at length
+gives way and sets free the contained sphere, which then swims about
+freely in the water (Pl. 5, Fig. 16), thus closely resembling _Synura_, or
+one of the Volvocineæ. After swimming about in this condition for a
+certain time, the sphere breaks up into the separate cells of which it
+is composed (Pl. 5, Fig. 17). As long as the individual cells remained
+together, they had undergone no changes of form, but after separating
+they show considerable contractility, and gradually alter their form,
+until they become undistinguishable from true _Amœbæ_ (Pl. 5, Fig. 18).
+Finally, according to Haeckel, these amœboid bodies, after living for
+a certain time in this condition, return to a state of rest, again
+contract into a spherical form, and secrete round themselves a
+structureless envelope. The life history of some other low organisms, as
+for instance _Gregarina_, is of a similar character.
+
+It may be said, and said truly, that the difference between such beings
+as these and the _Campodea_, or Tardigrade, is immense. But if it be
+considered incredible that even during the long lapse of geological time
+such great changes should have taken place as are implied in the belief
+that there is genetic connection between them and these lower groups,
+let us consider what happens under our eyes in the development of each
+one of these little creatures in the proverbially short space of their
+individual life.
+
+I will take for instance the first stages, and for the sake of brevity
+only the first stages, of the life-history of a Tardigrade.[77] As shown
+in Fig. 60, the egg is at first a round body or cell, with a clear
+central nucleus—the germinal vesicle; it increases in size, and after
+a while the yolk and the germinal vesicle divide into two (Fig. 61),
+then into four (Fig. 62), and so on, just as we have seen to be the case
+in _Magosphæra_. From the minute cells (Fig. 63) arising through this
+process of yolk-segmentation, the body of the Tardigrade is then built
+up.[78]
+
+[Illustration: FIG. 60, Egg of Tardigrade, Kaufmann, Zeit f. Wiss.
+Zool. 1851, Pl. 1. 61, Egg of Tardigrade after the yolk has subdivided.
+62, Egg of Tardigrade in the next stage. 63, Egg of Tardigrade more
+advanced.]
+
+Though I will not now attempt to point out the full bearing of these
+facts on the study of embryology generally, yet I cannot resist calling
+attention to the similarity of the development of _Magosphæra_ with the
+first stages of development of other animals, because it appears to me
+to possess a significance, the importance of which it would be difficult
+to overestimate.
+
+Among the Zoophytes Prof. Allman thus describes[79] the process in
+_Laomedea_, as representing the Hydroids (Pl. 6, Fig. 1, represents the
+young egg):—“The first step observable in the segmentation-process is
+the cleavage of the yolk into two segments (Pl. 6, Fig. 2), immediately
+followed by the cleavage of these into other two, so that the vitellus
+is now composed of four cleavage spheres (Pl. 6, Fig. 3).” These spheres
+again divide (Pl. 6, Fig. 4) and subdivide, thus at length forming
+minute cells, of which the body of the embryo is built up.
+
+In Pl. 6, Figs. 5-9 represent the corresponding stages in the
+development of a small parasitic worm—the _Filaria mustelarum_—as
+given by Van Beneden.[80] The first process is that within the egg,
+which represents, so to say, the encysted condition of _Magosphæra_, the
+yolk divides itself into two balls (Pl. 6, Fig. 6), then into four,
+eight, and so on, the cells thus constituted finally forming the young
+worm. I have myself observed the same stages in the eggs of the very
+remarkable and abnormal _Sphærularia bombi_.[81]
+
+Among the Echinoderms M. Derbès thus describes the first stages (Pl. 6,
+Figs. 10-13) in the development of the egg of an _Echinus_ (_Echinus
+esculentus_):—“Le jaune commence à se segmenter, d’abord en deux, puis
+en quatre et ainsi de suite, chacune des nouvelles cellules se
+partageant à son tour en deux.”[82] Sars has observed the same thing in
+the starfish.[83]
+
+[Illustration: PLATE. 6.]
+
+In the Rotatoria, as shown by Huxley in _Lacinularia_,[84] and by
+Williamson in _Melicerta_,[85] the yolk is at first a single globular
+mass, the first changes which take place in it being as follows:—“The
+central nucleus becomes drawn out and subdivides into two, this division
+being followed by a corresponding segmentation of the yolk. The same
+process is repeated again and again, until at length the entire yolk is
+converted into a mass of minute cells.” Among the Crustacea the total
+segmentation of the yolk occurs among the Copepoda, Rhizocephala, and
+Cirripedia. Sars has described the same process in one of the
+nudibranchiate mollusca[86] (_Tritonia_), Müller in Entochocha,[87]
+Haeckel in Ascidia,[88] Lacaze Duthiers in _Dentalium_.[89] Figures 18 to
+21, Pl. 6, are taken from Koren and Danielssen’s[90] memoir on the
+development of _Purpura lapillus_.
+
+Figs. 22-24 show the same stages in a fish (_Amphioxus_) as given by
+Haeckel, and it is unnecessary to point out the great similarity.
+
+Lastly, figures 25 to 29, Pl. 6, are given by Dr. Allen Thomson,[91] as
+illustrating the first stages in the development of the vertebrata.
+
+I might have given many other examples, but the above are probably
+sufficient, and will show that the processes which constitute the
+life-history of the lowest organized beings very closely resemble the
+first stages in the development of more advanced groups; that as Allen
+Thomson has truly observed,[92] “the occurrence of segmentation and the
+regularity of its phenomena are so constant that we may regard it as one
+of the best established series of facts in organic nature.”
+
+It is true that normal yolk-segmentation is not universal in the animal
+kingdom; that there are great groups in which the yolk does not divide
+in this manner,—perhaps owing to some difference in its relation to the
+germinal vesicle, or perhaps because one of the suppressed stages in
+embryological development, many examples might be given, not only in
+zoology, but, as I may state on the authority of Dr. Hooker, in botany
+also. But, however, this may be, it is surely not uninteresting, nor
+without significance, to find that changes which constitute the
+life-history of the lowest creatures for the initial stages even of the
+highest.
+
+Returning, in conclusion, to the immediate subject of this work, I have
+pointed out that many beetles and other insects are derived from larvæ
+closely resembling _Campodea_.
+
+Since, then, individual insects are certainly in many cases developed
+from larvæ closely resembling the genus _Campodea_, why should it be
+regarded as incredible that insects as a group have gone through similar
+stages? That the ancestors of beetles under the influence of varying
+external conditions, and in the lapse of geological ages, should have
+undergone changes which the individual beetle passes through under our
+own eyes and in the space of a few days, is surely no wild or
+extravagant hypothesis. Again, other insects come from vermiform larvæ
+much resembling the genus _Lindia_, and it has been also repeatedly shown
+that in many particulars the embryo of the more specialized forms
+resembles the full-grown representatives of lower types. I conclude,
+therefore, that the Insecta generally are descended from ancestors
+resembling the existing genus _Campodea_, and that these again have arisen
+from others belonging to a type represented more or less closely by the
+existing genus _Lindia_.
+
+Of course it may be argued that these facts have not really the
+significance which they seem to me to possess. It may be said that when
+Divine power created insects, they were created with these remarkable
+developmental processes. By such arguments the conclusions of geologists
+were long disputed. When God made the rocks, it was tersely said, He
+made the fossils in them. No one, I suppose, would now be found to
+maintain such a theory; and I believe the time will come when it will be
+generally admitted that the structure of the embryo, and its
+developmental changes, indicate as truly the course of organic
+development in ancient times as the contents of rocks and their sequence
+teach us the past history of the earth itself.
+
+
+FOOTNOTES:
+
+[1] Darwin’s “Researches into the Geology and Natural History of the
+Countries visited by H.M.S. _Beagle_,” p. 326.
+
+[2] Introduction to Entomology, vi. p. 50.
+
+[3] Manual of Entomology, p. 30.
+
+[4] Linnean Journal, vol. xi.
+
+[5] Introduction to the Modern Classification of Insects, p. 17.
+
+[6] Linnean Transactions, 1863—“On the Development of _Chloëon_.”
+
+[7] The figures on the first four plates are principally borrowed from
+Mr. Westwood’s excellent “Introduction to the Modern Classification of
+Insects.”
+
+[8] “Sur la Domestication des _Clavigers_ par les Fourmis.” Bull. de la
+Soc. d’Anthropologie de Paris, 1868, p. 315.
+
+[9] Westwood’s Introduction, vol. i. p. 36.
+
+[10] Westwood’s Introduction, vol. ii. p. 52.
+
+[11] Die Fortpflanzung und Entwickelung der Pupiparen. Von Dr. R.
+Leuckart. Halle. 1848.
+
+[12] Ann. des Sci. Nat., sér. 4, tome vii. See also _Natural History
+Review_, April 1862.
+
+[13] Ann. and Mag. of Nat. Hist. 1852.
+
+[14] Zeits. für Wiss. Zool. 1869.
+
+[15] Transactions of the Linnean Society, 1863.
+
+[16] Lectures on the Anatomy, &c. of the Invertebrate Animals.
+
+[17] Untersuchungen über die Entwickelung und den Bau der Gliederthiere,
+1854.
+
+[18] Linnean Transactions, vol. xxii. 1858.
+
+[19] “Embryological Studies on Hexapodous Insects.” Peabody Academy of
+Science. Third Memoir.
+
+[20] Mém. de l’Acad. Imp. des Sci. de St. Pétersbourg. 1869.
+
+[21] Observationes de Prima Insectorum Genesi, p. 14.
+
+[22] Mém. de l’Acad. Imp. des Sci. de St. Pétersbourg. tome xvi. 1871,
+p. 35.
+
+[23] Recherches sur l’Evolution des Araignées.
+
+[24] Philosophical Transactions, 1841.
+
+[25] Monog. of the Gymnoblastic or Tubularian Hydroids. See also Hincks,
+British Hydroid Zoophytes. Pl. x.
+
+[26] Loc. cit. p. 315.
+
+[27] Philosophical Transactions, 1859, p. 589.
+
+[28] “Facts for Darwin,” Eng. Trans. p. 127.
+
+[29] Rolleston, “Forms of Animal Life,” p. 146.
+
+[30] A. Agassiz, “Embryology of the Starfish,” p. 25; “Embryology of
+Echinoderms.” Mem. of Am. Ac. of Arts and Sciences N.S. vol. ix. p. 9.
+
+[31] Ueber die Gattungen der Seeigellarven. Siebente Abhandlung. Kön.
+Akad. d. Wiss. zu Berlin. Von Joh. Müller, 1855, Pl. iii. fig. 3.
+
+[32] Huxley, Introduction to the Classification of Animals, p. 45.
+
+[33] Philosophical Transactions, 1865 and 1866.
+
+[34] Loc. cit. Zweit. Abh. Pl. i., figs. 8 and 9.
+
+[35] Thomson, on the Embryology of the Echinodermata, _Natural History
+Review_, 1863, p. 415. See also Agassiz, “Embryology of the Starfish,”
+p. 62.
+
+[36] A. Agassiz, Embryology of Echinoderms, p. 18.
+
+[37] Hincks. British Hydroid Zoophytes, pp. 120-147.
+
+[38] Zeits. für Wiss. Zool. 1864, p. 228.
+
+[39] Introduction to Entomology, 6th ed. vol. i. p. 61.
+
+[40] Métamorphoses de l’Homme et des Animaux, p. 133. See also
+Carpenter, Principles of Physiology. 1851, p. 389.
+
+[41] Darwin, Origin of Species, 4th ed. p. 532.
+
+[42] Principles of Biology, vi. p. 349.
+
+[43] For differences in larva consequent on variation in the external
+condition, see _ante_, p. 61.
+
+[44] See Hincks. British Hydroid Zoophytes, P. lxii. Agassiz, Sea-side
+Studies, p. 43.
+
+[45] See Newport, Phil. Trans., 1832.
+
+[46] Linnean Transactions, 1862.
+
+[47] Origin of Species, 4th ed., pp. 14 and 97.
+
+[48] On the Alternation of Generations. By J. J. Steenstrup. Trans. by
+C. Busk, Esq. Ray Society. 1842.
+
+[49] Zeit. für Wiss. Zool. 1863.
+
+[50] Mém. de l’Acad. Imp. de St. Pétersbourg. vol. xv. 1870.
+
+[51] Of course all animals in which the sexes are distinct are in one
+sense dimorphic.
+
+[52] “There is no such thing as a true case of ‘alternation of
+generations in the animal kingdom;’ there is only an alternation of true
+generation with the totally distinct process of gemmation or
+fission.”—HUXLEY _on Animal Individuality_, Ann. and Mag. of Nat. Hist.
+June 1852.
+
+[53] Prince Hohenstiel Schwangau, p. 68.
+
+[54] Journal of the Royal Institution. April 1873.
+
+[55] “Embryology of Echinoderms,” l. c. p. 15.
+
+[56] Mr. and Mrs. Agassiz: “Sea-side Studies,” p. 139.
+
+[57] l. c. p. 138.
+
+[58] Wien. Zool. Bot. Gesells, 1869.
+
+[59] Linnean Transactions, 1863.
+
+[60] Linnean Transactions, 1866, vol. xxv.
+
+[61] Linnean Transactions, vol. xxiv. p. 65.
+
+[62] Siebold und Kolliker’s Zeitschr. f. Wiss. Zool., 1864.
+
+[63] Linnean Journal, vol. xi.
+
+[64] Facts for Darwin, p. 120.
+
+[65] A still nearer approach is afforded by the genus _Peripatus_, which
+since the above was written has been carefully described, especially by
+Moseley and Hutton. There are several species, scattered over the
+southern hemisphere. In general appearance they look like a link between
+a caterpillar and a centipede. They have a pair of antennæ, two pairs of
+jaws, and (according to the species) from fourteen to thirty-three pairs
+of legs. They breathe by means of tracheæ, which open diffusely all over
+the body.
+
+[66] Unters. üb. die Entwick, und den Bau der Gliederthiere, p. 73.
+
+[67] Linnean Transactions, v. xxii.
+
+[68] Facts for Darwin, trans. by Dallas, p. 118. See also Darwin,
+“Origin of Species,” p. 530. 4th ed.
+
+[69] Mem. Peabody Academy of Science, v. I. No, 3.
+
+[70] Wien. Zool. Bott. Gesells. 1869, p. 310.
+
+[71] See also the descriptions given by Dujardin (Ann. des Sci. Nat.
+1851, v. xv.) and Claparède (Anat. und Entwickl. der Wirbel osen Thiere)
+of the interesting genus _Echinoderes_, which these two eminent
+naturalists unite in regarding as intermediate between the Annelides and
+the Crustacea.
+
+[72] “On a New Rotifer.” _Monthly Microscopical Journal_, Sept. 1871.
+
+[73] Generelle Morphologie, vol. ii. p. 79.
+
+[74] Monographie der Moneren, p. 43.
+
+[75] Gegenbaur. Grund. d. Vergleich. Anat. p. 210. See also Dr. M. S.
+Schultze, Beiträge zur Naturg. der. Turbellarien. 1851. Pl. vi. fig. 1.
+
+[76] Monographieder Moneren, p. 10.
+
+[77] See Kauffmann, Ueber die Entwickelung and systematische Stellung
+der Tardigraden. Zeits. f. Wiss. Zool. 1851, p. 220.
+
+[78] It is true that among the Insecta generally the first stages of
+development differ in appearance considerably from those above
+described; those of _Platygaster_, as figured by Ganin (ante Figs. 17-22),
+being very exceptional.
+
+[79] Monograph of the Gymnoblastic or Tubularian Hydroids, by G. J.
+Allman, Ray Soc. 1871, p. 86.
+
+[80] Mém. sur les Vers Intestinaux, 1858.
+
+[81] Natural History Review, 1861, p. 44.
+
+[82] Ann. des Sci. Nat. 1847, p. 90.
+
+[83] Fauna littoralis Norvegiæ, pl. viii.
+
+[84] Trans. of the Microsc. Soc. of London, 1851.
+
+[85] Quarterly Journal of Microsc. Science, 1853.
+
+[86] Wiegmann’s Archiv., 1840, p. 196.
+
+[87] Ueber die Erzeugung von Schnecken in Holothurier. Berlin, Bericht,
+1851. Ann. Nat. Hist. 1852, v. ix. Müller’s Archiv., 1852.
+
+[88] Natürliche Schöpfungsgeschichte, pl. x.
+
+[89] Ann. des Sci. Nat. 1853, p. 89.
+
+[90] Ann. des Sci. Nat. 1857, pl. vi.
+
+[91] Cyclopædia of Anatomy and Physiology. Art. Ovum, p. 4.
+
+[92] Thomson, loc. cit. Article, Ovum, p. 139.
+
+
+   THE END.
+
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