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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-14 20:09:45 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-14 20:09:45 -0700 |
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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 <a href = "http://www.gutenberg.org">www.gutenberg.org</a></pre> +<p>Title: On the Origin and Metamorphoses of Insects</p> +<p>Author: Sir John Lubbock</p> +<p>Release Date: December 3, 2011 [eBook #38207]</p> +<p>Language: English</p> +<p>Character set encoding: ISO-8859-1</p> +<p>***START OF THE PROJECT GUTENBERG EBOOK ON THE ORIGIN AND METAMORPHOSES OF INSECTS***</p> +<p> </p> +<h4 class="pg">E-text prepared by Bryan Ness, Turgut Dincer,<br /> + and the Online Distributed Proofreading Team<br /> + (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br /> + from page images generously made available by<br /> + Internet Archive/American Libraries<br /> + (<a href="http://www.archive.org/details/americana">http://www.archive.org/details/americana</a>)</h4> +<p> </p> +<table border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10"> + <tr> + <td valign="top"> + Note: + </td> + <td> + Images of the original pages are available through + Internet Archive/American Libraries. See + <a href="http://www.archive.org/details/onoriginmetamorp00lubb"> + http://www.archive.org/details/onoriginmetamorp00lubb</a> + </td> + </tr> +</table> +<p> </p> +<div class="box"> +<p>Transcriber's note:<br /><br /> +All non-italic genus names in the text have been italicized.</p> +</div> +<p> </p> +<hr class="full" /> +<p> </p> + +<div class="figcenter" style="width: 350px;"><a name="logo" id="logo"></a> +<img src="images/logo_001.png" width="350" height="155" +alt="NATURE SERIES" title="" /></div> + +<h1>ORIGIN AND METAMORPHOSES</h1> + +<p class="right"><i>OF INSECTS.</i></p> +<p> </p> +<div class="figcenter" style="width: 500px;"><a name="frontispiece" id="frontispiece"></a> +<img src="images/frontispiece.png" width="500" height="727" +alt="frontispiece" title="" /></div> + +<div class="u"><i>NATURE SERIES</i></div> +<h3>ON THE</h3> +<h1>ORIGIN AND METAMORPHOSES</h1> +<h1>OF INSECTS</h1> + +<h4>BY</h4> + +<h3><span class="smcap">SIR JOHN LUBBOCK, Bart.</span>, M.P., F.R.S., D.C.L., LL.D.</h3> +<h6>PRINCIPAL OF THE LONDON WORKING MEN’S COLLEGE; PRESIDENT OF THE LONDON<br /> +CHAMBER OF COMMERCE; AND VICE-CHAIRMAN OF THE LONDON COUNTY COUNCIL</h6> + +<h3><i>WITH NUMEROUS ILLUSTRATIONS</i><br /></h3> +<p> </p> +<p> </p> +<p> </p> + +<h4><span class="oldtype">London</span></h4> +<h4>MACMILLAN AND CO.</h4> +<h5>AND NEW YORK</h5> +<h5>1890</h5> +<h6><i>The Right of Translation and Reproduction is Reserved</i></h6> + +<h6><span class="smcap">Richard Clay and Sons, Limited,<br /> +london and bungay.</span></h6> + +<h6><i>First Edition 1873. Reprinted 1874.<br /> +New Edition 1890.</i></h6> +<p><span class='pagenum'><a name="Page_vii" id="Page_vii">vii</a></span></p> + +<hr /> + +<h3>PREFACE.</h3> + +<p>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<span class='pagenum'><a name="Page_viii" id="Page_viii">viii</a></span> +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 +<i>Nature</i>, 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.</p> + +<p>The papers above referred to are as follows:—</p> + +<p class="indent">1. On <i>Labidocera</i>.—Annals and Magazine of Natural History, vol. +xi., 1853.</p> + +<p class="indent">2. On Two New Sub-genera of Calanidæ.—Annals and Magazine of +Natural History, vol. xii., 1853.</p> +<p><span class='pagenum'><a name="Page_ix" id="Page_ix">ix</a></span></p> +<p class="indent">3. On Two New Species of Calanidæ.—Annals and Magazine of +Natural History, vol. xii., No. lxvii., 1853.</p> + +<p class="indent">4. On Two New Species of Calanidæ.—Annals and Magazine of +Natural History, vol. xii., No. lxix., 1853.</p> + +<p class="indent">5. On some Arctic Calanidæ.—Annals and Magazine of Natural +History, 1854.</p> + +<p class="indent">6. On the Freshwater Entomostraca of South America.—Transactions +of the Entomological Society, vol. iii., 1855.</p> + +<p class="indent">7. On some New Entomostraca.—Transactions of the Entomological +Society, vol. iv., 1856.</p> + +<p class="indent">8. On some Marine Entomostraca found at Weymouth.—Annals and +Magazine of Natural History, vol. xx., 1857.</p> + +<p class="indent">9. On the Respiration of Insects.—Entomological Annual, 1857.</p> + +<p class="indent">10. An Account of the Two Methods of Reproduction in <i>Daphnia</i>.—Transactions +of the Royal Society, 1857.</p> + +<p class="indent">11. On the Ova and Pseudova of Insects.—Transactions of the Royal +Society, 1858.</p> + +<p class="indent">12. On the Arrangement of the Cutaneous Muscles of <i>Pygæra Bucephala</i>.—Linnean +Society’s Transactions, vol. xxii., 1858.</p> + +<p class="indent">13. On the Freshwater Entomostraca of South America.—Entomological +Society’s Transactions, 1858.</p> + +<p class="indent">14. On <i>Coccus Hesperidum</i>.—Royal Society Proceedings, vol. ix., +1858.</p> + +<p class="indent">15. On the Distribution of Tracheæ in Insects.—Linnean Society’s +Transactions, vol. xxiii., 1860.</p> + +<p class="indent">16. On the Generative Organs and on the Formation of the Egg in +Annulosa. Transactions of the Royal Society, 1861.</p> + +<p class="indent">17. On <i>Sphærularia Bombi</i>.—Natural History Review, 1861.</p> + +<p class="indent">18. On some Oceanic Entomostraca.—Linnean Society’s Transactions, +vol. xxiii., 1860.</p> + +<p class="indent">19. On the Thysanura. Part 1.—Linnean Society’s Transactions, 1862.</p> + +<p class="indent">20. On the Development of <i>Lonchoptera</i>.—Entomological Society’s +Transactions, 1862.</p> + +<p class="indent">21. On the Thysanura. Part 2.—Linnean Society’s Transactions, 1862.</p> + +<p class="indent">22. On the Development of <i>Chloëon</i>. Part 1.—Linnean Society’s +Transactions, 1863.</p> + +<p class="indent">23. On Two Aquatic Hymenoptera.—Linnean Society’s Transactions, +1863.</p> + +<p class="indent">24. On some little-known Species of Freshwater Entomostraca.—Linnean +Society’s Transactions, vol. xxiv., 1863.</p> + +<p class="indent">25. On <i>Sphærularia Bombi</i>.—Natural History Review, 1864.</p> + +<p><span class='pagenum'><a name="Page_x" id="Page_x">x</a></span></p> + +<p class="indent">26. On the Development of <i>Chloëon</i>. Part 2.—Linnean Society’s +Transactions, 1865.</p> + +<p class="indent">27. Metamorphoses of Insects.—Journal of the Royal Institution, 1866.</p> + +<p class="indent">28. On <i>Pauropus</i>.—Linnean Society’s Transactions, 1866.</p> + +<p class="indent">29. On the Thysanura. Part 3.—Linnean Society’s Transactions, 1867.</p> + +<p class="indent">30. Address to the Entomological Society.—Entomological Society’s +Transactions, 1867.</p> + +<p class="indent">31. On the Larva of <i>Micropeplus Staphilinoides</i>.—Entomological +Society’s Transactions, 1868.</p> + +<p class="indent">32. On the Thysanura. Part 4.—Linnean Society’s Transactions, 1869.</p> + +<p class="indent">33. Addresses to the Entomological Society.—Entomological Society’s +Transactions, 1867-1868.</p> + +<p class="indent">34. On the Origin of Insects.—Journal of the Linnean Society, vol. xi.</p> + +<p class="indent">35. Opening Address to the Biological Section of the British Association.—British +Association Report, 1872.</p> + +<p class="indent">36. Observations on Ants, Bees, and Wasps. Part 1.—Journal of the +Linnean Society, 1873.</p> + +<p class="indent">37. On British Wild Flowers considered in relation to Insects, 1874.</p> + +<p class="indent">38. Observations on Ants, Bees, and Wasps. Part 2.—Journal of the +Linnean Society, 1874.</p> + +<p class="indent">39. Observations on Ants, Bees, and Wasps. Part 3.—Journal of the +Linnean Society, 1875.</p> + +<p class="indent">40. Observations on Ants, Bees, and Wasps. Part 4.—Journal of the +Linnean Society, 1877.</p> + +<p class="indent">41. On some Points in the Anatomy of Ants.—Quekett Lecture, +1877.—Microscopical Journal.</p> + +<p class="indent">42. On the Colors of Caterpillars.—Entomological Society’s Transactions, +1878.</p> + +<p class="indent">43. Observations on Ants, Bees, and Wasps. Part 5.—Journal of the +Linnean Society, 1878.</p> + +<p class="indent">44. Observations on Ants, Bees, and Wasps. Part 6.—Journal of the +Linnean Society, 1879.</p> + +<p class="indent">45. On the Anatomy of Ants.—Linnean Society’s Transactions, 1880.</p> + +<p class="indent">46. Observations on Ants, Bees, and Wasps. Part 7.—Journal of the +Linnean Society, 1880.</p> + +<p class="indent">47. Observations on Ants, Bees, and Wasps. Part 8.—Journal of the +Linnean Society, 1881.</p> + +<p class="indent">48. On Fruits and Seeds.—Journal of the Royal Institution, 1881.</p> + +<p class="indent">49. Observations on Ants, Bees, and Wasps. Part 9.—Journal of the +Linnean Society, 1881.</p> + +<p class="indent">50. On the Limits of Vision among some of the lower Animals.—Journal +of the Linnean Society, 1881.</p> + +<p class="indent">51. Observations on Ants, Bees, and Wasps. Part 10.—Journal of the +Linnean Society, 1882.</p> + +<p><span class='pagenum'><a name="Page_xi" id="Page_xi">xi</a></span></p> + +<hr /> +<h3>CONTENTS.<br /><br /></h3> + +<table width="100%" summary="contents" border="0"> +<tr> +<td class="t100" colspan="2">CHAPTER I.</td> +</tr><tr> +<td class="t100s" colspan="2">THE CLASSIFICATION OF INSECTS.</td> +</tr><tr> +<td class="t85">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</td> +<td class="t15"><i>page </i><a href="#Page_1">1-26</a></td> +</tr> +<tr> +<td class="t100" colspan="2">CHAPTER II.</td> +</tr><tr> +<td class="t100s" colspan="2">THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND +STRUCTURE OF LARVÆ.</td> +</tr><tr> +<td class="t85">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 <i>Sirex</i>; of <i>Tenthredo</i>; of +Ichneumons; of Bees.—Rudimentary legs of Bee Embryo.—Beetles, +<i>Weevils</i>, <i>Scolytus</i>, <i>Crioceris</i>, <i>Sitaris</i>, Metamorphoses of Pteromalidæ. +<i>Platygaster</i>, <i>Polynema</i>.—Influence of external conditions.—Developmental +and adaptive Metamorphoses</td> +<td class="t15"><i>page </i><a href="#Page_27">27-41</a></td> +</tr> +<tr> +<td class="t100" colspan="2">CHAPTER III.</td> +</tr><tr> +<td class="t100s" colspan="2">THE NATURE OF METAMORPHOSES.</td> +</tr><tr> +<td class="t85">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 <i>Phryganea</i>; of <i>Aphis</i>; of +<span class='pagenum'><a name="Page_xii" id="Page_xii">xii</a></span><i>Chrysopa</i>.—Libellulidæ.—<i>Donacia</i>.—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.</td> +<td class="t15"><i>page </i><a href="#Page_41">41-62</a></td> +</tr> +<tr> +<td class="t100" colspan="2">CHAPTER IV</td> +</tr><tr> +<td class="t100s" colspan="2">THE ORIGIN OF METAMORPHOSES.</td> +</tr><tr> +<td class="t85">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 <i>Campodea</i> 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 <i>Cecidomyia</i>.—In alternation +of generations one form always agamic.—Dimorphism and Dieidism.—Summary +and Conclusions</td> +<td class="t15"><i>page </i><a href="#Page_62">62-81</a></td> +</tr> +<tr> +<td class="t100" colspan="2">CHAPTER V</td> +</tr><tr> +<td class="t100s" colspan="2">THE ORIGIN OF INSECTS.</td> +</tr><tr> +<td class="t85">The Origin of Insects.—Mistaken views of Darwinian theory.—Natural +selection a <i>vera causa</i>.—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.—<i>Campodea</i> +the modern representative of the Insect-stock.—<i>Campodea</i>, +perhaps derived from Tardigrade.—Vermiform or Apod type of +larva.—Views of Fritz Müller, Brauer, and Packard.—Represents +a still earlier ancestor.—Modern representatives.—<i>Notommata</i>, +<i>Albertia</i>, <i>Lindia.</i>—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</td> +<td class="t15"><i>page </i><a href="#Page_82">82-108</a></td> +</tr> +</table> + +<hr /> + +<p><span class='pagenum'><a name="Page_xiii" id="Page_xiii">xiii</a></span></p> + +<h3>DESCRIPTION OF THE PLATES.</h3> + +<h4><a href="#Page_8">PLATE I. p. 7.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1. Cricket. Westwood, Intro. to the Modern Classification of Insects, +vol. i. p. 440.</p> + +<p class="indent">2. Earwig. Westwood, loc. cit. vol. i. p. 399.</p> + +<p class="indent">3. <i>Aphis</i>. Packard, Guide to the Study of Insects, pp. 521, 522.</p> + +<p class="indent">4. <i>Scolytus</i>. Westwood, loc. cit. vol. i. p. 350.</p> + +<p class="indent">5. <i>Anthrax</i>. Westwood, loc. cit. vol. ii. p. 538.</p> + +<p class="indent">6. <i>Balaninus</i>.</p> + +<p class="indent">7. <i>Cynips</i>. Westwood, loc. cit. vol. ii. p. 121.</p> + +<p class="indent">8. Ant (<i>Formica</i>). Westwood, loc. cit. vol. ii. p. 218.</p> + +<p class="indent">9. Wasp. Ormerod, Nat. Hist. of Wasps, pl. i. fig. 1.</p> + +<h4><a href="#Page_9">PLATE II. p. 8.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1. Larva of Cricket. Westwood, loc. cit. vol. i. p. 440.</p> + +<p class="indent">2. Larva of <i>Aphis</i>. Packard, loc. cit. pp. 521, 522.</p> + +<p class="indent">3. Larva of Earwig. Westwood, loc. cit. vol. i. p. 399.</p> + +<p class="indent">4. Larva of <i>Scolytus</i>. Westwood, loc. cit. vol. i. p. 350.</p> + +<p class="indent">5. Larva of <i>Anthrax</i>. Westwood, loc. cit. vol. ii. p. 546.</p> + +<p class="indent">6. Larva of <i>Balaninus</i>.</p> + +<p class="indent">7. Larva of <i>Cynips</i>. Westwood, loc. cit. vol. ii. p. 121.</p> + +<p class="indent">8. Larva of Ant (<i>Formica</i>). Westwood, loc. cit. vol. ii. p. 226.</p> + +<p class="indent">9. Larva of Wasp. Newport, Art. Insecta, Todd’s Cycl. Anat. and +Phys., p. 871.</p> + +<h4><a href="#Page_14">PLATE III. p. 14.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1. <i>Chloëon</i>. Linn. Trans. 1866.</p> + +<p class="indent">2. <i>Meloë</i>. Spry and Shuckard, Coleoptera Delineated, pl. 56.</p> + +<p class="indent">3. <i>Calepteryx</i>.</p> + +<p class="indent">4. <i>Sitaris</i>. Spry and Shuckard, loc. cit. pl. 56.</p> + +<p class="indent">5. <i>Campodea</i>. Suites à Buffon. Aptéres.</p> + +<p><span class='pagenum'><a name="Page_xiv" id="Page_xiv">xiv</a></span></p> + +<p class="indent">6. <i>Acilius</i>. Westwood, loc. cit. vol. i. p. 100.</p> + +<p class="indent">7. <i>Termes</i>. Westwood, loc. cit. vol. ii. p. 12.</p> + +<p class="indent">8. <i>Stylops</i>. Duncan, Met. of Insects, p. 387; Packard, p. 482.</p> + +<p class="indent">9. <i>Thrips</i>. Westwood, loc. cit. vol. ii. p. 1.</p> + +<h4><a href="#Page_15">PLATE IV. p. 15.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1. Larva of <i>Chloëon</i>. Linn. Trans. 1863.</p> + +<p class="indent">2. Larva of <i>Meloë</i>. Chapuis and Candèze, Mem. Soc. Roy. Liége, +1853, pp. 1, 7.</p> + +<p class="indent">3. Larva of <i>Calepteryx</i>. Dufour, Ann. Sci. Nat. 1852.</p> + +<p class="indent">4. Larva of <i>Sitaris</i>. Duncan, Met. of Insects, p. 309.</p> + +<p class="indent">5. Larva of <i>Campodea</i>. Gervais' Suites à Buffon. Aptéres.</p> + +<p class="indent">6. Larva of <i>Acilius</i>. Westwood, loc. cit. vol. i. p. 100.</p> + +<p class="indent">7. Larva of <i>Termes</i>. Duncan, loc. cit. p. 348.</p> + +<p class="indent">8. Larva of <i>Stylops</i>. Westwood, Trans. Ent. Soc. 1839, vol. ii. +pl. xv. fig. 13a.</p> + +<p class="indent">9. Larva of <i>Thrips</i>. Westwood, loc. cit. vol. ii. p. i.</p> + +<h4><a href="#Page_8">PLATE V. p. 99.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1-5. <i>Protamœba</i>.</p> + +<p class="indent">6-9. <i>Protamyxa aurantiaca</i>. Haeckel Beit. zur. Monog. der Moneren, +pl. 1.</p> +<p class="indent">10-18. <i>Magosphœra planula</i>. Haeckel, loc. cit. pl. v.</p> + +<h4><a href="#Page_8">PLATE VI. p. 105.</a></h4> + +<p class="small">FIG.</p> + +<p class="indent">1-4. Yolk-segmentation in <i>Laomedea</i>. After Allman. Mon. of +Tubularian Hydroids. Ray Society.</p> + +<p class="indent">5-9. Yolk-segmentation in <i>Filaria</i>. After Van Beneden. Mem. +sur les Vers Intestinaux.</p> + +<p class="indent">10-13. Yolk-segmentation in <i>Echinus</i>. After Derbes. Ann. des. Sci. +Nat. 1847.</p> + +<p class="indent">14-17. Yolk-segmentation in <i>Lacinularia</i>. After Huxley. J. of Mic. +Sci. 1853.</p> + +<p class="indent">18-21. Yolk-segmentation in <i>Purpura</i>. After Koren and Danielssen. +Ann. des. Sci. Nat. 1853.</p> + +<p class="indent">22-24. Yolk-segmentation in <i>Amphioxus</i>. After Haeckel. Naturliche +Schöpfungsgeschichte, pl. x.</p> + +<p class="indent">25-29. Yolk-segmentation in Vertebrate. After Allen Thompson. +Art. Ovum. Cyclop. of Anatomy and Physiology.</p> + +<p><span class='pagenum'><a name="Page_xv" id="Page_xv">xv</a></span></p> + +<hr /> + +<h3>DESCRIPTION OF THE FIGURES.</h3> + +<p class="small">FIG.</p> + +<p class="indent">1. Larva of the Cockchafer (<i>Melolontha</i>)</p> + +<p class="indent">2. Larva of <i>Cetonia</i>.</p> + +<p class="indent">3. Larva of <i>Trox</i>.</p> + +<p class="indent">4. Larva of <i>Oryctes</i>.</p> + +<p class="indent">5. Larva of <i>Aphodius</i>.</p> + +<p class="indent">6. Larva of <i>Lucanus</i>.</p> + +<p class="indent">7. Larva of <i>Brachytarsus</i>.</p> + +<p class="indent">8. Larva of <i>Crioceris</i>.</p> + +<p class="indent">9. Larva of <i>Sitaris humeralis.</i></p> + +<p class="indent">10. Larva of <i>Sitaris humeralis</i>, in the second stage.</p> + +<p class="indent">11. Larva of <i>Sitaris humeralis</i>, in the third stage.</p> + +<p class="indent">12. Larva of <i>Sitaris humeralis</i>, in the fourth stage.</p> + +<p class="indent">13. Pupa of <i>Sitaris</i>.</p> + +<p class="indent">14. Larva of <i>Sirex</i>.</p> + +<p class="indent">15. Egg of <i>Rhynchites</i>, showing the parasitic larva.</p> + +<p class="indent">16. The parasitic larva, more magnified.</p> + +<p class="indent">17. Egg of <i>Platygaster</i>.</p> + +<p class="indent">18. Egg of <i>Platygaster</i>, showing the central cell.</p> + +<p class="indent">19. Egg of <i>Platygaster</i>, after the division of the central cell.</p> + +<p class="indent">20. Egg of <i>Platygaster</i>, more advanced.</p> + +<p class="indent">21. Egg of <i>Platygaster</i>, more advanced.</p> + +<p class="indent">22. Egg of <i>Platygaster</i>, showing the rudiment of the embryo.</p> + +<p class="indent">23. Larva of <i>Platygaster</i>.—<i>mo</i>, mouth; <i>a</i>, antenna; <i>kf</i>, hooked feet; +<i>r</i>, toothed process; <i>lfg</i>, lateral process; <i>f</i>, branches of the tail.</p> + +<p class="indent">24. Larva of another species of <i>Platygaster</i>. (The letters indicate the +same parts as in the preceding figure.)</p> + +<p class="indent">25. Larva of a third species of <i>Platygaster</i>. (The letters indicate the +same parts as in the preceding figure.)</p> + +<p class="indent">26. Larva of <i>Platygaster</i> in the second stage.—<i>mo</i>, mouth; <i>slkf</i>, +œsophagus; <i>gsae</i>, supra-œsophagal ganglion; <i>lm</i>, muscles; +<i>bsm</i>, nervous system; <i>gagh</i>, rudiments of the reproductive glands.</p> + +<p class="indent">27. Larva of <i>Platygaster</i> in the third stage.—<i>mo</i>, mouth; <i>ma</i>, mandibles; +<i>gsae</i>, supra-œsophagal ganglion; <i>slk</i>, œsophagus; <i>ag</i>, +ducts of the salivary glands; <i>bnm</i>, ventral nervous system; <i>sp</i>, +salivary glands; <i>msl</i>, stomach; <i>im</i>, imaginal discs; <i>tr</i>, tracheæ; +<i>fk</i>, fatty tissue; <i>ed</i>, intestine; <i>ga</i>, rudiments of reproductive +organs; <i>ew</i>, wider portion of intestine; <i>ao</i>, posterior opening.</p> + +<p class="indent">28. Embryo of <i>Polynema</i>.</p> + +<p class="indent">29. Larva of <i>Polynema</i>.—<i>asch</i>, rudiments of the antennæ; <i>flsch</i>, of +the wings; <i>bsch</i>, of the legs; <i>vfg</i>, lateral projections; <i>gsch</i>, +rudiments of the ovipositor; <i>fk</i>, fatty tissue.</p> + +<p class="indent"><span class='pagenum'><a name="Page_xvi" id="Page_xvi">xvi</a></span> +30. Egg of <i>Phryganea</i> (Mystacides).—<i>A</i><sup>1</sup>, mandibular segment; <i>C</i><sup>1</sup>-<i>C</i><sup>5</sup>, +maxillary, labial, and three thoracic segments; <i>D</i>, abdomen.</p> + +<p class="indent">31. Egg of <i>Phryganea</i> somewhat more advanced.—<i>b</i>, mandibles; <i>c</i>, +maxillæ; <i>cfs</i>, rudiments of the three pairs of legs.</p> + +<p class="indent">32. Egg of <i>Pholcus opilionides</i>, showing the Protozonites.</p> + +<p class="indent">33. Embryo of <i>Julus</i>.</p> + +<p class="indent">34. Colony of <i>Bougainvillea fruticosa</i>, natural size, attached to the underside +of a piece of floating timber.</p> + +<p class="indent">35. Portion of the same, more magnified.</p> + +<p class="indent">36. The Medusa from the same species.</p> + +<p class="indent">37. Larva of Prawn, Nauplius stage.</p> + +<p class="indent">38. Larva of Prawn, more advanced, Zoëa stage.</p> + +<p class="indent">39. Larva of <i>Echino-cidaris œquituberculata</i> seen from above ✕ 6/10.</p> + +<p class="indent">40. Larva of <i>Echinus</i> ✕ 100.—<i>A</i>, front arm; <i>F</i>, arms of the mouth-process; +<i>B</i>, posterior side arm; <i>E</i><sup>1</sup>, accessory arm of the mouth-process; +<i>a</i>, mouth; <i>a</i><sup>1</sup>, œsophagus; <i>b</i>, stomach; <i>b</i><sup>1</sup>, intestine; +<i>o</i>, posterior orifice; <i>d</i>, ciliated bands; <i>f</i>, ciliated epaulets; +<i>c</i>, disc of future <i>Echinus</i>.</p> + +<p class="indent">41. <i>Comatula rosacea</i>.</p> + +<p class="indent">42. Larva of <i>Comatula rosacea</i>.</p> + +<p class="indent">43. Larva of <i>Comatula rosacea</i>, more advanced.</p> + +<p class="indent">44. Larva of <i>Comatula rosacea</i>, in the Pentacrinus state.</p> + +<p class="indent">45. Larva of Starfish (Bipinnaria), ✕ 100.</p> + +<p class="indent">46. Larva of Starfish (Bipinnaria), ✕ 100, seen from the side.—<i>a</i>, +mouth; <i>b</i>, œsophagus; <i>c</i>, stomach; <i>c</i><sup>1</sup>, intestine.</p> + +<p class="indent">47. Larva of another Bipinnaria, showing the commencement of the +Starfish.—<i>g</i>, canal of the ciliated sac; <i>i</i>, rudiments of tentacles; +<i>d</i>, ciliated band.</p> + +<p class="indent">48. Larva of Moth (<i>Agrotis</i>).</p> + +<p class="indent">49. Larva of Beetle (<i>Haltica</i>).</p> + +<p class="indent">50. Larva of Saw-fly (<i>Cimbex</i>).</p> + +<p class="indent">51. Larva of <i>Julus</i>.</p> + +<p class="indent">52. <i>Agrotis suffusa</i>.</p> + +<p class="indent">53. <i>Haltica</i>.</p> + +<p class="indent">54. <i>Cimbex</i>.</p> + +<p class="indent">55. <i>Julus</i>.</p> + +<p class="indent">56. Tardigrade.</p> + +<p class="indent">57. Larva of <i>Cecidomyia</i>.</p> + +<p class="indent">58. <i>Lindia torulosa</i>.</p> + +<p class="indent">59. <i>Prorhynchus stagnalis</i>.</p> + +<p class="indent">60. Egg of Tardigrade.</p> + +<p class="indent">61. Egg of Tardigrade, after the yolk has subdivided.</p> + +<p class="indent">62. Egg of Tardigrade, in the next stage.</p> + +<p class="indent">63. Egg of Tardigrade, more advanced.</p> + +<hr /> + +<p><span class='pagenum'><a name="Page_1" id="Page_1">1</a></span></p> + +<h4>ON THE</h4> +<h2>ORIGIN AND METAMORPHOSES<br /> +OF INSECTS.</h2> + +<h4>CHAPTER I.</h4> + +<h4><i>THE CLASSIFICATION OF INSECTS.</i></h4> + +<p>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.<a name="FNanchor_1" id="FNanchor_1"></a><a href="#Footnote_1" class="fnanchor">1</a> 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.</p> + +<p>Thus, according to Kirby and Spence,<a name="FNanchor_2" id="FNanchor_2"></a><a href="#Footnote_2" class="fnanchor">2</a> “The states +through which insects pass are four: the <i>egg</i>, the +<i>larva</i>, the <i>pupa</i>, and the <i>imago</i>.” Burmeister,<a name="FNanchor_3" id="FNanchor_3"></a><a href="#Footnote_3" class="fnanchor">3</a> also, +<span class='pagenum'><a name="Page_2" id="Page_2">2</a></span>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.</p> + +<p>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<span class='pagenum'><a name="Page_3" id="Page_3">3</a></span> +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.</p> + +<h4>ORDERS OF INSECTS ACCORDING TO WESTWOOD.<br /><br /></h4> + +<table width="80%" summary="orders of insects"> +<tr> +<td class="t30"> 1. <span class="smcap">Hymenoptera</span></td> +<td class="t70">Bees, Wasps, Ants, &c.</td> +</tr><tr> +<td class="t30"> 2. Strepsiptera</td> +<td class="t70"><i>Stylops</i>, <i>Zenos</i>, &c.</td> +</tr><tr> +<td class="t30"> 3. <span class="smcap">Coleoptera</span></td> +<td class="t70">Beetles.</td> +</tr><tr> +<td class="t30"> 4. Euplexoptera</td> +<td class="t70">Earwigs.</td> +</tr><tr> +<td class="t30"> 5. <span class="smcap">Orthoptera</span></td> +<td class="t70">Grasshoppers, Crickets, Cockroaches, &c.</td> +</tr><tr> +<td class="t30"> 6. Thysanoptera</td> +<td class="t70"><i>Thrips</i>.</td> +</tr><tr> +<td class="t30"> 7. <span class="smcap">Neuroptera</span></td> +<td class="t70"><i>Ephemeras</i>, &c.</td> +</tr><tr> +<td class="t30"> 8. Trichoptera</td> +<td class="t70"><i>Phryganea</i>.</td> +</tr><tr> +<td class="t30"> 9. <span class="smcap">Diptera</span></td> +<td class="t70">Flies and Gnats.</td> +</tr><tr> +<td class="t30">10. Aphaniptera</td> +<td class="t70">Fleas</td> +</tr><tr> +<td class="t30">11. <span class="smcap">Heteroptera</span></td> +<td class="t70">Bugs.</td> +</tr><tr> +<td class="t30">12. <span class="smcap">Homoptera</span></td> +<td class="t70"><i>Aphis</i>, <i>Coccus</i>, &c.</td> +</tr><tr> +<td class="t30">13. <span class="smcap">Lepidoptera</span></td> +<td class="t70">Butterflies and Moths.</td> +</tr> +</table> + +<p>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.</p> + +<p>The Euplexoptera or Earwigs are only too familiar +to most of us. Linnæus classed them among the<span class='pagenum'><a name="Page_4" id="Page_4">4</a></span> +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.</p> + +<p>The Thysanoptera, consisting of the Linnæan genus +<i>Thrips</i>, 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.</p> + +<p>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 <i>Phryganea</i> “forms +the connecting link between the Neuroptera and Lepidoptera.”</p> + +<p>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 “<i>la puce est un diptère sans +ailes</i>.” Westwood, however, regards it as constituting +a separate order.</p> + +<p>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<span class='pagenum'><a name="Page_5" id="Page_5">5</a></span> +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,<a name="FNanchor_4" id="FNanchor_4"></a><a href="#Footnote_4" class="fnanchor">4</a> the Collembola (<i>Podura</i>, +<i>Smynthurus</i>, &c.) occupy an intermediate position. +These two series are:—</p> + +<table width="70%" summary="orders of insects"> +<tr> +<td class="t50"><span class="smcap">Mandibulata.</span></td> +<td class="t50"><span class="smcap">Haustellata.</span></td> +</tr> +<tr> +<td class="t50">Hymenoptera.</td> +<td class="t50">Lepidoptera.</td> +</tr> +<tr> +<td class="t50">Strepsiptera.</td> +<td class="t50">Diptera.</td> +</tr> +<tr> +<td class="t50">Coleoptera.</td> +<td class="t50">Aphaniptera.</td> +</tr> +<tr> +<td class="t50">Euplexoptera.</td> +<td class="t50">Hemiptera.</td> +</tr> +<tr> +<td class="t50">Orthoptera.</td> +<td class="t50">Homoptera.</td> +</tr> +<tr> +<td class="t50">Trichoptera?</td> +<td class="t50"> </td> +</tr> +<tr> +<td class="t50">Thysanoptera?</td> +<td class="t50"> </td> +</tr> +</table> + +<p>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,<a name="FNanchor_5" id="FNanchor_5"></a><a href="#Footnote_5" class="fnanchor">5</a> “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.”</p> + +<p><span class='pagenum'><a name="Page_6" id="Page_6">6</a></span></p> + +<table width="70%" summary="orders of insects"> +<tr> +<td class="t50"><span class="smcap">Heteromorpha.</span></td> +<td class="t50"><span class="smcap">Haustellata.</span></td> +</tr> +<tr> +<td class="t50">Hymenoptera.</td> +<td class="t50">Euplexoptera.</td> +</tr> +<tr> +<td class="t50">Strepsiptera.</td> +<td class="t50">Orthoptera.</td> +</tr> +<tr> +<td class="t50">Coleoptera.</td> +<td class="t50">Hemiptera.</td> +</tr> +<tr> +<td class="t50">Trichoptera.</td> +<td class="t50">Homoptera.</td> +</tr> +<tr> +<td class="t50">Diptera.</td> +<td class="t50">Thysanoptera.</td> +</tr> +<tr> +<td class="t50">Aphaniptera.</td> +<td class="t50"> </td> +</tr> +<tr> +<td class="t50">Lepidoptera.</td> +<td class="t50"> </td> +</tr> +<tr> +<td class="t100" colspan="2">Neuroptera.</td> + +</tr> +</table> + +<p>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.</p> + +<p>I have, however, elsewhere<a name="FNanchor_6" id="FNanchor_6"></a><a href="#Footnote_6" class="fnanchor">6</a> 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.</p> + +<p>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, +<span class='pagenum'><a name="Page_7" id="Page_7">7</a></span>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 <a href="#p2">II.</a>, 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.</p> + +<p>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.</p> + +<p>The sawflies 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.</p> + +<p><span class='pagenum'><a name="Page_8" id="Page_8">8</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="p1" id="p1"></a> +<img src="images/plate_001.png" width="500" height="712" +alt="PLATE I." title="" /> + +<p class="center">PLATE I.<a name="FNanchor_7" id="FNanchor_7"></a><a href="#Footnote_7" class="fnanchor">7</a>—MATURE INSECTS.<br /><br /> + +Fig. 1, Cricket; 2, Earwig; 3, <i>Aphis</i>; 4, <i>Scolytus</i>; 5, <i>Anthrax</i>; 6, <i>Balaninus</i>; +7, <i>Cynips</i>; 8, Ant; 9, Wasp.</p></div> + +<p><span class='pagenum'><a name="Page_9" id="Page_9">9</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="p2" id="p2"></a> +<img src="images/plate_002.png" width="500" height="805" +alt="PLATE II." title="" /> + +<p class="center">PLATE II.—LARVÆ OF THE INSECTS REPRESENTED ON PLATE I.</p> + +<p class="caption">Fig. 1, Larva of Cricket; 2, Larva of <i>Aphis</i>; 3, Larva of Earwig; 4, Larva of +<i>Scolytus</i> (Beetle); 5, Larva of <i>Anthrax</i> (Fly); 6, Larva of <i>Balaninus</i> (Nut +Weevil); 7, Larva of <i>Cynips</i>; 8, Larva of Ant; 9, Larva of Wasp.</p></div> + +<p><span class='pagenum'><a name="Page_10" id="Page_10">10</a></span></p> + +<p>In the Cynipidæ (Plate <a href="#p1">I.</a>, 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 <a href="#p2">II.</a>, Fig. 7), +feeds. In some species each gall contains a single +larva; in others, several live together.</p> + +<p>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 <i>Cynus longipennis</i>. 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.</p> + +<p>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<span class='pagenum'><a name="Page_11" id="Page_11">11</a></span> +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. (<a href="#f14">15, 16</a>). These parasites assume very curious +forms in their larval state.</p> + +<p>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. <i>Cerceris bupresticida</i>, as its name +denotes, attacks beetles belonging to the genus <i>Buprestis</i>. +Now if the <i>Cerceris</i> 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<span class='pagenum'><a name="Page_12" id="Page_12">12</a></span> +the egg, it finds ready a sufficient store of wholesome +food.</p> + +<p>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 <a href="#p2">II.</a>, 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 <i>Aphis</i> +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<span class='pagenum'><a name="Page_13" id="Page_13">13</a></span> +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.</p> + +<p><span class='pagenum'><a name="Page_14" id="Page_14">14</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="p3" id="p3"></a> +<img src="images/plate_003.png" width="500" height="812" +alt="PLATE III." title="" /> + +<p class="center">PLATE III.--MATURE INSECTS.</p> + +<p class="caption">Fig. 1, <i>Chloëon;</i> 2, <i>Meloë</i> (after Shuckard); 3, <i>Calepteryx</i>; 4, <i>Sitaris</i> (after Shuckard); +5, <i>Campodea</i> (after Gervais); 6, <i>Acilius</i>; 7, <i>Termes</i>; 8, <i>Stylops</i> (female); +9, <i>Thrips</i>.</p></div> + + +<p><span class='pagenum'><a name="Page_15" id="Page_15">15</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="p4" id="p4"></a> +<img src="images/plate_004.png" width="500" height="844" +alt="PLATE IV." title="" /> + +<p class="caption">PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ON +<span class="smcap">PLATE III</span>.--Fig. 1, Larva of <i>Chloëon;</i> 2, Larva of <i>Meloë</i> (after Chapuis and Candèze); +3, Larva of <i>Calepteryx</i> (after Léon Dufour); 4, Larva of <i>Sitaris</i>; 5, Larva +of <i>Campodea</i>; 6, Larva of <i>Acilius</i>; 7, Larva of <i>Termes</i> (after Blanchard); 8, +Larva of <i>Stylops</i>; 9, Larva of <i>Thrips</i>.</p></div> + +<p>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.</p> + +<p>Some indeed of these beetles produce a secretion +which is licked by the ants like the honeydew; 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.</p> + +<p>M. Lespès, who regards these insects as true +domestic animals, has recorded<a name="FNanchor_8" id="FNanchor_8"></a><a href="#Footnote_8" class="fnanchor">8</a> some interesting +observations on the relations between one of them +(<i>Claviger Duvalii</i>) and the ants (<i>Lasius niger</i>) with +which it lives. This species of <i>Claviger</i> is never met +with except in ants' nests, though on the other hand +there are many communities of <i>Lasius</i> which possess +none of these beetles; and M. Lespès found that +when he placed <i>Clavigers</i> 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 +<span class='pagenum'><a name="Page_16" id="Page_16">16</a></span> +no doubt ingenious, and the fact curiously resembles +the experience of navigators who have endeavoured +to introduce domestic animals among barbarous +tribes; but M. Lepès 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.</p> + +<p>The order Strepsiptera are a small, but very remarkable +group of insects, parasitic on bees and wasps. +The larva (Pl. <a href="#p4">IV.</a>, 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. <a href="#p3">III.</a>, 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.</p> + +<p>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 <i>Weevils</i> +(Pl. <a href="#p2">II.</a>, Fig. 6), of <i>Scolytus</i> (Pl. <a href="#p2">II.</a>, 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<span class='pagenum'><a name="Page_17" id="Page_17">17</a></span> +<i>Cicindela</i> and <i>Carabus</i>, have six-legged, slender, carnivorous +larvæ; those of <i>Cicindela</i>, 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. <a href="#p4">IV.</a>, 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. (<a href="#f1">1-6</a>)—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.</p> + +<p>The metamorphoses of the Cantharidæ are very +remarkable, and will be described subsequently. +The larvæ are active and hexapod. The Phytophaga +(<i>Crioceris</i>, <i>Galeruca</i>, <i>Haltica</i>, <i>Chrysomela</i>, +&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.</p> + +<p>The larva of <i>Coccinella</i> (the Ladybird) is somewhat +depressed, of an elongated ovate form, with a<span class='pagenum'><a name="Page_18" id="Page_18">18</a></span> +small head, and moderately strong legs. It feeds +on Aphides.</p> + +<p>Thus, then, we see that there are among the Coleoptera +many different forms of larvæ. Macleay considered +that there were five principal types.</p> + +<p>1. Carnivorous hexapod larvæ, with an elongated, +more or less flattened body, six eyes on each side of +the head, and sharp falciform mandibles (<i>Carabus</i>, +<i>Dyticus</i>, &c.).</p> + +<p>2. Herbivorous hexapod larvæ, with fleshy, cylindrical +bodies, somewhat curved, so that they lie on +their side.</p> + +<p>3. Apod grub-like larvæ, with scarcely the rudiments +of antennæ (<i>Curculio</i>).</p> + +<p>4. Hexapod antenniferous larvæ, with a subovate +body, the second segment being somewhat larger +than the others (<i>Chrysomela</i>, <i>Coccinella</i>).</p> + +<p>5. Hexapod antenniferous larvæ, of oblong form, +somewhat resembling the former, but with caudal +appendages (<i>Meloë</i>, <i>Sitaris</i>).</p> + +<p>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.”<a name="FNanchor_9" id="FNanchor_9"></a><a href="#Footnote_9" class="fnanchor">9</a></p> + +<p>In the next three orders—namely, the Orthoptera +(grasshoppers, locusts, crickets, walking-stick insects, +<span class='pagenum'><a name="Page_19" id="Page_19">19</a></span>cockroaches, &c.), Euplexoptera (earwigs), and Thysanoptera, +a small group of insects well known to +gardeners under the name of <i>Thrips</i> (Pl. <a href="#p1">I.</a> and <a href="#p2">II.</a>, +Figs. <a href="#f1">1 and 2</a>)—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.</p> + +<p>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 <i>Raphidia</i>, though motionless at first, at length +acquires strength enough to walk, even while still +enclosed in the pupa skin, which is very thin.<a name="FNanchor_10" id="FNanchor_10"></a><a href="#Footnote_10" class="fnanchor">10</a></p> + +<p>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 +<span class='pagenum'><a name="Page_20" id="Page_20">20</a></span>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, <i>Termes lucifugus</i>, 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.<span class='pagenum'><a name="Page_21" id="Page_21">21</a></span> +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.</p> + +<p>Another interesting family of Neuroptera is that of +the Ephemeræ, or Mayflies (Pl. <a href="#p3">III.</a>, Fig. 1), so well +known to fishermen. The larvæ (Pl. <a href="#p4">IV.</a>, 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<span class='pagenum'><a name="Page_22" id="Page_22">22</a></span> +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.</p> + +<p>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.</p> + +<p>One other family of Neuroptera which I must<span class='pagenum'><a name="Page_23" id="Page_23">23</a></span> +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.</p> + +<p>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.</p> + +<p>The Diptera, or Flies, comprise insects with two +wings only, the hinder pair being represented by +minute club-shaped organs called “haltères.” Flies<span class='pagenum'><a name="Page_24" id="Page_24">24</a></span> +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.</p> + +<p>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,<a name="FNanchor_11" id="FNanchor_11"></a><a href="#Footnote_11" class="fnanchor">11</a> they are +true, though abnormal, larvæ.</p> + +<p>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.</p> + +<p>The Heteroptera, unlike the preceding orders of +insects, quit the egg in a form differing from that of +<span class='pagenum'><a name="Page_25" id="Page_25">25</a></span>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<span class='pagenum'><a name="Page_26" id="Page_26">26</a></span> +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 <i>Coccus</i> +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.</p> + +<p>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 metathorax, 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.</p> + +<hr /> + +<p><span class='pagenum'><a name="Page_27" id="Page_27">27</a></span></p> + + +<h4>CHAPTER II.</h4> + +<h4><i>THE INFLUENCE OF EXTERNAL CONDITIONS +ON THE FORM AND STRUCTURE OF LARVÆ.</i></h4> + +<p>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.</p> + +<p>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. <a href="#f1">1</a>) feed +on the roots of grass; those of <i>Cetonia aurata</i> (Fig. <a href="#f1">2</a>) +inhabit ants' nests; the larvæ of the genus <i>Trox</i> +(Fig. <a href="#f1">3</a>) are found on dry animal substances; of +<i>Oryctes</i> (Fig. <a href="#f1">4</a>) in tan-pits; of <i>Aphodius</i> (Fig. <a href="#f1">5</a>) in +dung; of <i>Lucanus</i> (the stag-beetle, Fig. <a href="#f1">6</a>) in wood.</p> + +<div class="figcenter" style="width: 500px;"><a name="f1" id="f1"></a> +<img src="images/fig_001-6.png" width="500" height="303" +alt="Figs. 1-6" title="" /> + +<p class="caption"><span class="smcap">Fig. 1</span>, Larva of the Cockchafer (<i>Melolontha</i>). (Westwood, Int. to the +Modern Classification of Insects, vol. i. p. 194.). 2, Larva of <i>Cetonia</i>. +3, Larva of <i>Trox</i>. 4, Larva of <i>Oryctes</i>. 5, Larva of <i>Aphodius</i> (Chapuis +and Candèze, Mém. Soc. Roy. Liège, 1853). 6, Larva of <i>Lucanus</i>. +(Packard, Guide to the Study of Insects, Fig. 403).</p></div> + +<p>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<span class='pagenum'><a name="Page_28" id="Page_28">28</a></span> +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 hex<span class='pagenum'><a name="Page_29" id="Page_29">29</a></span>apod +larvæ, and that the present apod condition of +these larvæ is not original, but results from their +mode of life.</p> + +<p>On the other hand, the larvæ of <i>Sirex</i> (Fig. <a href="#f14">14</a>) +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.</p> + +<div class="figcenter" style="width: 400px;"><a name="f7" id="f7"></a> +<img src="images/fig_007-8.png" width="400" height="125" +alt="figs. 7-8" title="" /> + +<p class="center"><span class="smcap">Fig. 7</span>, Larva of <i>Brachytarsus</i> (Ratzeburg, Forst. Insecten). 8, Larva of <i>Crioceris</i> +(Westwood, loc. cit.).</p></div> + +<p>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. <a href="#p2">II.</a>, +Fig. 6, represents the larva of the nut-weevil, <i>Balaninus</i> +(Pl. <a href="#p1">I.</a>, Fig. 6), and it will be seen that it closely +resembles Pl. <a href="#p2">II.</a>, Fig. 5, which represents that of a fly +(<i>Anthrax</i>), Pl. <a href="#p1">I.</a>, Fig. 5, and Pl. <a href="#p2">II.</a>, Figs. 7, 8, and 9, +which represent respectively those of a <i>Cynips</i> or +gall-fly (Pl. <a href="#p1">I.</a>, Fig. 7), an ant (Pl. <a href="#p1">I.</a>, Fig. 8), and wasp +(Pl. <a href="#p1">I.</a>, Fig. 9). Nor is <i>Balaninus</i> the only genus of +Coleoptera which affords us examples of this fact. +Thus in the genus <i>Scolytus</i> (Pl. <a href="#p1">I.</a>, Fig. 4), the larvæ +(Pl. <a href="#p2">II.</a>, Fig. 4), which, as already mentioned, feed on +the bark of the elm, closely resemble those just described, +as also do those of <i>Brachytarsus</i> (Fig. <a href="#f7">7</a>). On +the other hand, the larvæ of certain beetles feed on<span class='pagenum'><a name="Page_30" id="Page_30">30</a></span> +leaves, like the caterpillars of Lepidoptera; thus that +of <i>Crioceris Asparagi</i> (Fig. <a href="#f7">8</a>)—which, as its name +denotes, feeds on the asparagus—closely resembles +the larvæ of certain Lepidoptera, as for instance of +<i>Thecla spini</i>. From this point of view the transformations +of the genus <i>Sitaris</i> (Pl. <a href="#p3">III.</a>, Fig. 4), which +have been very carefully investigated by M. Fabre, +are peculiarly interesting.<a name="FNanchor_12" id="FNanchor_12"></a><a href="#Footnote_12" class="fnanchor">12</a></p> + +<div class="figcenter" style="width: 500px;"><a name="f9" id="f9"></a> +<img src="images/fig_009-13.png" width="500" height="324" +alt="Figs. 9-13" title="" /> + +<p class="caption"><span class="smcap">Fig. 9</span>, Larva of <i>Sitaris humeralis</i> (Fabre, Ann. des Sci. Nat., sér. +4, tome vii.). 10, Larva of <i>Sitaris humeralis</i>, in the second stage. +11, Larva of <i>Sitaris humeralis</i>, in the third stage. 12, Larva of <i>Sitaris humeralis</i>, in the fourth stage. 13, Pupa of <i>Sitaris</i>.</p></div> + +<p>The genus <i>Sitaris</i> (a small beetle allied to Cantharis, +the blister-fly, and to <i>Meloë</i>, the oil-beetle) is +parasitic on a kind of Bee (Anthophora), which excavates +subterranean galleries, each leading to a cell. +The eggs of the <i>Sitaris</i>, which are deposited at the +entrance of these galleries, are hatched at the end of +September or beginning of October; and M. Fabre not +<span class='pagenum'><a name="Page_31" id="Page_31">31</a></span>unnaturally expected that the young larvæ, which are +active little creatures with six serviceable legs (Fig. <a href="#f9">9</a>), +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 <i>Anthophoras</i>, 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 <i>Meloë</i>;. The larvæ of <i>Sitaris</i> 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 +<i>Sitaris</i>. The males of Anthophora emerge from the<span class='pagenum'><a name="Page_32" id="Page_32">32</a></span> +pupæ sooner than the females, and M. Fabre ascertained +that, as they come out of their galleries, the +little <i>Sitaris</i> 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 <i>Sitaris</i>. +The mystery was solved. At the moment when the +egg is laid the <i>Sitaris</i> 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 <i>Sitaris</i>, therefore, at its first meal, relieves itself +from its only rival. After eight days the egg is consumed, +and on the empty shell the <i>Sitaris</i> undergoes +its first transformation, and makes its appearance +in a very different form, as shown in Fig. <a href="#f9">10.</a></p> + +<p>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<span class='pagenum'><a name="Page_33" id="Page_33">33</a></span> +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. <a href="#f9">11</a>), 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. <a href="#f9">12</a>; +after this it becomes a pupa (Fig. <a href="#f9">13</a>) without any +remarkable peculiarities. Finally, after these wonderful +changes and adventures, in the month of August the +perfect <i>Sitaris</i> (Pl. <a href="#p3">III.</a>, Fig. 4) makes its appearance.</p> + +<p>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.</p> + +<p>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. <a href="#f14">14</a>), 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,<a name="FNanchor_13" id="FNanchor_13"></a><a href="#Footnote_13" class="fnanchor">13</a> who, +<span class='pagenum'><a name="Page_34" id="Page_34">34</a></span>having collected some of the transparent eggs of +a small Beetle (<i>Rhynchites betuleti</i>), 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. <a href="#f14">15</a> shows the egg of the Beetle, with the parasitic +larva, which is represented on a larger scale in +Fig. <a href="#f14">16.</a></p> + +<div class="figcenter" style="width: 500px;"><a name="f14" id="f14"></a> +<img src="images/fig_014-16.png" width="500" height="188" +alt="Figs. 14-16" title="" /> + +<p class="caption"><span class="smcap">Fig. 14</span>, Larva of <i>Sirex</i> (Westwood, loc. cit.). 15, Egg of <i>Rhynchites</i>, showing the +parasitic Larva in the interior. 16, the parasitic Larva more magnified.</p></div> + +<p>More recently this group has been studied by M. +Ganin,<a name="FNanchor_14" id="FNanchor_14"></a><a href="#Footnote_14" class="fnanchor">14</a> who thus describes the development of <i>Platygaster</i>. +The egg, as in allied Hymenopterous families, +for instance in <i>Cynips</i>, is elongated and club-shaped +(Fig. <a href="#f17">17</a>). After a while a large nucleated cell appears +in the centre (Fig. <a href="#f17">18</a>). This nucleated cell +divides (Fig. <a href="#f17">19</a>) 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. <a href="#f17">20 and 21</a>), which +gradually form a mulberry-like mass, thus giving +rise to the embryo (Fig. <a href="#f17">22</a>).</p> + +<p><span class='pagenum'><a name="Page_35" id="Page_35">35</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="f17" id="f17"></a> +<img src="images/fig_017-22.png" width="500" height="345" +alt="Figs. 17-22" title="" /> + +<p class="caption"><span class="smcap">Fig. 17</span>, Egg of <i>Platygaster</i> (after Ganin). 18, Egg of <i>Platygaster</i> +showing the central cell. 19, Egg of <i>Platygaster</i> after the division of +the central wall. 20, Egg of <i>Platygaster</i> more advanced. 21, Egg of +<i>Platygaster</i> more advanced. 22, Egg of <i>Platygaster</i> showing the rudiment +of the embryo.</p></div> + +<p>Ganin met with the larvæ of <i>Platygaster</i> in those +of a small gnat, <i>Cecidomyia</i>. Sometimes as many as +fifteen parasites occurred in one gnat, but as a rule +only one of these attained maturity. The three +species of <i>Platygaster</i> differ considerably in form, as +shown in Figs. <a href="#f23">23-25</a>. They creep about within +the larva of <i>Cecidomyia</i> by means of the strong +hooked feet, <i>kf</i>, 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. <a href="#f23">23</a>) changes its +skin, assuming the form represented in Fig. <a href="#f23">26</a>. 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<span class='pagenum'><a name="Page_36" id="Page_36">36</a></span> +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<span class='pagenum'><a name="Page_37" id="Page_37">37</a></span> +indicated only by the arrangement of the muscles. +<i>slkf</i> is the œsophagus leading into a wide stomach +which occupies nearly the whole body, <i>gsae</i> is the +rudiment of the supra-œsophageal ganglia, <i>bsm</i> 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.</p> + +<div class="figcenter" style="width: 500px;"><a name="f23" id="f23"></a> +<img src="images/fig_023-27.png" width="500" height="525" +alt="Figs. 23-27" title="" /> + +<p class="caption"><span class="smcap">Fig. 23</span>, Larva of <i>Platygaster</i> (after Ganin)—<i>mo</i>, mouth; <i>a</i>, antenna; +<i>kf</i>, hooked feet; <i>z</i>, toothed process; <i>lfg</i>, lateral process; <i>f</i>, branches +of the tail. 24, Larva of another species of <i>Platygaster</i>. The letters indicate +the same parts as in the preceding figure. 25, Larva of a third +species of <i>Platygaster</i>. The letters indicate the same parts as in the preceding +figures. 26, Larva of <i>Platygaster</i> in the second stage—<i>mo</i>, mouth; +<i>slkf</i>, œsophagus; <i>gsae</i>, supra-œsophageal ganglion; <i>lm</i>, muscles; +<i>bsm</i>, nervous system; <i>ga</i>, <i>gh</i>, rudiments of the reproductive glands. +27, Larva of <i>Platygaster</i> in the third stage—<i>mo</i>, mouth; <i>md</i>, +mandibles; <i>gsae</i>, supra-œsophageal ganglion; <i>slk</i>, œsophagus; <i>ag</i>, +ducts of the salivary glands; <i>bnm</i>, ventral nervous system; <i>sp</i>, salivary +glands; <i>msl</i>, stomach; <i>im</i>, imaginal discs; <i>tr</i>, tracheæ; <i>fk</i>, fatty +tissue; <i>ed</i>, intestine; <i>ga</i>, rudiments of reproductive organs; <i>ew</i>, wider +portion of intestine; <i>ao</i>, posterior opening.</p></div> + +<p>At the next moult the larva enters its third state, +which, as far as the external form (Fig. <a href="#f23">27</a>) 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 <i>Platygaster</i> do not appear to +differ materially from those of other parasitic Hymenoptera.</p> + +<p>An allied genus, <i>Polynema</i>, 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.<a name="FNanchor_15" id="FNanchor_15"></a><a href="#Footnote_15" class="fnanchor">15</a> It lays its eggs inside those +of Dragon-flies; and the embryo, as shown in +Fig. <a href="#f28">28</a>, 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 <i>Polynema</i> imbibes nourishment through its +whole surface, and increases rapidly in size. The +digestive canal gradually makes its appearance; the +<span class='pagenum'><a name="Page_38" id="Page_38">38</a></span>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. <a href="#f28">29</a>. The subsequent metamorphoses +of <i>Polynema</i> offer no special peculiarities.</p> + +<div class="figcenter" style="width: 500px;"><a name="f28" id="f28"></a> +<img src="images/fig_028-29.png" width="500" height="355" +alt="Figs. 28-29" title="" /> + +<p class="caption"><span class="smcap">Fig. 28</span>, Embryo of <i>Polynema</i> (after Ganin). 29, Larva of <i>Polynema</i>—<i>asch</i>, +rudiments of the antenna; <i>flsch</i>, rudiments of the wings; <i>bsch</i>, +rudiments of the legs; <i>vfg</i>. lateral projections; <i>gsch</i>, rudiments of +the ovipositor; <i>fk</i>, fatty tissue.</p></div> + +<p>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.</p> + +<p>The larvæ of insects are generally regarded as being +nothing more than immature states—as stages in the<span class='pagenum'><a name="Page_39" id="Page_39">39</a></span> +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.</p> + +<p>I conclude, therefore, that the form of the larva +in insects, whenever it departs from the hexapod<span class='pagenum'><a name="Page_40" id="Page_40">40</a></span> +<i>Campodea</i> 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.</p> + +<p>And, lastly, as a consequence, that metamorphoses +may be divided into two kinds, developmental and +adaptional or adaptive.</p> + +<hr /> + +<p><span class='pagenum'><a name="Page_41" id="Page_41">41</a></span></p> + + + +<h4>CHAPTER III.</h4> + +<h4><i>ON THE NATURE OF METAMORPHOSES.</i></h4> + +<p>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,<span class='pagenum'><a name="Page_42" id="Page_42">42</a></span> +labrum, labium, and, in fact, all the mouth parts of a +perfect insect.</p> + +<p>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 +<i>Chloëon</i>, 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.</p> + +<p>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<span class='pagenum'><a name="Page_43" id="Page_43">43</a></span> +without thoracic appendages; in the Orthoptera, on +the contrary, the legs are fully developed before the +young animal quits the egg.</p> + +<p>Prof. Owen,<a name="FNanchor_16" id="FNanchor_16"></a><a href="#Footnote_16" class="fnanchor">16</a> 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 <i>Carabi</i> and +<i>Petalocera</i>.”</p> + +<p>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 +<span class='pagenum'><a name="Page_44" id="Page_44">44</a></span>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.</p> + +<div class="figcenter" style="width: 500px;"><a name="f30" id="f30"></a> +<img src="images/fig_030-31.png" width="500" height="233" +alt="Figs. 30-31" title="" /> + +<p class="caption"><span class="smcap">Fig. 30</span>, Egg of <i>Phryganea</i> (Mystacides)—<i>A<sup>1</sup></i>, mandibular segment; <i>C<sup>1</sup></i> +to <i>C<sup>5</sup></i>, maxillary, labial, and three thoracic segments; <i>D</i>, abdomen +(after Zaddach). 31, Egg of <i>Phryganea</i> somewhat more advanced—<i>b</i>, +mandibles; <i>c</i>, maxillæ; <i>cfs</i>, rudiments of the three pairs of legs.</p></div> + +<p>Fig. <a href="#f30">30,</a> represents an egg of a May-fly (<i>Phryganea</i>), +as represented by Zaddach in his excellent +memoir,<a name="FNanchor_17" id="FNanchor_17"></a><a href="#Footnote_17" class="fnanchor">17</a> 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. <a href="#f30">31</a>, which, however, as will be seen, does +not in other respects show much advance on Fig. <a href="#f30">30.</a></p> + +<p>Again in <i>Aphis</i>, the embryology of which has been +so well worked out by Huxley,<a name="FNanchor_18" id="FNanchor_18"></a><a href="#Footnote_18" class="fnanchor">18</a> 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 +<span class='pagenum'><a name="Page_45" id="Page_45">45</a></span>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.”</p> + +<p>In <i>Chrysopa oculata</i>, one of the Hemerobiidæ, +Packard has described<a name="FNanchor_19" id="FNanchor_19"></a><a href="#Footnote_19" class="fnanchor">19</a> 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.</p> + +<p>In certain Dragon-flies (<i>Calepteryx</i>), and Hemiptera +(<i>Hydrometra</i>), the legs, according to Brandt,<a name="FNanchor_20" id="FNanchor_20"></a><a href="#Footnote_20" class="fnanchor">20</a> appear +at a still earlier stage.</p> + +<p>According to the observations of Kölliker,<a name="FNanchor_21" id="FNanchor_21"></a><a href="#Footnote_21" class="fnanchor">21</a> it +would appear that in the Coleopterous genus <i>Donacia</i> +the segments and appendages appear simultaneously.</p> + +<p><span class='pagenum'><a name="Page_46" id="Page_46">46</a></span></p><p>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 <i>Hydrophilus</i> +the appendages do not make their appearance until +after the segments.<a name="FNanchor_22" id="FNanchor_22"></a><a href="#Footnote_22" class="fnanchor">22</a></p> + +<p>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.</p> + +<div class="figcenter" style="width: 250px;"><a name="f32" id="f32"></a> +<img src="images/fig_032.png" width="250" height="257" +alt="Fig. 32" title="" /></div> + +<p class="center"><span class="smcap">Fig. 32.</span>—Egg of <i>Pholcus opilionides</i> (after Claparède).</p> + +<p>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. <a href="#f32">32</a>), representing +an early stage in the development of a spider +(<i>Pholcus</i>) after Claparède,<a name="FNanchor_23" id="FNanchor_23"></a><a href="#Footnote_23" class="fnanchor">23</a> who says, “C’est à ce +<span class='pagenum'><a name="Page_47" id="Page_47">47</a></span>moment qu’a lieu la formation des <i>protozonites</i> 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.”</p> + +<div class="figcenter" style="width: 250px;"><a name="f33" id="f33"></a> +<img src="images/fig_033.png" width="250" height="293" +alt="Fig. 33" title="" /></div> + +<p class="center"><span class="smcap">Fig. 33.</span>—Embryo of <i>Julus</i> (after Newport).</p> + +<p>Among Centipedes the development of <i>Julus</i> has +been described by Newport.<a name="FNanchor_24" id="FNanchor_24"></a><a href="#Footnote_24" class="fnanchor">24</a> 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. <a href="#f33">33</a>), 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<span class='pagenum'><a name="Page_48" id="Page_48">48</a></span> +on the undersides of the segments to which +they belong.</p> + +<p>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.</p> + +<div class="figcenter" style="width: 350px;"><a name="f34" id="f34"></a> +<img src="images/fig_034.png" width="350" height="452" +alt="Fig. 34" title="" /> + +<p class="center"><span class="smcap">Fig. 34.</span>—Colony of <i>Bougainvillea fruticosa</i>, natural size, to the underside +of a piece of floating timber (after Allman).</p></div> + +<p>For instance, among the Hydroida, in the great +majority of cases, the egg produces a body more +or less resembling the common <i>Hydra</i> 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,<a name="FNanchor_25" id="FNanchor_25"></a><a href="#Footnote_25" class="fnanchor">25</a> and of which the genus +<span class='pagenum'><a name="Page_49" id="Page_49">49</a></span> +<i>Ægina</i> 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. <a href="#f34">34</a> represents, after Allman, a colony of +<i>Bougainvillea fruticosa</i> of the natural size. It is a +British species, which is found growing on buoys, +floating timber, &c., and, says Allman,<a name="FNanchor_26" id="FNanchor_26"></a><a href="#Footnote_26" class="fnanchor">26</a> “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 +<span class='pagenum'><a name="Page_50" id="Page_50">50</a></span> +Medusæ in all stages of development (Fig. <a href="#f35">35</a>), 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 <i>ectothèque</i> (external +layer); others, again, just casting off this thin +outer pellicle, and others completely freed from it,<span class='pagenum'><a name="Page_51" id="Page_51">51</a></span> +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.”</p> + +<div class="figcenter" style="width: 500px;"><a name="f35" id="f35"></a> +<img src="images/fig_035.png" width="500" height="566" +alt="Fig. 35" title="" /> + +<p class="center"><span class="smcap">Fig. 35.</span>—Portion of colony of <i>Bougainvillea fruticosa</i>, more magnified.</p></div> + +<div class="figcenter" style="width: 250px;"><a name="f36" id="f36"></a> +<img src="images/fig_036.png" width="250" height="436" +alt="Fig. 36" title="" /></div> + +<p class="center"><span class="smcap">Fig. 36.</span>—The Medusa form of the same species.</p> + +<p>Fig. <a href="#f36">36</a> 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.</p> + +<p>On the other hand, there are groups in which<span class='pagenum'><a name="Page_52" id="Page_52">52</a></span> +the Medusiform stage becomes less and less important.</p> + +<div class="figcenter" style="width: 500px;"><a name="f37" id="f37"></a> +<img src="images/fig_037-38.png" width="500" height="412" +alt="Figs. 37-38" title="" /> + +<p class="center"><span class="smcap">Fig. 37</span>, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn, +more advanced, Zoëa stage.</p></div> + +<p>The great majority of the higher Crustacea go +through well-marked metamorphoses. Figs. <a href="#f37">37 and +38</a> represent two stages in the development of the +prawn. In the first (Fig. <a href="#f37">37</a>), 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<span class='pagenum'><a name="Page_53" id="Page_53">53</a></span> +forms, and O. F. Müller gave them the name of Nauplius. +So also, the second or Zoëa form (Fig. <a href="#f37">38</a>) was +at first supposed to be a mature animal, until its true +nature was discovered by Vaughan Thompson.</p> + +<p>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<a name="FNanchor_27" id="FNanchor_27"></a><a href="#Footnote_27" class="fnanchor">27</a> in the case of the common crab +(<i>Carcinus mænas</i>).</p> + +<p>All crabs, as far as we know, with the exception of +a species of land crab (<i>Gegarcinus</i>), described by +Westwood, pass through a stage more or less resembling +that shown in Fig. <a href="#f37">38</a>. 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, <i>Tanais Dulongii</i>, 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 <i>Mysis</i>, says F. Müller,<a name="FNanchor_28" id="FNanchor_28"></a><a href="#Footnote_28" class="fnanchor">28</a> +“there is still a trace of the Nauplius stage; being +<span class='pagenum'><a name="Page_54" id="Page_54">54</a></span>transferred back to a period when it had not to +provide for itself, the Nauplius has become degraded +into a mere skin; in <i>Ligia</i> this larva-skin has lost +the traces of limbs, and in <i>Philoscia</i> it is scarcely +demonstrable.”</p> + +<p>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.”<a name="FNanchor_29" id="FNanchor_29"></a><a href="#Footnote_29" class="fnanchor">29</a> +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.”<a name="FNanchor_30" id="FNanchor_30"></a><a href="#Footnote_30" class="fnanchor">30</a></p> + +<div class="figcenter" style="width: 350px;"><a name="f39" id="f39"></a> +<img src="images/fig_039.png" width="350" height="437" +alt="Fig. 39" title="" /> + +<p class="center"><span class="smcap">Fig. 39.</span>—Larva of <i>Echino-cidaris</i>, seen from above ✕ 6/10 (after Müller).</p></div> + +<p>Fig. <a href="#f39">39</a> represents the larva of a sea-egg (<i>Echino-cidaris</i>) +after Müller.<a name="FNanchor_31" id="FNanchor_31"></a><a href="#Footnote_31" class="fnanchor">31</a> The body is transparent, +shaped somewhat like a double easel, but with two +long horns in front, which, as well as the posterior +<span class='pagenum'><a name="Page_55" id="Page_55">55</a></span>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.<a name="FNanchor_32" id="FNanchor_32"></a><a href="#Footnote_32" class="fnanchor">32</a></p> + +<p><span class='pagenum'><a name="Page_56" id="Page_56">56</a></span></p> + +<div class="figcenter" style="width: 350px;"><a name="f40" id="f40"></a> +<img src="images/fig_040.png" width="350" height="442" +alt="Fig. 40" title="" /> + +<p class="caption"><span class="smcap">Fig. 40</span>, Larva of <i>Echinus</i>, ✕ 100. <i>A</i>, front arm; <i>F</i>, arms of the mouth +process; <i>B</i>, posterior side arm; <i>E<sub>1</sub></i>, accessory arm of the mouth process; +<i>a</i>, mouth; <i>a´</i>, œsophagus; <i>b</i>, stomach; <i>b´</i>, intestine; <i>o</i>, posterior +orifice; <i>d</i>, ciliated bands; <i>f</i>, ciliated epaulets; <i>c</i>, disc of future <i>Echinus</i> +(after Müller).</p></div> + +<p>Fig. <a href="#f40">40</a> represents a larva, probably of another sea-egg +(<i>Echinus lividus</i>), from the Mediterranean, and +shows the commencement of the sea-egg within the +body of the larva. The capital letters denote the +different arms: <i>a</i> is the mouth, <i>a´</i> the œsophagus, <i>b</i> +the stomach, <i>b´</i> the intestine, <i>f</i> the ciliated lobes or +epaulets, <i>c</i> the young sea-egg.</p> + +<div class="figcenter" style="width: 500px;"><a name="f41" id="f41"></a> +<img src="images/fig_041.png" width="500" height="485" +alt="Fig. 41" title="" /> + +<p class="center"><span class="smcap">Fig. 41.</span>—<i>Comatula rosacea</i> (after Forbes).</p></div> + +<p>The development of the beautiful <i>Comatula rosacea</i> +(Fig. <a href="#f41">41</a>) has been described in the “Philosophical +Transactions,” by Prof. Wyville Thomson and Dr. +Carpenter.<a name="FNanchor_33" id="FNanchor_33"></a><a href="#Footnote_33" class="fnanchor">33</a> The larva quits the egg, as shown in +Fig. <a href="#f42">42</a>, in the form of an oval body about 1/30 inch +<span class='pagenum'><a name="Page_57" id="Page_57">57</a></span>in length, something like a barrel, surrounded by +four bands orops 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. <a href="#f42">43</a>) 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.</p> + +<p><span class='pagenum'><a name="Page_58" id="Page_58">58</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="f42" id="f42"></a> +<img src="images/fig_042-44.png" width="500" height="789" +alt="Figs. 42-44" title="" /> + +<p class="caption"><span class="smcap">Fig. 42</span>, Larva of <i>Comatula rosacea</i> (after Thomson). 43, Larva of <i>Comatula +rosacea</i>, more advanced. 44, Larva of <i>Comatula rosacea</i>, in the +Pentacrinus state.</p></div> + +<p><span class='pagenum'><a name="Page_59" id="Page_59">59</a></span></p> + +<p>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. <a href="#f42">44</a>, make +their appearance.</p> + +<p>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 <i>Comatula</i> 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 <i>Comatula</i>. 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 +<i>Comatula</i>.</p> + +<p>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 <i>Bipinnaria asterigera</i> (Figs. <a href="#f45">45-47</a>), 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.</p> + +<p>Figs. <a href="#f45">45 and 46</a> represent the front and side view<span class='pagenum'><a name="Page_60" id="Page_60">60</a></span> +of a Bipinnaria found by Müller<a name="FNanchor_34" id="FNanchor_34"></a><a href="#Footnote_34" class="fnanchor">34</a> near Marseilles. +<i>a</i> is the mouth, <i>b</i> the œsophagus, <i>c</i> the stomach, <i>c</i>´ +the intestine. Fig. <a href="#f45">47</a> represents a somewhat older +specimen, in which the Starfish (<i>k</i>) is already beginning +to make its appearance.</p> + +<div class="figcenter" style="width: 500px;"><a name="f45" id="f45"></a> +<img src="images/fig_045-47.png" width="500" height="305" +alt="Figs. 45-47" title="" /> + +<p class="caption"><span class="smcap">Fig. 45</span>, Larva of Starfish (Bipinnaria), ✕ 100 (after Müller). 46, Larva of +Starfish (Bipinnaria), ✕ 100, seen from the side—<i>a</i>, mouth; <i>b</i>, œsophagus; +<i>c</i>, stomach; <i>c´</i>, intestine. 47, Larva of another Bipinnaria, +showing the commencement of the Starfish—<i>g</i>, canal of the ciliated sac; +<i>i</i>, rudiments of tentacles; <i>d</i>, ciliated band.</p></div> + +<p>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 <i>Asteracanthion</i>—in which development +may be said to be direct—the organs and +appendages special to the Pseudembryo being in +abeyance; while in another genus, Pteraster, they +are reduced to a mere investing membrane.<a name="FNanchor_35" id="FNanchor_35"></a><a href="#Footnote_35" class="fnanchor">35</a></p> + +<p><span class='pagenum'><a name="Page_61" id="Page_61">61</a></span></p><p>Among the Ophiurans also we find two well-marked +types of development. Some passing through metamorphoses, +while others, as for instance <i>Ophiopholis +bellis</i>, “is developed very much after the method of +<i>Asteracanthion Mülleri</i>, without passing through the +Plutean stage.”<a name="FNanchor_36" id="FNanchor_36"></a><a href="#Footnote_36" class="fnanchor">36</a></p> + +<p>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 <i>Comatula</i>, 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. <a href="#f42">43</a>), 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,<a name="FNanchor_37" id="FNanchor_37"></a><a href="#Footnote_37" class="fnanchor">37</a> and among the insects +themselves, in Flies;<a name="FNanchor_38" id="FNanchor_38"></a><a href="#Footnote_38" class="fnanchor">38</a> 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.</p> + +<hr /> + +<p><span class='pagenum'><a name="Page_62" id="Page_62">62</a></span></p> + +<h4>CHAPTER IV.</h4> + +<h4><i>ON THE ORIGIN OF METAMORPHOSES.</i></h4> + +<p>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;”<a name="FNanchor_39" id="FNanchor_39"></a><a href="#Footnote_39" class="fnanchor">39</a> 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.”</p> + +<p>But there are some insects—as, for instance, the +<span class='pagenum'><a name="Page_63" id="Page_63">63</a></span>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.</p> + +<p>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.</p> + +<p>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.</p> + +<p>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<span class='pagenum'><a name="Page_64" id="Page_64">64</a></span> +not come within the popular ken. “La larve,” says, +Quatrefages, “n’est qu’un embryon à vie indépendante.”<a name="FNanchor_40" id="FNanchor_40"></a><a href="#Footnote_40" class="fnanchor">40</a> +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.”<a name="FNanchor_41" id="FNanchor_41"></a><a href="#Footnote_41" class="fnanchor">41</a> +“Each organism,” says Herbert Spencer,<a name="FNanchor_42" id="FNanchor_42"></a><a href="#Footnote_42" class="fnanchor">42</a> +“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.”</p> + +<p>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 +<span class='pagenum'><a name="Page_65" id="Page_65">65</a></span>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.”</p> + +<p>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 (<i>Macropus major</i>), 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.</p> + +<p>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<span class='pagenum'><a name="Page_66" id="Page_66">66</a></span> +species the development of the egg presents certain +differences.<a name="FNanchor_43" id="FNanchor_43"></a><a href="#Footnote_43" class="fnanchor">43</a></p> + +<p>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.</p> + +<p>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.<a name="FNanchor_44" id="FNanchor_44"></a><a href="#Footnote_44" class="fnanchor">44</a></p> + +<p>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 nomen<span class='pagenum'><a name="Page_67" id="Page_67">67</a></span>clature +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.</p> + +<p>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.</p> + +<p>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<span class='pagenum'><a name="Page_68" id="Page_68">68</a></span> +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 <i>Chloëon</i>, 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.</p> + +<p>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<span class='pagenum'><a name="Page_69" id="Page_69">69</a></span> +gradual, the mouth of the so-called larva resembles +that of the perfect insect, and the principal difference +consists in the presence of wings.</p> + +<p>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.<a name="FNanchor_45" id="FNanchor_45"></a><a href="#Footnote_45" class="fnanchor">45</a></p> + +<p>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: +<span class='pagenum'><a name="Page_70" id="Page_70">70</a></span>others—as, for instance, <i>Raphidia</i>—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.</p> + +<p>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<span class='pagenum'><a name="Page_71" id="Page_71">71</a></span> +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.</p> + +<p>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 <i>Campodea</i> and the +Collembola (<i>Podura</i>, &c.) are peculiarly interesting. +There are in insects three principal types of mouth:—</p> + +<div class="blockquot"><p>First, the mandibulate;</p> + +<p>Secondly, the suctorial; and</p> + +<p>Thirdly, that of <i>Campodea</i> and the Collembola +generally,</p></div> + +<p>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<span class='pagenum'><a name="Page_72" id="Page_72">72</a></span> +abeyance; while, on the other hand, if masticatory +jaws were an advantage, the opposite process would +take place.</p> + +<p>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 <i>vice versâ</i>. 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<span class='pagenum'><a name="Page_73" id="Page_73">73</a></span> +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 <i>Campodea</i> and the <i>Collembola</i> 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.</p> + +<p>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.</p> + +<p><span class='pagenum'><a name="Page_74" id="Page_74">74</a></span> +In the larva of <i>Chloëon</i> (Pl. <a href="#p4">IV.</a>, Fig. 1), for instance, +which in other respects so singularly resembles +<i>Campodea</i> (Pl. <a href="#p3">III.</a>, 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 <i>Chloëon</i> 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.</p> + +<p>That wings may be of use to insects under water +is proved by the very interesting case of <i>Polynema +natans</i>,<a name="FNanchor_46" id="FNanchor_46"></a><a href="#Footnote_46" class="fnanchor">46</a> 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 +<span class='pagenum'><a name="Page_75" id="Page_75">75</a></span>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,<a name="FNanchor_47" id="FNanchor_47"></a><a href="#Footnote_47" class="fnanchor">47</a> 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 <i>Chloëon</i> in form, but +had thoracic as well as abdominal branchiæ.</p> + +<p>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.<a name="FNanchor_48" id="FNanchor_48"></a><a href="#Footnote_48" class="fnanchor">48</a></p> + +<p>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.</p> + +<p>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 cater<span class='pagenum'><a name="Page_76" id="Page_76">76</a></span>pillar, +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.</p> + +<p>Recently, however, Prof. Wagner<a name="FNanchor_49" id="FNanchor_49"></a><a href="#Footnote_49" class="fnanchor">49</a> 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 <i>Chironomus</i> in which the pupæ +lay eggs.<a name="FNanchor_50" id="FNanchor_50"></a><a href="#Footnote_50" class="fnanchor">50</a></p> + +<p>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, +<span class='pagenum'><a name="Page_77" id="Page_77">77</a></span>known as alternation of generations, may have originated.</p> + +<p>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.</p> + +<p>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<span class='pagenum'><a name="Page_78" id="Page_78">78</a></span> +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 <i>Sitaris</i> (<i>ante</i>, p. 30) we +even find that, the conditions of life not being +uniform throughout the larval period, the larva itself +undergoes metamorphoses.</p> + +<p>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.</p> + +<p>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.<span class='pagenum'><a name="Page_79" id="Page_79">79</a></span></p> + +<p>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.</p> + +<p>Moreover, it is evident that we have in the animal +kingdom two kinds of dimorphism.</p> + +<p>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 <i>Primula</i> 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 <i>Lythrum</i>, +in which there are three distinct forms, and which +therefore offers an instance of polymorphism.<a name="FNanchor_51" id="FNanchor_51"></a><a href="#Footnote_51" class="fnanchor">51</a></p> + +<p>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 +<span class='pagenum'><a name="Page_80" id="Page_80">80</a></span>is composed of dissimilar links. Many instances of +this second form of dimorphism have been described +under the name of alternation of generations.</p> + +<p>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.<a name="FNanchor_52" id="FNanchor_52"></a><a href="#Footnote_52" class="fnanchor">52</a></p> + +<p>In order, then, to have some name for these remarkable +phenomena, and to distinguish them from those +cases in which the <i>mature</i> 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.</p> + +<p>The conclusions, then, which I think we may draw +from the preceding considerations, are:—</p> + +<p>1. That the occurrence of metamorphoses arises +from the immaturity of the condition in which some +animals quit the egg.</p> + +<p>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 refer<span class='pagenum'><a name="Page_81" id="Page_81">81</a></span>ence +to its immediate wants, rather than to its final +form.</p> + +<p>3. That metamorphoses may therefore be divided +into two kinds, developmental and adaptional or +adaptive.</p> + +<p>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.</p> + +<p>5. The immobility of the pupa or chrysalis depends +on the rapidity of the changes going on in it.</p> + +<p>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.</p> + +<p>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.</p> + +<hr /> + +<p><span class='pagenum'><a name="Page_82" id="Page_82">82</a></span></p> + +<h4>CHAPTER V.</h4> + +<h4><i>ON THE ORIGIN OF INSECTS.</i></h4> + +<p>"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:—</p> + +<div class="poem"> +<div class="stanza"> +<span class="i05">"That mass man sprang from was a jelly lump</span> +<span class="i0">Once on a time; he kept an after course</span> +<span class="i0">Through fish and insect, reptile, bird, and beast,</span> +<span class="i0">Till he attained to be an ape at last,</span> +<span class="i0">Or last but one."<a name="FNanchor_53" id="FNanchor_53"></a><a href="#Footnote_53" class="fnanchor">53</a></span> +</div></div> + +<p>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 +<span class='pagenum'><a name="Page_83" id="Page_83">83</a></span> +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 <i>vera causa</i>, 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.</p> + +<p>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 in<span class='pagenum'><a name="Page_84" id="Page_84">84</a></span>stance +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.</p> + +<p>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.</p> + +<p>In other instances Palæontology throws much light +on this question. Leidy has shown that the milk-teeth +of the genus <i>Equus</i> resemble the permanent +teeth of the ancient <i>Anchitherium</i>, while the milk-teeth +of <i>Anchitherium</i> again approximate to the dental system +of the still earlier <i>Merychippus</i>. Rütimeyer, while +calling attention to this interesting observation, adds +that the milk-teeth of <i>Equus caballus</i> in the same way, +and still more those of <i>E. fossilis</i>, resemble the permanent +teeth of <i>Hipparion</i>.</p> + +<p>"If we were not acquainted with the horse," says +Flower,<a name="FNanchor_54" id="FNanchor_54"></a><a href="#Footnote_54" class="fnanchor">54</a> “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 palæon<span class='pagenum'><a name="Page_85" id="Page_85">85</a></span>tology +has revealed that in the structure of its skull, +its teeth, its limbs, the horse is nothing more than a +modified <i>Palæotherium</i>; and though still with gaps +in certain places, many of the intermediate stages +of these modifications are already known to us, being +the <i>Palæotherium</i>, <i>Anchitherium</i>, <i>Merychippus</i>, and +<i>Hipparion</i>.”</p> + +<p>"All Echinoids," says A. Agassiz,<a name="FNanchor_55" id="FNanchor_55"></a><a href="#Footnote_55" class="fnanchor">55</a> “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.</p> + +<p>Again<a name="FNanchor_56" id="FNanchor_56"></a><a href="#Footnote_56" class="fnanchor">56</a> “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 +<span class='pagenum'><a name="Page_86" id="Page_86">86</a></span>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<a name="FNanchor_57" id="FNanchor_57"></a><a href="#Footnote_57" class="fnanchor">57</a> 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.”</p> + +<p>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.</p> + +<p><span class='pagenum'><a name="Page_87" id="Page_87">87</a></span></p> + +<p>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.</p> + +<p>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. <a href="#p4">IV</a> I have figured the +larvæ of an <i>Ephemera</i> (Fig. 1), of a <i>Meloë</i> (Fig. 2), +of a Dragon-fly (Fig. 3), of a <i>Sitaris</i> (Fig. 4), of a +<i>Campodea</i> (Fig. 5), of a <i>Dyticus</i> (Fig. 6), of a Termite +(Fig. 7), of a <i>Stylops</i> (Fig. 8), and of a <i>Thrips</i> (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. <a href="#f48">48-55</a>, where +Figs. <a href="#f48">48-51</a> represent the larval states of the mature +forms represented in Figs. <a href="#f52">52-55</a>. Fig. <a href="#f48">48</a> is the +larva of a moth, <i>Agrotis suffusa</i> (Fig. <a href="#f52">52</a>); Fig. <a href="#f48">49</a> of<span class='pagenum'><a name="Page_88" id="Page_88">88</a></span> +a beetle, <i>Haltica</i> (Fig. <a href="#f52">53</a>); Fig. <a href="#f48">50</a> of a Saw-fly, +<i>Cimbex</i> (Fig. <a href="#f54">54</a>); and Fig. <a href="#f48">51</a> of a Centipede, <i>Julus</i> +(Fig. <a href="#f55">55</a>).</p> + +<div class="figcenter" style="width: 450px;"><a name="f48" id="f48"></a> +<img src="images/fig_048-51.png" width="450" height="305" +alt="Figs. 48-51" title="" /> + +<p class="caption"><span class="smcap">Fig. 48</span>, Larva of Moth (<i>Agrotis suffusa</i>), after Packard. 49, Larva of Beetle +(<i>Haltica</i>), after Westwood. 50, Larva of Sawfly (<i>Cimbex</i>), Brischke and +Zaddach. Beob. ub d. arten. der Blatt und Holzwespen, Fig. 8. 51, Larva of +<i>Julus</i>. Newport, Philos. Transactions, 1841.</p></div> + +<p>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<a name="FNanchor_58" id="FNanchor_58"></a><a href="#Footnote_58" class="fnanchor">58</a> +and I<a name="FNanchor_59" id="FNanchor_59"></a><a href="#Footnote_59" class="fnanchor">59</a> have pointed out that two types of larvæ, +which I have proposed to call <i>Campodea</i>-form and +<i>Lindia</i>-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 <i>Meloë</i>s are derived from a form +resembling Pl. <a href="#p2">II</a>, Fig. 2, it is surely no rash hypothesis +to suggest that the genus itself may have been so.</p> + +<p><span class='pagenum'><a name="Page_89" id="Page_89">89</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="f52" id="f52"></a> +<img src="images/fig_052-53.png" width="500" height="225" +alt="Figs. 52-53" title="" /> + +<p class="center"><span class="smcap">Fig. 52</span>, <i>Agrotis suffusa</i> (after Packard). 53, <i>Haltica</i> (after Westwood).</p></div> + +<div class="figcenter" style="width: 250px;"><a name="f54" id="f54"></a> +<img src="images/fig_054.png" width="250" height="240" +alt="Fig. 54" title="" /> + +<p class="center"><span class="smcap">Fig. 54</span>, <i>Cimbex</i>, Brischae and Zaddach. l.c. T. 2, Fig. 9.</p></div> + +<div class="figcenter" style="width: 400px;"><a name="f55" id="f55"></a> +<img src="images/fig_055.png" width="400" height="63" +alt="Fig. 55" title="" /> + +<p class="center"><span class="smcap">Fig. 55.</span> <i>Julus</i> (after Gervais).</p></div> + +<p>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<span class='pagenum'><a name="Page_90" id="Page_90">90</a></span> +as regards the wings, Pl. <a href="#p4">IV</a>, Fig. 4, representing the +larva of a small beetle named <i>Sitaris</i>, answers very +well to this description. Many other Beetles are +developed from larvæ closely resembling those of +<i>Meloë</i> (Pl. <a href="#p4">IV</a>, Fig. 2), and <i>Sitaris</i> (Pl. <a href="#p4">IV</a>, Fig. 4); in +fact—except those species the larvæ of which, as, for +instance of the <i>Weevils</i> (Pl. <a href="#p2">II</a>, Fig. 6), are internal +feeders, and do not require legs—we may say that +the Coleoptera generally are derived from larvæ of +this type.</p> + +<p>I will now pass to a second order, the Neuroptera. +Pl. <a href="#p4">IV</a>, Fig. 1, represents the larva of <i>Chloëon</i>, a +species the metamorphoses of which I described +some years ago in the Linnean Transactions,<a name="FNanchor_60" id="FNanchor_60"></a><a href="#Footnote_60" class="fnanchor">60</a> and +it is obvious that in essential points it closely resembles +the form to which I have just alluded.</p> + +<p>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.</p> + +<p>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 <i>Sirex</i> (Fig. <a href="#f9">9</a>), +and of the Saw-flies (Fig. <a href="#f48">50</a>) have well-developed +thoracic legs.</p> + +<p>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. <a href="#f48">48</a>), +<span class='pagenum'><a name="Page_91" id="Page_91">91</a></span>have generally been classed with the maggots of Flies, +<i>Weevils</i>, &c., rather than with the more active form +of larva just adverted to. This seems to me, as I +have already pointed out,<a name="FNanchor_61" id="FNanchor_61"></a><a href="#Footnote_61" class="fnanchor">61</a> 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.</p> + +<p>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<a name="FNanchor_62" id="FNanchor_62"></a><a href="#Footnote_62" class="fnanchor">62</a> has +shown, very abnormal in other respects.</p> + +<p>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 <i>Campodea</i> (Pl. <a href="#p3">III</a>, +Fig. 5) still lives; it inhabits damp earth, and +closely resembles the larva of <i>Chloëon</i> (Pl. <a href="#p2">II</a>, Fig. 1), +constituting, indeed, a type which, as shown in Pl. 4, +<span class='pagenum'><a name="Page_92" id="Page_92">92</a></span>occurs in many orders of insects. It is true that the +mouth-parts of <i>Campodea</i> 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<a name="FNanchor_63" id="FNanchor_63"></a><a href="#Footnote_63" class="fnanchor">63</a> pointed out, its mouth-parts +are intermediate between the mandibulate +and haustellate types; a fact which seems to me +most suggestive.</p> + +<p>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 <i>Campodea</i>, 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.</p> + +<p>If these views are correct, the genus <i>Campodea</i> +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.</p> + +<p>From what lower group the <i>Campodea</i> type was +itself derived is a question of great difficulty. Fritz +Müller indeed says,<a name="FNanchor_64" id="FNanchor_64"></a><a href="#Footnote_64" class="fnanchor">64</a> “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 +<span class='pagenum'><a name="Page_93" id="Page_93">93</a></span>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 <i>Campodea</i> type may possibly have been +derived from a less highly developed one, resembling +the modern Tardigrade,<a name="FNanchor_65" id="FNanchor_65"></a><a href="#Footnote_65" class="fnanchor">65</a> a (Fig. <a href="#f56">56</a>) smaller and much +less highly organized being than <i>Campodea</i>. 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.</p> + +<div class="figcenter" style="width: 400px;"><a name="f56" id="f56"></a> +<img src="images/fig_056.png" width="400" height="140" +alt="Fig. 56" title="" /> + +<p class="center"><span class="smcap">Fig. 56</span>, Tardigrade (after Dujardin).</p></div> + +<p>These legs, however, as will be seen, are reduced +to mere projections. But for them, the Tardigrada +<span class='pagenum'><a name="Page_94" id="Page_94">94</a></span>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,<a name="FNanchor_66" id="FNanchor_66"></a><a href="#Footnote_66" class="fnanchor">66</a> 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 <i>Phryganea</i>, only once found a specimen +in this stage, which also, according to the researches +of Huxley,<a name="FNanchor_67" id="FNanchor_67"></a><a href="#Footnote_67" class="fnanchor">67</a> seems to be little more than indicated +in <i>Aphis</i>. 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.</p> + +<p>Fritz Müller has expressed the opinion<a name="FNanchor_68" id="FNanchor_68"></a><a href="#Footnote_68" class="fnanchor">68</a> 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 +<span class='pagenum'><a name="Page_95" id="Page_95">95</a></span>before larvæ and pupæ.” This opinion has been +adopted by Mr. Packard<a name="FNanchor_69" id="FNanchor_69"></a><a href="#Footnote_69" class="fnanchor">69</a> in his “Embryological +Studies on Hexapodous Insects.”</p> + +<p>M. Brauer<a name="FNanchor_70" id="FNanchor_70"></a><a href="#Footnote_70" class="fnanchor">70</a> 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 +<i>Sitaris</i>.</p> + +<div class="figcenter" style="width: 350px;"><a name="f57" id="f57"></a> +<img src="images/fig_057-58.png" width="350" height="532" +alt="Figs. 57-58" title="" /> + +<p class="center"><span class="smcap">Fig. 57</span>, Larva of <i>Cecidomyia</i> (After Packard). 58, <i>Lindia +torulosa</i> (after Dujardin).</p></div> + +<p>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 <i>Campodea</i> 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 grublike 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 <i>Campodea</i> type is +itself derived from earlier vermiform ancestors. +Nicolas Wagner has shown in the case of a small +gnat, allied to <i>Cecidomyia</i>, that even now, in some +instances, the vermiform larvæ possess the power of +reproduction. Such a larva (as, for instance, Fig. <a href="#f57">57</a>) +very closely resembles some of the Rotatoria, such +for instance as <i>Albertia</i> or <i>Notommata</i>, which however +<span class='pagenum'><a name="Page_96" id="Page_96">96</a></span> +possess vibratile cilia. There is, indeed, +one genus—<i>Lindia</i> (Fig. <a href="#f57">58</a>)—in which these ciliæ +are altogether absent, and which, though resembling +<i>Macrobiotus</i> 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<span class='pagenum'><a name="Page_97" id="Page_97">97</a></span> +Dujardin figures a specimen containing an egg, and +seems to have no doubt that it is a mature form.<a name="FNanchor_71" id="FNanchor_71"></a><a href="#Footnote_71" class="fnanchor">71</a></p> + +<p>For the next descending stage we must, I think, +look among the Infusoria, through such genera as +<i>Chætonotus</i> or <i>Ichthydium</i>. Other forms of the +Rotatoria, such for instance as <i>Rattulus</i>, and still +more the very remarkable species discovered in 1871 +by Mr. Hudson,<a name="FNanchor_72" id="FNanchor_72"></a><a href="#Footnote_72" class="fnanchor">72</a> and described under the name of +<i>Pedalion mira</i>, seem to lead to the Crustacea through +the Nauplius form. Dr. Cobbold tells me that he +regards the <i>Gordii</i> as the lowest of the Scolecida; +Mr. E. Ray Lankester considers some of the Turbellaria, +such genera as <i>Mesostomum</i>, <i>Vortex</i>, &c., to be +the lowest of existing worms; excluding the parasitic +groups. Haeckel<a name="FNanchor_73" id="FNanchor_73"></a><a href="#Footnote_73" class="fnanchor">73</a> 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.</p> + +<div class="figcenter" style="width: 400px;"><a name="f59" id="f59"></a> +<img src="images/fig_059.png" width="400" height="101" +alt="Fig. 59" title="" /> + +<p class="center"><span class="smcap">Fig. 59</span>, <i>Prorhynchus stagnaus</i>.<a name="FNanchor_75" id="FNanchor_75"></a><a href="#Footnote_75" class="fnanchor">75</a></p></div> + +<p>We may take, as an illustration of the lower worms, +the genus <i>Prorhynchus</i> (Fig. <a href="#f59">59</a>), which consists of +a hollow cylindrical body, containing a straight +simple tube, the digestive organ.</p> + +<p>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 repre<span class='pagenum'><a name="Page_98" id="Page_98">98</a></span>sent +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.</p> + +<p>Thus the form which Haeckel has described<a name="FNanchor_74" id="FNanchor_74"></a><a href="#Footnote_74" class="fnanchor">74</a> under +the name <i>Protamœba primitiva</i>, Pl. <a href="#p5">V</a>, 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 <i>Amœba</i>, from which, +however, <i>Protamœba</i> 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. <a href="#p5">V</a>, Fig. 2), its form approximates more +and more to that of an hour-glass (Pl. <a href="#p5">V</a>, Fig. 3), and +at length the two halves separate, and each commences +an independent existence (Pl. <a href="#p5">V</a>, Fig. 5).</p> + +<p><span class='pagenum'><a name="Page_99" id="Page_99">99</a></span></p> + +<div class="figcenter" style="width: 500px;"><a name="p5" id="p5"></a> +<img src="images/plate_005.png" width="500" height="779" +alt="PLATE V." title="" /> + +<p class="center"><span class="smcap">PLATE V.</span></p> + +<p class="caption"><span class="smcap">Figs. 1-5</span>, <i>Protamœba</i> 6-9, <i>Protamyxa aurantiaca</i>, Haeckel, Beit. zur Monog. +der Moneren, pl. 1; 10-18, <i>Magosphœra planula</i>, Haeckel, loc. cit. pl. 5.]<span class='pagenum'><a name="Page_100" id="Page_100">100</a></span></p></div> + +<p>In the true <i>Amœbas</i>, 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 <i>Amœbas</i>, +as in <i>Protamœba</i>, multiplication takes place by self-division, +and nothing corresponding to sexual reproduction +has yet been discovered.</p> + +<p>Somewhat more advanced, but still of great simplicity, +is the <i>Protomyxa aurantiaca</i> (Pl. <a href="#p5">V</a>, Fig. 8), discovered +by Haeckel<a name="FNanchor_76" id="FNanchor_76"></a><a href="#Footnote_76" class="fnanchor">76</a> on dead shells of <i>Spirula</i>, where +it appears as a minute orange speck, which shows well +against the clear white of the <i>Spirula</i>. 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. <a href="#p5">V</a>, Fig. 9), after which the containing +membrane burst, and the separate spherules, losing +their globular form, crept out as small Amœbæ (Pl. <a href="#p5">V</a>, +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. <a href="#p5">V</a>, 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.</p> +<p><span class='pagenum'><a name="Page_101" id="Page_101">101</a></span></p> +<p>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.</p> + +<p>As another illustration I may take the <i>Magosphæra +planula</i>, discovered by Haeckel on the coast of +Norway.</p> + +<p>In one stage of its existence (Pl. <a href="#p5">V</a>, 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 <i>Amœba</i> just described. It does +not, however, remain always in this condition. After +a while it contracts into a spherical form (Pl. <a href="#p5">V</a>, Fig. +ii), and secretes round itself a structureless envelope, +which, with the nucleus, gives it a very close resemblance +to a minute egg.</p> + +<p>Gradually the nucleus divides, and the protoplasm +also separates into two spherules (Pl. <a href="#p5">V</a>, Fig. 12); these +two subdivide into four (Pl. <a href="#p5">V</a>, 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. <a href="#p5">V</a>, +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 <i>Magosphæra</i>, 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<span class='pagenum'><a name="Page_102" id="Page_102">102</a></span> +then swims about freely in the water (Pl. <a href="#p5">V</a>, Fig. 16), +thus closely resembling <i>Synura</i>, 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. <a href="#p5">V</a>, 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. <a href="#p5">V</a>, 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 <i>Gregarina</i>, +is of a similar character.</p> + +<p>It may be said, and said truly, that the difference +between such beings as these and the <i>Campodea</i>, 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.</p> + +<p>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.<a name="FNanchor_77" id="FNanchor_77"></a><a href="#Footnote_77" class="fnanchor">77</a> As shown in Fig. <a href="#f60">60</a>, the egg is at +first a round body or cell, with a clear central nucleus—the germinal +<span class='pagenum'><a name="Page_103" id="Page_103">103</a></span>vesicle; it increases in size, and after +a while the yolk and the germinal vesicle divide into +two (Fig. <a href="#f60">61</a>), then into four (Fig. <a href="#f60">62</a>), and so on, just +as we have seen to be the case in <i>Magosphæra</i>. From +the minute cells (Fig. <a href="#f60">63</a>) arising through this process +of yolk-segmentation, the body of the Tardigrade is +then built up.<a name="FNanchor_78" id="FNanchor_78"></a><a href="#Footnote_78" class="fnanchor">78</a></p> + +<div class="figcenter" style="width: 500px;"><a name="f60" id="f60"></a> +<img src="images/fig_060-63.png" width="500" height="149" +alt="Figs. 60-63" title="" /> + +<p class="caption"><span class="smcap">Fig. 60</span>, 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.</p></div> + +<p>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 <i>Magosphœra</i> 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.</p> + +<p>Among the Zoophytes Prof. Allman thus describes<a name="FNanchor_79" id="FNanchor_79"></a><a href="#Footnote_79" class="fnanchor">79</a> +the process in <i>Laomedea</i>, as representing the Hydroids +(Pl. <a href="#p6">VI</a>, Fig. 1, represents the young egg):—"The first +step observable in the segmentation-process is the +<span class='pagenum'><a name="Page_104" id="Page_104">104</a></span>cleavage of the yolk into two segments (Pl. <a href="#p6">VI</a>, Fig. 2), +immediately followed by the cleavage of these into +other two, so that the vitellus is now composed of +four cleavage spheres (Pl. <a href="#p6">VI</a>, Fig. 3)." These spheres +again divide (Pl. <a href="#p6">VI</a>, Fig. 4) and subdivide, thus at +length forming minute cells, of which the body of the +embryo is built up.</p> + +<p>In Pl. <a href="#p6">VI</a>, Figs. 5-9 represent the corresponding +stages in the development of a small parasitic worm—the +<i>Filaria mustelarum</i>—as given by Van Beneden.<a name="FNanchor_80" id="FNanchor_80"></a><a href="#Footnote_80" class="fnanchor">80</a> +The first process is that within the egg, which represents, +so to say, the encysted condition of <i>Magosphœra</i>, +the yolk divides itself into two balls (Pl. <a href="#p6">VI</a>, +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 <i>Sphærularia +bombi</i>.<a name="FNanchor_81" id="FNanchor_81"></a><a href="#Footnote_81" class="fnanchor">81</a></p> + +<p>Among the Echinoderms M. Derbès thus describes +the first stages (Pl. <a href="#p6">VI</a>, Figs. 10-13) in the development +of the egg of an <i>Echinus</i> (<i>Echinus esculentus</i>):—"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."<a name="FNanchor_82" id="FNanchor_82"></a><a href="#Footnote_82" class="fnanchor">82</a> +Sars has observed the same thing in the star-fish.<a name="FNanchor_83" id="FNanchor_83"></a><a href="#Footnote_83" class="fnanchor">83</a></p> + +<div class="figcenter" style="width: 500px;"><a name="p6" id="p6"></a> +<img src="images/plate_006.png" width="500" height="840" +alt="PLATE VI." title="" /> + +<p class="center">PLATE. 6.</p></div> + +<p>In the Rotatoria, as shown by Huxley in <i>Lacinularia</i>,<a name="FNanchor_84" id="FNanchor_84"></a><a href="#Footnote_84" class="fnanchor">84</a> +and by Williamson in <i>Melicerta</i>,<a name="FNanchor_85" id="FNanchor_85"></a><a href="#Footnote_85" class="fnanchor">85</a> the yolk is at +<span class='pagenum'><a name="Page_106" id="Page_106">106</a></span><span class='pagenum'><a name="Page_105" id="Page_105">105</a></span>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<a name="FNanchor_86" id="FNanchor_86"></a><a href="#Footnote_86" class="fnanchor">86</a> (<i>Tritonia</i>), Müller in Entochocha,<a name="FNanchor_87" id="FNanchor_87"></a><a href="#Footnote_87" class="fnanchor">87</a> +Haeckel in Ascidia,<a name="FNanchor_88" id="FNanchor_88"></a><a href="#Footnote_88" class="fnanchor">88</a> Lacaze Duthiers in <i>Dentalium</i>.<a name="FNanchor_89" id="FNanchor_89"></a><a href="#Footnote_89" class="fnanchor">89</a> +Figures 18 to 21, Pl. <a href="#p6">VI</a>, are taken from Koren and +Danielssen’s<a name="FNanchor_90" id="FNanchor_90"></a><a href="#Footnote_90" class="fnanchor">90</a> memoir on the development of <i>Purpura +lapillus</i>.</p> + +<p>Figs. <a href="#p6">22-24</a> show the same stages in a fish +(<i>Amphioxus</i>) as given by Haeckel, and it is unnecessary +to point out the great similarity.</p> + +<p>Lastly, figures 25 to 29, Pl. 6, are given by Dr. +Allen Thomson,<a name="FNanchor_91" id="FNanchor_91"></a><a href="#Footnote_91" class="fnanchor">91</a> as illustrating the first stages in the +development of the vertebrata.</p> + +<p>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 +<span class='pagenum'><a name="Page_107" id="Page_107">107</a></span>groups; that as Allen Thomson has truly observed,<a name="FNanchor_92" id="FNanchor_92"></a><a href="#Footnote_92" class="fnanchor">92</a> +“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.”</p> + +<p>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.</p> + +<p>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 +<i>Campodea</i>.</p> + +<p>Since, then, individual insects are certainly in many +cases developed from larvæ closely resembling the +genus <i>Campodea</i>, 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 +<span class='pagenum'><a name="Page_108" id="Page_108">108</a></span> +days, is surely no wild or extravagant hypothesis. +Again, other insects come from vermiform larvæ +much resembling the genus <i>Lindia</i>, 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 <i>Campodea</i>, and that these again have +arisen from others belonging to a type represented +more or less closely by the existing genus <i>Lindia</i>.</p> + +<p>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.</p> + +<h3>FOOTNOTES:</h3> + +<div class="footnote"><p><a name="Footnote_1" id="Footnote_1"></a><a href="#FNanchor_1"><span class="label">1</span></a> Darwin’s “Researches into the Geology and Natural History of the +Countries visited by H.M.S. <i>Beagle</i>,” p. 326.</p></div> + +<div class="footnote"><p><a name="Footnote_2" id="Footnote_2"></a><a href="#FNanchor_2"><span class="label">2</span></a> Introduction to Entomology, vi. p. 50.</p></div> + +<div class="footnote"><p><a name="Footnote_3" id="Footnote_3"></a><a href="#FNanchor_3"><span class="label">3</span></a> Manual of Entomology, p. 30.</p></div> + +<div class="footnote"><p><a name="Footnote_4" id="Footnote_4"></a><a href="#FNanchor_4"><span class="label">4</span></a> Linnean Journal, vol. xi.</p></div> + +<div class="footnote"><p><a name="Footnote_5" id="Footnote_5"></a><a href="#FNanchor_5"><span class="label">5</span></a> Introduction to the Modern Classification of Insects, p. 17.</p></div> + +<div class="footnote"><p><a name="Footnote_6" id="Footnote_6"></a><a href="#FNanchor_6"><span class="label">6</span></a> Linnean Transactions, 1863—"On the Development of <i>Chloëon</i>."</p></div> + +<div class="footnote"><p><a name="Footnote_7" id="Footnote_7"></a><a href="#FNanchor_7"><span class="label">7</span></a> The figures on the first four plates are principally borrowed from +Mr. Westwood’s excellent “Introduction to the Modern Classification +of Insects.”</p></div> + +<div class="footnote"><p><a name="Footnote_8" id="Footnote_8"></a><a href="#FNanchor_8"><span class="label">8</span></a> “Sur la Domestication des <i>Clavigers</i> par les Fourmis.” Bull. de +la Soc. d’Anthropologie de Paris, 1868, p. 315.</p></div> + +<div class="footnote"><p><a name="Footnote_9" id="Footnote_9"></a><a href="#FNanchor_9"><span class="label">9</span></a> Westwood’s Introduction, vol. i. p. 36.</p></div> + +<div class="footnote"><p><a name="Footnote_10" id="Footnote_10"></a><a href="#FNanchor_10"><span class="label">10</span></a> Westwood’s Introduction, vol. ii. p. 52.</p></div> + +<div class="footnote"><p><a name="Footnote_11" id="Footnote_11"></a><a href="#FNanchor_11"><span class="label">11</span></a> Die Fortpflanzung und Entwickelung der Pupiparen. Von Dr. +R. Leuckart. Halle. 1848.</p></div> + +<div class="footnote"><p><a name="Footnote_12" id="Footnote_12"></a><a href="#FNanchor_12"><span class="label">12</span></a> Ann. des Sci. Nat., sér. 4, tome vii. See also <i>Natural History +Review</i>, April 1862.</p></div> + +<div class="footnote"><p><a name="Footnote_13" id="Footnote_13"></a><a href="#FNanchor_13"><span class="label">13</span></a> Ann. and Mag. of Nat. Hist. 1852.</p></div> + +<div class="footnote"><p><a name="Footnote_14" id="Footnote_14"></a><a href="#FNanchor_14"><span class="label">14</span></a> Zeits. für Wiss. Zool. 1869.</p></div> + +<div class="footnote"><p><a name="Footnote_15" id="Footnote_15"></a><a href="#FNanchor_15"><span class="label">15</span></a> Transactions of the Linnean Society, 1863.</p></div> + +<div class="footnote"><p><a name="Footnote_16" id="Footnote_16"></a><a href="#FNanchor_16"><span class="label">16</span></a> Lectures on the Anatomy, &c. of the Invertebrate Animals.</p></div> + +<div class="footnote"><p><a name="Footnote_17" id="Footnote_17"></a><a href="#FNanchor_17"><span class="label">17</span></a> Untersuchungen über die Entwickelung und den Bau der Gliederthiere, +1854.</p></div> + +<div class="footnote"><p><a name="Footnote_18" id="Footnote_18"></a><a href="#FNanchor_18"><span class="label">18</span></a> Linnean Transactions, vol. xxii. 1858.</p></div> + +<div class="footnote"><p><a name="Footnote_19" id="Footnote_19"></a><a href="#FNanchor_19"><span class="label">19</span></a> “Embryological Studies on Hexapodous Insects.” Peabody Academy +of Science. Third Memoir.</p></div> + +<div class="footnote"><p><a name="Footnote_20" id="Footnote_20"></a><a href="#FNanchor_20"><span class="label">20</span></a> Mém. de l’Acad. Imp. des Sci. de St. Pétersbourg. 1869.</p></div> + +<div class="footnote"><p><a name="Footnote_21" id="Footnote_21"></a><a href="#FNanchor_21"><span class="label">21</span></a> Observationes de Prima Insectorum Genesi, p. 14.</p></div> + +<div class="footnote"><p><a name="Footnote_22" id="Footnote_22"></a><a href="#FNanchor_22"><span class="label">22</span></a> Mém. de l’Acad. Imp. des Sci. de St. Pétersbourg. tome xvi. 1871, +p. 35.</p></div> + +<div class="footnote"><p><a name="Footnote_23" id="Footnote_23"></a><a href="#FNanchor_23"><span class="label">23</span></a> Recherches sur l’Evolution des Araignées.</p></div> + +<div class="footnote"><p><a name="Footnote_24" id="Footnote_24"></a><a href="#FNanchor_24"><span class="label">24</span></a> Philosophical Transactions, 1841.</p></div> + +<div class="footnote"><p><a name="Footnote_25" id="Footnote_25"></a><a href="#FNanchor_25"><span class="label">25</span></a> Monog. of the Gymnoblastic or Tubularian Hydroids. See also +Hincks, British Hydroid Zoophytes. Pl. x.</p></div> + +<div class="footnote"><p><a name="Footnote_26" id="Footnote_26"></a><a href="#FNanchor_26"><span class="label">26</span></a> Loc. cit. p. 315.</p></div> + +<div class="footnote"><p><a name="Footnote_27" id="Footnote_27"></a><a href="#FNanchor_27"><span class="label">27</span></a> Philosophical Transactions, 1859, p. 589.</p></div> + +<div class="footnote"><p><a name="Footnote_28" id="Footnote_28"></a><a href="#FNanchor_28"><span class="label">28</span></a> “Facts for Darwin,” Eng. Trans. p. 127.</p></div> + +<div class="footnote"><p><a name="Footnote_29" id="Footnote_29"></a><a href="#FNanchor_29"><span class="label">29</span></a> Rolleston, “Forms of Animal Life,” p. 146.</p></div> + +<div class="footnote"><p><a name="Footnote_30" id="Footnote_30"></a><a href="#FNanchor_30"><span class="label">30</span></a> 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.</p></div> + +<div class="footnote"><p><a name="Footnote_31" id="Footnote_31"></a><a href="#FNanchor_31"><span class="label">31</span></a> Ueber die Gattungen der Seeigellarven. Siebente Abhandlung. Kön. +Akad. d. Wiss. zu Berlin. Von Joh. Müller, 1855, Pl. iii. fig. 3.</p></div> + +<div class="footnote"><p><a name="Footnote_32" id="Footnote_32"></a><a href="#FNanchor_32"><span class="label">32</span></a> Huxley, Introduction to the Classification of Animals, p. 45.</p></div> + +<div class="footnote"><p><a name="Footnote_33" id="Footnote_33"></a><a href="#FNanchor_33"><span class="label">33</span></a> Philosophical Transactions, 1865 and 1866.</p></div> + +<div class="footnote"><p><a name="Footnote_34" id="Footnote_34"></a><a href="#FNanchor_34"><span class="label">34</span></a> Loc. cit. Zweit. Abh. Pl. i., figs. 8 and 9.</p></div> + +<div class="footnote"><p><a name="Footnote_35" id="Footnote_35"></a><a href="#FNanchor_35"><span class="label">35</span></a> Thomson, on the Embryology of the Echinodermata, <i>Natural +History Review</i>, 1863, p. 415. See also Agassiz, “Embryology of the +Starfish,” p. 62.</p></div> + +<div class="footnote"><p><a name="Footnote_36" id="Footnote_36"></a><a href="#FNanchor_36"><span class="label">36</span></a> A. Agassiz, Embryology of Echinoderms, p. 18.</p></div> + +<div class="footnote"><p><a name="Footnote_37" id="Footnote_37"></a><a href="#FNanchor_37"><span class="label">37</span></a> Hincks. British Hydroid Zoophytes, pp. 120-147.</p></div> + +<div class="footnote"><p><a name="Footnote_38" id="Footnote_38"></a><a href="#FNanchor_38"><span class="label">38</span></a> Zeits. für Wiss. Zool. 1864, p. 228.</p></div> + +<div class="footnote"><p><a name="Footnote_39" id="Footnote_39"></a><a href="#FNanchor_39"><span class="label">39</span></a> Introduction to Entomology, 6th ed. vol. i. p. 61.</p></div> + +<div class="footnote"><p><a name="Footnote_40" id="Footnote_40"></a><a href="#FNanchor_40"><span class="label">40</span></a> Métamorphoses de l’Homme et des Animaux, p. 133. See also +Carpenter, Principles of Physiology. 1851, p. 389.</p></div> + +<div class="footnote"><p><a name="Footnote_41" id="Footnote_41"></a><a href="#FNanchor_41"><span class="label">41</span></a> Darwin, Origin of Species, 4th ed. p. 532.</p></div> + +<div class="footnote"><p><a name="Footnote_42" id="Footnote_42"></a><a href="#FNanchor_42"><span class="label">42</span></a> Principles of Biology, vi. p. 349.</p></div> + +<div class="footnote"><p><a name="Footnote_43" id="Footnote_43"></a><a href="#FNanchor_43"><span class="label">43</span></a> For differences in larva consequent on variation in the external condition, +see <i>ante</i>, p. 61.</p></div> + +<div class="footnote"><p><a name="Footnote_44" id="Footnote_44"></a><a href="#FNanchor_44"><span class="label">44</span></a> See Hincks. British Hydroid Zoophytes, P. lxii. Agassiz, Sea-side +Studies, p. 43.</p></div> + +<div class="footnote"><p><a name="Footnote_45" id="Footnote_45"></a><a href="#FNanchor_45"><span class="label">45</span></a> See Newport, Phil. Trans., 1832.</p></div> + +<div class="footnote"><p><a name="Footnote_46" id="Footnote_46"></a><a href="#FNanchor_46"><span class="label">46</span></a> Linnean Transactions, 1862.</p></div> + +<div class="footnote"><p><a name="Footnote_47" id="Footnote_47"></a><a href="#FNanchor_47"><span class="label">47</span></a> Origin of Species, 4th ed., pp. 14 and 97.</p></div> + +<div class="footnote"><p><a name="Footnote_48" id="Footnote_48"></a><a href="#FNanchor_48"><span class="label">48</span></a> On the Alternation of Generations. By J. J. Steenstrup. Trans. +by C. Busk, Esq. Ray Society. 1842.</p></div> + +<div class="footnote"><p><a name="Footnote_49" id="Footnote_49"></a><a href="#FNanchor_49"><span class="label">49</span></a> Zeit. für Wiss. Zool. 1863.</p></div> + +<div class="footnote"><p><a name="Footnote_50" id="Footnote_50"></a><a href="#FNanchor_50"><span class="label">50</span></a> Mém. de l’Acad. Imp. de St. Pétersbourg. vol. xv. 1870.</p></div> + +<div class="footnote"><p><a name="Footnote_51" id="Footnote_51"></a><a href="#FNanchor_51"><span class="label">51</span></a> Of course all animals in which the sexes are distinct are in one +sense dimorphic.</p></div> + +<div class="footnote"><p><a name="Footnote_52" id="Footnote_52"></a><a href="#FNanchor_52"><span class="label">52</span></a> “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.”—<span class="smcap">Huxley</span> +<i>on Animal Individuality</i>, Ann. and Mag. of Nat. Hist. +June 1852.</p></div> + +<div class="footnote"><p><a name="Footnote_53" id="Footnote_53"></a><a href="#FNanchor_53"><span class="label">53</span></a> Prince Hohenstiel Schwangau, p. 68.</p></div> + +<div class="footnote"><p><a name="Footnote_54" id="Footnote_54"></a><a href="#FNanchor_54"><span class="label">54</span></a> Journal of the Royal Institution. April 1873.</p></div> + +<div class="footnote"><p><a name="Footnote_55" id="Footnote_55"></a><a href="#FNanchor_55"><span class="label">55</span></a> “Embryology of Echinoderms,” l. c. p. 15.</p></div> + +<div class="footnote"><p><a name="Footnote_56" id="Footnote_56"></a><a href="#FNanchor_56"><span class="label">56</span></a> Mr. and Mrs. Agassiz: “Seaside Studies,” p. 139.</p></div> + +<div class="footnote"><p><a name="Footnote_57" id="Footnote_57"></a><a href="#FNanchor_57"><span class="label">57</span></a> l. c. p. 138.</p></div> + +<div class="footnote"><p><a name="Footnote_58" id="Footnote_58"></a><a href="#FNanchor_58"><span class="label">58</span></a> Wien. Zool. Bot. Gesells, 1869.</p></div> + +<div class="footnote"><p><a name="Footnote_59" id="Footnote_59"></a><a href="#FNanchor_59"><span class="label">59</span></a> Linnean Transactions, 1863.</p></div> + +<div class="footnote"><p><a name="Footnote_60" id="Footnote_60"></a><a href="#FNanchor_60"><span class="label">60</span></a> Linnean Transactions, 1866, vol. xxv.</p></div> + +<div class="footnote"><p><a name="Footnote_61" id="Footnote_61"></a><a href="#FNanchor_61"><span class="label">61</span></a> Linnean Transactions, vol. xxiv. p. 65.</p></div> + +<div class="footnote"><p><a name="Footnote_62" id="Footnote_62"></a><a href="#FNanchor_62"><span class="label">62</span></a> Siebold und Kolliker’s Zeitschr. f. Wiss. Zool., 1864.</p></div> + +<div class="footnote"><p><a name="Footnote_63" id="Footnote_63"></a><a href="#FNanchor_63"><span class="label">63</span></a> Linnean Journal, vol. xi.</p></div> + +<div class="footnote"><p><a name="Footnote_64" id="Footnote_64"></a><a href="#FNanchor_64"><span class="label">64</span></a> Facts for Darwin, p. 120.</p></div> + +<div class="footnote"><p><a name="Footnote_65" id="Footnote_65"></a><a href="#FNanchor_65"><span class="label">65</span></a> A still nearer approach is afforded by the genus <i>Peripatus</i>, 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.</p></div> + +<div class="footnote"><p><a name="Footnote_66" id="Footnote_66"></a><a href="#FNanchor_66"><span class="label">66</span></a> Unters. üb. die Entwick, und den Bau der Gliederthiere, p. 73.</p></div> + +<div class="footnote"><p><a name="Footnote_67" id="Footnote_67"></a><a href="#FNanchor_67"><span class="label">67</span></a> Linnean Transactions, v. xxii.</p></div> + +<div class="footnote"><p><a name="Footnote_68" id="Footnote_68"></a><a href="#FNanchor_68"><span class="label">68</span></a> Facts for Darwin, trans. by Dallas, p. 118. See also Darwin, +“Origin of Species,” p. 530. 4th ed.</p></div> + +<div class="footnote"><p><a name="Footnote_69" id="Footnote_69"></a><a href="#FNanchor_69"><span class="label">69</span></a> Mem. Peabody Academy of Science, v. I. No, 3.</p></div> + +<div class="footnote"><p><a name="Footnote_70" id="Footnote_70"></a><a href="#FNanchor_70"><span class="label">70</span></a> Wien. Zool. Bott. Gesells. 1869, p. 310.</p></div> + +<div class="footnote"><p><a name="Footnote_71" id="Footnote_71"></a><a href="#FNanchor_71"><span class="label">71</span></a> 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 <i>Echinoderes</i>, which these two eminent +naturalists unite in regarding as intermediate between the Annelides and +the Crustacea.</p></div> + +<div class="footnote"><p><a name="Footnote_72" id="Footnote_72"></a><a href="#FNanchor_72"><span class="label">72</span></a> “On a New Rotifer.” <i>Monthly Microscopical Journal</i>, Sept. 1871.</p></div> + +<div class="footnote"><p><a name="Footnote_73" id="Footnote_73"></a><a href="#FNanchor_73"><span class="label">73</span></a> Generelle Morphologie, vol. ii. p. 79.</p></div> + +<div class="footnote"><p><a name="Footnote_74" id="Footnote_74"></a><a href="#FNanchor_74"><span class="label">74</span></a> Monographie der Moneren, p. 43.</p></div> + +<div class="footnote"><p><a name="Footnote_75" id="Footnote_75"></a><a href="#FNanchor_75"><span class="label">75</span></a> Gegenbaur. Grund. d. Vergleich. Anat. p. 210. See also +Dr. M. S. Schultze, Beiträge zur Naturg. der. Turbellarien. 1851. +Pl. vi. fig. 1.</p></div> + +<div class="footnote"><p><a name="Footnote_76" id="Footnote_76"></a><a href="#FNanchor_76"><span class="label">76</span></a> Monographieder Moneren, p. 10.</p></div> + +<div class="footnote"><p><a name="Footnote_77" id="Footnote_77"></a><a href="#FNanchor_77"><span class="label">77</span></a> See Kauffmann, Ueber die Entwickelung and systematische Stellung +der Tardigraden. Zeits. f. Wiss. Zool. 1851, p. 220.</p></div> + +<div class="footnote"><p><a name="Footnote_78" id="Footnote_78"></a><a href="#FNanchor_78"><span class="label">78</span></a> It is true that among the Insecta generally the first stages of development +differ in appearance considerably from those above described; +those of <i>Platygaster</i>, as figured by Ganin (ante Figs. <a href="#f17">17-22</a>), being +very exceptional.</p></div> + +<div class="footnote"><p><a name="Footnote_79" id="Footnote_79"></a><a href="#FNanchor_79"><span class="label">79</span></a> Monograph of the Gymnoblastic or Tubularian Hydroids, by G. J. +Allman, Ray Soc. 1871, p. 86.</p></div> + +<div class="footnote"><p><a name="Footnote_80" id="Footnote_80"></a><a href="#FNanchor_80"><span class="label">80</span></a> Mém. sur les Vers Intestinaux, 1858.</p></div> + +<div class="footnote"><p><a name="Footnote_81" id="Footnote_81"></a><a href="#FNanchor_81"><span class="label">81</span></a> Natural History Review, 1861, p. 44.</p></div> + +<div class="footnote"><p><a name="Footnote_82" id="Footnote_82"></a><a href="#FNanchor_82"><span class="label">82</span></a> Ann. des Sci. Nat. 1847, p. 90.</p></div> + +<div class="footnote"><p><a name="Footnote_83" id="Footnote_83"></a><a href="#FNanchor_83"><span class="label">83</span></a> Fauna littoralis Norvegiæ, pl. viii.</p></div> + +<div class="footnote"><p><a name="Footnote_84" id="Footnote_84"></a><a href="#FNanchor_84"><span class="label">84</span></a> Trans. of the Microsc. Soc. of London, 1851.</p></div> + +<div class="footnote"><p><a name="Footnote_85" id="Footnote_85"></a><a href="#FNanchor_85"><span class="label">85</span></a> Quarterly Journal of Microsc. Science, 1853.</p></div> + +<div class="footnote"><p><a name="Footnote_86" id="Footnote_86"></a><a href="#FNanchor_86"><span class="label">86</span></a> Wiegmann’s Archiv., 1840, p. 196.</p></div> + +<div class="footnote"><p><a name="Footnote_87" id="Footnote_87"></a><a href="#FNanchor_87"><span class="label">87</span></a> Ueber die Erzeugung von Schnecken in Holothurier. Berlin, Bericht, +1851. Ann. Nat. Hist. 1852, v. ix. Müller’s Archiv., 1852.</p></div> + +<div class="footnote"><p><a name="Footnote_88" id="Footnote_88"></a><a href="#FNanchor_88"><span class="label">88</span></a> Natürliche Schöpfungsgeschichte, pl. x.</p></div> + +<div class="footnote"><p><a name="Footnote_89" id="Footnote_89"></a><a href="#FNanchor_89"><span class="label">89</span></a> Ann. des Sci. Nat. 1853, p. 89.</p></div> + +<div class="footnote"><p><a name="Footnote_90" id="Footnote_90"></a><a href="#FNanchor_90"><span class="label">90</span></a> Ann. des Sci. Nat. 1857, pl. vi.</p></div> + +<div class="footnote"><p><a name="Footnote_91" id="Footnote_91"></a><a href="#FNanchor_91"><span class="label">91</span></a> Cyclopædia of Anatomy and Physiology. Art. Ovum, p. 4.</p></div> + +<div class="footnote"><p><a name="Footnote_92" id="Footnote_92"></a><a href="#FNanchor_92"><span class="label">92</span></a> Thomson, loc. cit. Article, Ovum, p. 139.</p></div> + +<h4>THE END.</h4> +<hr /> +<h6>RICHARD CLAY AND SONS, LIMITED, LONDON AND BUNGAY.</h6> +<hr /> +<h3><span class='pagenum'><a name="Page_109" id="Page_109">109</a></span>BY THE SAME AUTHOR.</h3> + +<div class="blockquot"><p>PRE-HISTORIC TIMES. 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