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Originally appearing in Volume V13, Page 436 of the 1911 Encyclopedia Britannica.
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RELATIONSHIPS AND PHYLOGENY The Hexapoda form a very clearly defined class of the Arthropoda, and many recent writers have suggested that they must have. arisen independently of other Arthropods from annelid worms, and that the Arthropoda must, therefore, be regarded as an " unnatural, polyphyletic assemblage. The cogent arguments against this view are set forth in the article on Arthropoda. A near relationship between the Apterygota and the Crustacea has been ably advocated by H. J. Hansen (1893). It is admitted on all hands that the Hexapoda are akin to the Chilopoda. Verhoeff has lately (1904) put forward the view that there are really six segments in the hexapodan thorax and twenty in the abdomen—the cerci belonging to the seventeenth abdominal segment thus showing a close agreement with the centipede Scolopendra. On the other hand, G. H. Carpenter (1899, 1902-1904) has lately endeavoured to show an exact numerical correspondence in segmentation between the Hexapoda, the Crustacea, the Arachnid a, and the most primitive of the Diplopoda. On either view it may be believed that the Hexapoda arose with the allied classes from a primitive arthropod stock, while the relationships of the class are with the Crustacea, the Chilopoda and the Diplopoda, rather than with the Arachnida. Nature of Primitive Hexapoda.—Two divergent views have been held as to the nature of the original hexapod stock. Some of those zoologists who look to Peripatus, or a similar worm-like form, as representing the direct ancestors of the Hexapoda have laid stress on a larva like the caterpillar of a moth or saw-fly as representing a primitive stage. On the other hand, the view of F. Muller and F. Brauer, that the Thysanura represent more nearly than any other existing insects the ancestors of the class, has been accepted by the great majority of students. And there can be little doubt that this belief is justified. The caterpillar, or the maggot, is a specialized larval form characteristic of the most highly developed orders, while the campodeiform larva is the starting-point for the more primitive insects. The occurrence in the hypermetamorphic Coleoptera (see supra) of a campodeiform preceding an eruciform stage in the life-history is most suggestive. Taken in connexion with the likeness of the young among the more generalized orders to the adults, it indicate, clearly a thysanuroid starting-point for the evolution of the hexapod orders. And we must infer further that the specialization of the higher orders has been accompanied by an increase in the extent of the metamorphosis—a very exceptional condition among animals generally, as has been ably pointed out by L. C. Mall (1895). Origin of Wings.—The post-embryonic growth of Hexapods with or without metamorphosis is accompanied in most cases by the acquisition of wings. These organs; thus acquired during the lifetime of the individual, must have been in some way acquired during the evolution of the class. Many students of the group, following Brauer, have regarded the Apterygota as representing the original wingless progenitors of the Pterygota, and the many primitive characters shown by the former group lend support to this view. On the other hand, it has been argued that the presence of wings in a vast majority of the Hexapoda suggests their presence in the ancestors of the whole class. It is most unlikely that wings have been acquired independently by various orders of Hexapoda, and if we regard the Thysanura as the slightly modified representatives of a primitively wingless stock, we must postulate the acquisition of wings by-some early offshoot of that stock, an offshoot whence the whole group of the Pterygota took its rise. How wings were acquired by these primitive Pterygota must remain for the present a subject for speculation. Insect wings are specialized outgrowths of certain thoracic segments, and are quite unrepresented in any other class of Arthropods. They are not, therefore, like the wings of birds, modified from some pre-existing structures (the fore-limbs) common to their phylum; they are new and peculiar structures. Comparison of the tracheated wings with the paired tracheated outgrowths on the abdominal segments of the aquatic campodeiform larva of may-flies (see fig. 27) led C. Gegenbaur to the brilliant suggestion that wings might be regarded as specialized and transformed gills. But a survey of the Hexapoda as a whole, and especially a comparative study of the tracheal system, can hardly leave room for doubt that this system is primitively adapted for atmospheric breathing, and that the presence of tracheal gills in larvae must be regarded as a special adaptation for temporary aquatic life. The origin of insect wings remains, therefore, a mystery, deepened by the difficulty of imagining any probable use for thoracic outgrowths, comparable to the wing-rudiments of the Exopterygota, in the early stages of their evolution. Origin of Metamorphosis.—In connexion with the question whether metamorphosis has been gradually acquired, we have to consider two aspects, viz. the bionomic nature of metamorphosis, and to what extent it existed in primitive insects. Bionomically, metamorphosis may be defined as the sum of adaptations that have gradually fitted the larva (caterpillar or maggot) for one kind of life, the fly for another. So that we may conclude that the factors of evolution would favour its development. With regard to its occurrence in primitive insects, our knowledge of the geological record is most imperfect, but so far as it goes it supports the conclusion that holometabolism (i.e. extreme metamorphosis) is a comparatively recent phenomenon of insect life. None of the groups of existing Endopterygota have been traced with certainty farther back than the Mesozoic epoch, and all the numerous Palaeozoic insect-fossils seem to belong to forms that possessed only imperfect metamorphosis. The only doubt arises from the existence of insect remains, referred to the order Coleoptera, in the Silesian Culm of Steinkunzendorf near Reichenbach. The oldest larva known, Mortnolucoides artieulatus, is from the New Red Sandstone of Connecticut; it belongs to the Sialidae, one of the lowest forms of Holometabola. It is now, in fact, generally admitted that metamorphosis has been acquired comparatively recently, and Scudder in his review of the earliest fossil insects states that " their meta-morphoses were simple and incomplete, the young leaving the egg with the form of the parent, but without wings, the assumption of which required no quiescent stage before maturity." It has been previously remarked that the phenomena of holometabolism are connected with the development of wings inside the body (except in the case of the fleas, where there are no wings in the perfect insect). Of existing insects 90% belong to the Endopterygota. At the same time we have noevidence that any Endopterygota existed amongst Palaeozoic insects, so that the phenomena of endopterygotism are comparatively recent, and we are led to infer that the Endopterygota owe their origin to the older Exopterygota. In Endopterygota the wings commence their development as invaginations of the hypodermis, while in Exopterygota the wings begin—and always remain—as external folds or evaginations. The two modes of growth are directly opposed, and at first sight it appears that this fact negatives the view that Endopterygota have been derived from Exopterygota. Only three hypotheses as to the origin of Endopterygota can be suggested as possible, viz.:—(1) That some of the Palaeozoic insects, though we infer them to have been exopterygotous, were really endopterygotous, and were the actual ancestors of the existing Endopterygota; (2) that Endopterygota are not descended from Exopterygota, but were derived directly from ancestors that were never winged; (3) that the predominant division—i.e. Endopterygota—of insects of the present epoch are descended from the predominant—if not the sole—group that existed in the Palaeozoic epoch, viz. the Exopterygota. The first hypothesis is not negatived by direct evidence, for we do not actually know the ontogeny of any of the Palaeozoic insects; it is, however, rendered highly improbable by the modern views as to the nature and origin of wings in insects, and by the fact that the Endopterygota include none of the lower existing forms of insects. The second hypothesis—to the effect that Endopterygota are the descendants of apterous insects that had never possessed wings (i.e. the Apterygogenea of Brauer and others, though we prefer the shorter term Apterygota)—is rendered improbable from the fact that existing Apterygota are related to Exopterygota, not to Endopterygota, and by the knowledge that has been gained as to the morphology and development of wings, which suggest that—if we may so phrase it—were an apterygotous insect gradually to develop wings, it would be on the exopterygotous system. From all points of view it appears, therefore, probable that Endopterygota are descended from Exopterygota, and we are brought to the question as to the way in which this has occurred. It is almost impossible to believe that any species of insect that has for a long period developed the wings outside the body could change this mode of growth suddenly for an internal mode of development of the organs in question, for, as we have already explained, the two modes of growth are directly opposed. The explanation has to be sought in another direction. Now there are many forms of Exopterygota in which the creatures are almost or quite destitute of wings. This phenomenon occurs among species found at high elevations, among others found in arid or desert regions, and in some cases in the female sex only, the male being winged and the female wingless. This last state is very frequent in Blattidae, which were amongst the most abundant of Palaeozoic insects. The wingless forms in question are always allied to winged forms, and there is every reason to believe that they have been really derived from winged forms. There are also insects (fleas, &c.) in which metamorphosis of a " complete " character exists, though the insects never develop wings. These cases render it highly probable that insects may in some circumstances become wing-less, though their ancestors were winged. Such insects have been styled anapterygotous. In these facts we have one possible clue to the change from exopterygotism to endopterygotism, namely, by an intermediate period of anapterygotism. Although we cannot yet define the conditions under which exopterygotous wings are suppressed or unusually developed, yet we know that such fluctuations occur. There are, in fact, existing forms of Exopterygota that are usually wingless, and that nevertheless appear in certain seasons or localities with wings. We are therefore entitled to assume that the suppressed wings of Exopterygota tend to reappear; and, speaking of the past, we may say that if after a period of suppression the wings began to reappear as hypodermal buds while a more rigid pressure was exerted by the cuticle, the growth of the buds would necessarily be inwards, and we should have incipient endopterygotism. The change that is required to transform Exopterygota into Endopterygota is merely that a cell of hypodeimis should proliferate inwards instead of outwards, or that a minute hypo-dermal evaginated bud should be forced to the interior of the body by the pressure of a contracted cuticle. If it should be objected that the wings so developed would be rudimentary, and that there would be nothing to encourage their development into perfect functional organs, we may remind the reader that we have already pointed out that imperfect wings of Exopterygota do, even at the present time under certain conditions, become perfect organs; and we may also add that there are, even among existing Endopterygota, species in which the wings are usually vestiges and yet sometimes become perfectly developed. In fact, almost every condition that is required for the change from exopterygotism to endopterygotism exists among the insects that surround us. But it may perhaps be considered improbable that organs like the wings, having once been lost, should have been re-acquired on the large scale suggested by the theory just put forward. If so, there is an alternative method by which the endopterygotous may have arisen from the exopterygotous condition. The sub-imago of the Ephemeroptera suggests that a moult, after the wings had become functional, was at one time general among the Hexapoda, and that the resting nymph of the Thysanoptera or the pupa of the Endopterygota represents a formerly active stage in the life-history. Further, although the wing-rudiments appear externally in an early instar of an exopterygotous insect, the earliest instars are wingless and wing-rudiments have been previously developing beneath the cuticle, growing however outwards, not inwards as in the larva of an endopterygote. The change from an exopterygote to an endopterygote development could, therefore, be brought about by the gradual postponement to a later and later instar of the appearance of the wing-rudiments outside the body, and their correlated growth inwards as imaginal disks. For in the post-embryonic development of the ancestors of the Endopterygota we may imagine two or three instars with wing-rudiments to have existed, the last represented by the sub-imago of the may-flies. As the life-conditions and feeding-habits of the larva and imago become constantly more divergent, the appearance of the wing-rudiments would be postponed to the pre-imaginal instar, and that instar would become pre-dominantly passive. Relationships of the Orders.—Reasons have been given for regarding the Thysanura as representing, more nearly than any other living group, the primitive stock of the Hexapoda. It is believed that insects of this group are represented among Silurian fossils. We may conclude, therefore, that they were pre-ceded, in Cambrian times or earlier, by Arthropods possessing well developed appendages on all the trunk-segments. Of such Arthropods the living Symphyla—of which the delicate little Scutigerella is a fairly well-known example—give us some representation. No indications beyond those furnished by comparative anatomy help us to unravel the phylogeny of the Collembola. In most respects, the shortened abdomen, for example, they are more specialized than the Thysanura, and most of the features in which they appear to be simple, such as the absence of a tracheal system and of compound eyes, can be explained as the result of degradation. In their insunken mouth and their jaws retracted within the head-capsule, the Collembola resemble the entotrophous division of the Thysanura (see APTERA), from which they are probably descended. From the thysanuroid stock of the Apterygota, the Exopterygota took their rise. We have undoubted fossil evidence that winged insects lived in the Devonian and became numerous in the Carboniferous period. These ancient Exopterygota were synthetic in type, and included insects that may, with probability, be regarded as ancestral to most of the existing orders. It is hard to arrange the Exopterygota in a linear series, for some of the orders that are remarkably primitive in some respects are rather highly specialized in others. As regards wing-structure, the Isoptera with the two pairs closelysimilar are the most primitive of all winged insects; while in the paired mesodermal genital ducts, the elongate cerci and the conspicuous maxillulae of their larvae the Ephemeroptera retain notable ancestral characters. But the vestigial jaws, numerous Malpighian tubes, and specialized wings of may-flies forbid us to consider the order as on the whole primitive. So the Dermaptera, which retain distinct maxillulae and have no ectodermal genital ducts, have either specialized or aborted wings and a large number of Malpighian tubes. The Corrodentia retain vestigial maxillulae and two pairs of Malpighian tubes, but the wings are somewhat specialized in the Copeognatha and absent in the degraded and parasitic Mallophaga. The Plecoptera and Orthoptera agree in their numerous Malpighian tubes and in the development of a folding anal area in the hind-wing. .As shown by the number and variety of species, the Orthoptera are the most dominant order of this group. Eminently terrestrial in habit, the differentiation of their fore-wings and hind-wings can be traced from Carboniferous, isopteroid ancestors through intermediate Mesozoic forms. The Plecoptera resemble the Ephemeroptera and Odonata in the aquatic habits of their larvae, and by the occasional presence of tufted thoracic gills in the imago exhibit an aquatic character unknown in any other winged insects. The Odonata are in many imaginal and larval characters highly specialized; yet they probably arose with the Ephemeroptera as a divergent offshoot of the same primitive isopteroid stock which developed more directly into the living Isoptera, Plecoptera, Dermaptera and Orthoptera. All these orders agree in the possession of biting mandibles, while their second maxillae have the inner and outer lobes usually distinct. The Hemiptera, with their piercing mandibles and first maxillae and with their second maxillae fused to form a jointed beak, stand far apart from them. This order can be traced with certainty back to the early Jurassic epoch, while the Permian fossil Eugereon , and the living order—specially modified in many respects—of the Thysanoptera indicate steps by which the aberrant suctorial and piercing mouth of the Hemiptera may have been developed from the biting mouth of primitive Isopteroids, by the elongation of some parts and the suppression of others. The Anoplura may probably be regarded as a degraded offshoot of the Hemiptera. The importance of great cardinal features of the life-history as indicative of relationship leads us to consider the Endopterygota as a natural assemblage of orders. The occurrence of weevils—among the most specialized of the Coleoptera—in Triassic rocks shows us that this great order of metabolous insects had become differentiated into its leading families at the dawn of the Mesozoic era, and that we must go far back into the Palaeozoic for the origin of the Endopterygota. In this view we are confirmed by the impossibility of deriving the Endopterygota from any living order of Exopterygota. We conclude, therefore, that the primitive stock of the former sub-class became early differentiated from that of the latter. So widely have most of the higher orders of the Hexapoda now diverged from each other, that it is exceedingly difficult in most cases to trace their relationships with any confidence. The Neuroptera, with their similar fore- and hind-wings and their campodeiform larvae, seem to stand nearest to the presumed isopteroid ancestry, but the imago and larva are often specialized. The campodeiform larvae of many Coleoptera are indeed far more primitive than the neuropteran larvae, and suggest to us that the Coleoptera—modified as their wing-structure has become—arose very early from the primitive metabolous stock. The antiquity of the Coleoptera is further shown by the great diversity of larval form and habit that has arisen in the order, and the proof afforded by the hypermetamorphic beetles that the campodeiform preceded the eruciform larva has already been emphasized. In all the remaining orders of the Endopterygota the larva is eruciform or vermiform. The Mecaptera, with their pre-dominantly longitudinal wing-nervuration, serve as a link between the Neuroptera and the Trichoptera, their retention of small cerci being an archaic character which stamps them as synthetic in type, but does not necessarily remove them from orders which agree with them in most points of structure but which have lost the cerci. The standing of the Trichoptera in a position almost ancestral to the Lepidoptera is one of the assured results of recent morphological study, the mobile mandibulate pupa and the imperfectly auctorial maxillae of the Trichoptera reappearing in the lowest families of the Lepidoptera. This latter order, which is not certainly known to have existed before Tertiary times, has become the most highly specialized of all insects in the structure of the pupa. Diptera of the sub-order Orthorrhapha occur in the Lias and Cyclorrhapha in the Kimmeridgian. The order must therefore be ancient, and as no evidence is forthcoming as to the mode of reduction of the hind-wings, nor as to the stages by which the suctorial mouth-organs became specialized, it is difficult to trace the exact relationship of the group, but the presence of cerci and a degree of correspondence in the nervuration of the fore-wings suggest the Mecaptera as possible allies. There seems no doubt that the auctorial mouth-organs of the Diptera have arisen quite independently from those of the Lepidoptera, for in the former order the sucker is formed from the second maxillae, in the latter from the first. The eruciform larva of the Orthorrhapha leads on to the headless vermiform maggot of the Cyclorrhapha, and in the latter sub-order we find meta-morphosis carried to its extreme point, the muscid flies being the most highly specialized of all the Hexapoda as regards structure, while their maggots are the most degraded of all insect larvae. The Siphonaptera appear by the form of the larva and the nature of the metamorphosis to be akin to the Orthorrhapha—in which division they have indeed been included by many students. They differ from the Diptera, however, in the general presence of palps to both pairs of maxillae, and in the absence of a hypopharynx, so it is possible that their relationship to the Diptera is less close than has been supposed. The affinities of the Hymenoptera afford another problem of much difficulty. They differ from other Endopterygota in the multiplication of their Malpighian tubes, and from all other Hexapoda in the union of the first abdominal segment with the thorax. Specialized as they are in form, development and habit, they retain mandibles for biting, and in their lower sub-order—the Symphyta—the maxillae are hardly more modified than those of the Orthoptera. From the evidence of fossils it seems that the higher sub-order—Apocrita—can be traced back to the Lias, so that we believe the Hymenoptera to be more ancient than the Diptera, and far more ancient than the Lepidoptera. They afford an example—paralleled in other classes of the animal kingdom—of an order which, though specialized in some respects, retains many primitive characters, and has won its way to dominance rather by perfection of behaviour, and specially by the development of family life and helpful socialism, than by excessive elaboration of structure. We would trace the Hymenoptera back therefore to the primitive endopterygote stock. The specialization of form in the constricted abdomen and in the suctorial " tongue " that characterizes the higher families of the order is correlated with the habit of careful egg-laying and provision of food for the young. In some way it is assured among the highest of the Hexapoda—the Lepidoptera, Diptera and Hymenoptera—that the larva finds itself amid a rich food-supply. And thus perfection of structure and instinct in the imago has been accompanied by degradation in the larva, and by an increase in the extent of transformation and in the degree of reconstruction before and during the pupal stage. The fascinating difficulties presented to the student by the metamorphosis of the Hexapoda are to some extent explained, as he ponders over the evolution of the class. tIIBLIOGRAPHY.—References to the older classical writings on the Hexapoda are given in the article on Entomology. At present about a thousand works and papers are published annually, and in this place it is possible to enumerate only a few of the most important among (mostly) recent memoirs that bear upon the Hexapoda generally. Further references will be found appended to the special articles on the orders (APTERA, COLEOPTERA, &e). General Works.—A. S. Packard, Text-book of Entomology (London, 1898) ; V. Graber, Die Insekten (Munich, 1877—1879) ; D. Sharp, Cambridge Natural History, vols. v., vi. (London, 1895–1899) ; L. C. Miall and A. Denny, Structure and Life-history of the Cockroach (London, 1886) ; B. T. Lowne, The Anatomy, Physiology, Morphology and Development of the Blow fly (2 vols., London, 1890-1895); G. H. Carpenter, Insects: their Structure and Life (London, 1899) ; L. F. Henneguy, Les Insectes (Paris, 1904) ; J. W. Folsom, Entomology (New York and London, 1906) ; A. Berlese, Gli Insetti (Milan, 1906), &c. (Extensive bibliographies will be found in several of the above.) Head and Appendages.—J. C. Savigny, Memoires sur les animaux sans vertebras (Paris, 1816) ; C. Janet, Essai sur la constitution morphologique de la teete de l'insecte (Paris, 1899) ; J. H. Comstock and C. Kochi (American Naturalist, xxxvi., 1902) ; V. L. Kellogg (ibid.) ; W. A. Riley (American Naturalist, xxxviii., 1904) ; F. Meinert (Entom. Tidsskr. i., t88o) ; H. J. Hansen (Zoo'. Anz. xvi., 1893) ; J. B. Smith (Trans. Amer. Phil. Soc. xix., 1896) ; H. Holmgren (Zeitsch. wiss. Zoolog. lxxvi., 1904) ; K. W. Verhoeff (Abhandl. K. Leop.-Carol. Akad. lxxxiv., 1905). Thorax, Legs and Wings.—K. W. Verhoeff (Abhandl. K. Leo.-Carol. Akad, xxxxii., 1903) ; F. Voss (Zeits. wins. Zool. lxxviu., 1905) ; F. Dahl (Arch. f. Naturgesch. 1, 1884) ; J. Demoor (Arch. de biol. x., 189o) ; J. Redtenbacher (Ann. Kais. naturhist. Museum, Wien, i., I886); R. von Lendenfeld (S. B. Akad. Wissens., Wien, Ixxxiii., 1881); J. H. Comstock and J. G. Needham (Amer. Nat., xxxii., xxxiii., 1898–1899); C. W. Woodworth (Univ. California Entom. Bull. i., 1906). Abdomen and Appendages.-E. Haase (Morph. Jahrb. xv., 1889); R. Heymons (Morph. Jahrb. xxiv.. 1896; Abhandl. K. Leop.-Carol. Akad. lxxiv., 1899) ; K. W. Verhoeff (Zoo'. Anz. xix., xx., 1896–1897) ; S. A. Peytoureau, Contribution a ''etude de la morphologie de l'armure genitale des insectes (Bordeaux, 1895) ; H. Dewitz (Zeits. wiss. Zool. xxv., xxviii., 1874, 1877) ; E. Zander (ibid. lxvi., lxvii., 1899–1900). Nervous System.—H. Viallanes (Ann. Sci. Nat. Zool. [6], xvii., xviii., xix., [7] ii., iv., 1884–1887) ; S. J. Hickson (Quart. Journ. Mier. Sci. xxv., 1885); W. Patten (Journ. Morph. i., ii., 1887–1888) ; F. Plateau (Mem. Acad. Belg. xliii., 1888) ; V. Graber (Arch. mike. Anat. xx., xxi., 1882). Respiratory System.—J. A. Palmen, Zur Morphologie des Tracheensystems (Leipzig, 1877) ; F. Plateau (Mem. Acad. Belg. xiv., 1884) ; L. C. Miall, Natural History of Aquatic Insects (London, 1895). Digestive System, &c.—L. Dufour (Ann. Sci. Nat., 1824–186o) ; V. Faussek ('Lei's. wiss. Zoo/. xlv., 1887). Malpighian Tubes,—E. Schindler (Zeits. wiss. Zool. xxx., 1878) ; W. M. Wheeler (Psyche vi., 1893) ; L. Cuenot (Arch. de biol. xiv., 1895). Reproductive Organs.—H. V. Wielowiejski (Zoo'. Anz. ix., 1886) ; J. A. Palmeri, Uber paarige Ausfiihrungsgange der Geschlechtsorgane (iei Insekten (Helsingfors, 1884) ; H. Henking (Zeits. wiss. Zool. xlix., liv., 189o–189z); F. Leydig (Zool. Jahrb. Anat. iii., 1889). Embryology.—F. Blochmann (Morph. Jahrb. xii., 1887) ; A. Kovalevsky (Mem. Acad. St-Petersbourg, xvi., 1871; Zeits. wiss. Zool. xlv., 1887) ; V. Graber (Denksch. Akad. Wissens., Wien, Ivi., 1889) ; K. Heider, Die Embryonalentwicklung von Ilydrophilus piceus (Jena, 1889) ; W. M. Wheeler (Journ. Morph. iii., 1889--1893); E. Korschelt and K. Heider, Handbook of the Comparative Embryology of Invertebrates (trans. M. Bernard), (vol. iii., London, 1899) ; R. Heymons, Die Embryonalentwicklung von Dermapteren and Orthopteren (Jena, 1895) (also Zeits. wiss. Zook 1891, lxii., 1897; Anhang zu den Abhandl. K. Akad. d. Wissens., Berlin, 1896); A. Lecaillon (Arch. d'anat. mice. ii., 1898) ; J. Carricbre and O. Burger (Abhandl. K. Leop.-Carol. Akad. lxix., 1897) ; K. Escherich (ibid. lxxvii., 1901) ; F. Schwangart (Zeits. wiss. Zool. lxxvi., 0.04); R. Ritter (ib. 189o) ; E. Metchnikoff (ib. xvi., 1866) ; Uzel (Zool. Anz. xx., 1897) ; J. W. Folsom (Bull. Illus. Comp. Zool. Harvard., xxxvi., 1900). Parthenogenesis and Paedogenesis.—T. H. Huxley (Trans. Linn. Soc. xxii., 1858) ; R. Leuckart, Zur Kenntnis des Generationswechsels and der Parthogenesis bei den Insekten (Frankfurt, 1858); N. Wagner (Zeits. wiss. Zook xv., 1865) ; L. F. Henneguy (Bull. Soc. Philomath. [91, i. 1899) ; A. Petrunkevich (Zoo'. Jahrb. Anat. xiv., xvii., 1901–1903) ; P. Marchal (Arch. zoo'. exp. et gen. [4], ii., 1904) ; L. Doncaster (Quart. Journ. Micr. Sci. xlix., li., 1906–1907). Growth and Metamorphosis.—A. Weismann (Zeits. wiss. Zool. xiv., 1863–1864); F. Brauer (Verb. zool.-bot. Gesellsch., Wien, xix., 1869) ; Sir J. Lubbock (Lord Avebury), Origin and Metamorphosis of Insects (London, 1874) ; L. C. Miall (Nature, 1895) ; L. C. Miall and A. R. Hammond, Structure and Life-history of the Harlequin-fly (Oxford, 1900) ; J. Gonin (Bull. Soc. Vaud. Sci. Nat. xxx., 1894) ; C. de Bruyne (Arch. de biol. xv.( 1898); D. Sharp (Proc. Inter. Zool. Congress, 1898) ; E. B. Poulton (Trans. Linn. Soc. v., 1891) ; T. A. Chapman (Trans. Ent. Soc., 1893). Classification.—F. Brauer (S. B. Akad. Wiss., Wien, xci., 1885) ; A. S. Packard (Amer. Nat. xx.; 1886); C. BOrner, A. Handlirsch, F. Klapalek (Zool. Anz. xxvii., 1904) ; G. Enderlein (Zoo'. Anz. xxvi., 1903). Palaeontology.—S. H. Scudder, in Zittel's Palaeontology (French trans., vol. ii., Paris, 1887, and Eng. trans., vol. i., London, 1900) ; C. Brongniart, Insectes fossiles des temps primaires (St-Etienne, 1894) ; A. Handlirsch, Die fossilen Insekten and die Phylogenie der rezenten Formen (Leipzig, 1906). Phylogeny.—Brauer, Lubbock, Sharp, Burner, &c. (opp. cit.); P. Mayer (Jena, Zeits. Naturw. x., 1876) ; B. Grassi (Atti R. Accad. dei Lincei, Roma [41, iv., 1888, and Archiv ital. biol. xi., 1889); F. Muller, Facts and Arguments for Darwin (trans. W. S. Dallas, London, 1869) ; N. Zograf (Congr. Zool. Int., 1892) ; E. R. Lankester (Quart. Journ. Micr. Sci. xlvii., 1904) ; G. H. Carpenter (Proc. R. Irish Acad. xxiv., 1903; Quart. Journ. Micr. Sci. xlix., 1905). (D.S.*; G.H.0 )

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