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RELATIONSHIPS] 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 suctorial 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 suctorial 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 metamorphosis 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.

—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 (,, &c.).

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, Mémoires sur les animaux sans vertèbres (Paris, 1816); C. Janet, Essai sur la constitution morphologique de la tête 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., 1880); H. J. Hansen (Zool. 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. Leop.-Carol. Akad. lxxxii., 1903); F. Voss (Zeits. wiss. Zool. lxxviii., 1905); F. Dahl (Arch. f. Naturgesch. 1, 1884); J. Demoor (Arch. de biol. x., 1890); J. Redtenbacher (Ann. Kais. naturhist. Museum, Wien, i., 1886); R. von Lendenfeld (S. B. Akad. Wissens., Wien, lxxxiii., 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 (Zool. Anz. xix., xx., 1896–1897); S. A. Peytoureau, Contribution à l’étude de la morphologie de l’armure génitale 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. Micr. Sci. xxv., 1885); W. Patten (Journ. Morph. i., ii., 1887–1888); F. Plateau (Mém. Acad. Belg. xliii., 1888); V. Graber (Arch. mikr. Anat. xx., xxi., 1882).

Respiratory System.—J. A. Palmén, Zur Morphologie des Tracheensystems (Leipzig, 1877); F. Plateau (Mém. Acad. Belg. xiv., 1884); L. C. Miall, Natural History of Aquatic Insects (London, 1895).

Digestive System, &c.—L. Dufour (Ann. Sci. Nat., 1824–1860); V. Faussek (Zeits. wiss. Zool. xlv., 1887).

Malpighian Tubes.—E. Schindler (Zeits. wiss. Zool. xxx., 1878); W. M. Wheeler (Psyche vi., 1893); L. Cuénot (Arch. de biol. xiv., 1895).

Reproductive Organs.—H. V. Wielowiejski (Zool. Anz. ix., 1886); J. A. Palmén, Über paarige Ausführungsgänge der Geschlechtsorgane bei Insekten (Helsingfors, 1884); H. Henking (Zeits. wiss. Zool. xlix., li., liv., 1890–1892); F. Leydig (Zool. Jahrb. Anat. iii., 1889).

Embryology.—F. Blochmann (Morph. Jahrb. xii., 1887); A. Kovalevsky (Mém. Acad. St-Pétersbourg, xvi., 1871; Zeits. wiss. Zool. xlv., 1887); V. Graber (Denksch. Akad. Wissens., Wien, lvi., 1889); K. Heider, Die Embryonalentwicklung von Hydrophilus piceus (Jena, 1889); W. M. Wheeler (Journ. Morph. iii., viii., 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 und Orthopteren (Jena, 1895) (also Zeits. wiss. Zool. liii., 1891, lxii., 1897; Anhang zu den Abhandl. K. Akad. d. Wissens., Berlin, 1896); A. Lécaillon (Arch. d’anat. micr. ii., 1898); J. Carrière and O. Burger (Abhandl. K. Leop.-Carol. Akad. lxix., 1897); K. Escherich (ibid. lxxvii., 1901); F. Schwangart (Zeits. wiss. Zool. lxxvi., 1904); R. Ritter (ib. li., 1890); E. Metchnikoff (ib. xvi., 1866); H. Uzel (Zool. Anz. xx., 1897); J. W. Folsom (Bull. Mus. Comp. Zool. Harvard., xxxvi., 1900).

Parthenogenesis and Paedogenesis.—T. H. Huxley (Trans. Linn. Soc. xxii., 1858); R. Leuckart, Zur Kenntnis des Generationswechsels und der Parthogenesis bei den Insekten (Frankfurt, 1858); N. Wagner (Zeits. wiss. Zool. xv., 1865); L. F. Henneguy (Bull. Soc. Philomath. [9], i. 1899); A. Petrunkevich (Zool. Jahrb. Anat. xiv., xvii., 1901–1903); P. Marchal (Arch. zool. exp. et gén. [4], ii., 1904); L. Doncaster (Quart. Journ. Micr. Sci. xlix., li., 1906–1907).

Growth and Metamorphosis.—A. Weismann (Zeits. wiss. Zool. xiii., xiv., 1863–1864); F. Brauer (Verh. zool.-bot. Gesellsch., Wien, xix., 1869); Sir J. Lubbock (Lord Avebury), Origin and Metamorphosis of Insects (London, 1874); L. C. Miall (Nature, liii., 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. Börner, A. Handlirsch, F. Klapalek (Zool. Anz. xxvii., 1904); G. Enderlein (Zool. Anz. xxvi., 1903).

Palaeontology.—S. H. Scudder, in Zittel’s Palaeontology (French