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. BIBLIOGRAPHY.--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, &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.)