seem as appropriate now as the original balance was when Dr A. D. Imms' textbook was first published over fifty years ago. There are 35 new figures, all based on published illustrations, the sources of which are acknowledged in the captions. We are grateful to the authors concerned and also to Miss K. Priest of Messrs Chapman & Hall, who saved us from many errors and omissions, and to Mrs R. G. Davies for substantial help in preparing the bibliographies and checking references. London O. W. R. R. G. D. May 1976 Part III THEORDERSOFINSECTS THE CLASSIFICATION AND PHYLOGENY OF INSECTS The classification of insects has passed through many changes and with the growth of detailed knowledge an increasing number of orders has come to be recognized. Handlirsch (1908) and Wilson and Doner (1937) have reviewed the earlier attempts at classification, among which the schemes of Brauer (1885), Sharp (1899) and Borner (1904) did much to define the more distinctive recent orders. In 1908 Handlirsch published a more revolutionary system, incorporating recent and fossil forms, which gave the Collembola, Thysanura and Diplura the status of three independent Arthropodan classes and considered as separate orders such groups as the Sialoidea, Raphidioidea, Heteroptera and Homoptera. He also split up the old order Orthoptera, gave its components ordinal rank and regrouped them with some of the other orders into a subclass Orthopteroidea and another subclass Blattaeformia.

The present edition may be regarded as a descendant, much changed and greatly enlarged, of the late Dr A. D. Imms' Outlines of Entomology, first published in 1942. This went through three further editions without much change, but after the death of the original author a fifth, revised edition by Professor O. W. Richards and myself appeared in 1959 and a sixth in 1978. The book now appears in a considerably extended version in which I have tried to provide a more balanced introduction to the whole field of modern entomology by dealing with several aspects of the subject not discussed at all in previous editions. Thus, in addition to innumerable lesser changes in the sections on insect structure, function, development, classification and phylogeny, I have completely recast the earlier chapter on some important modes of life in insects. This now includes a far wider range of biological topics well exemplified by the insects and should, I hope, appeal not only to those already dedicated to entomology but also to others with more general biological interests. A completely new chapter on the biology of insect populations has also been added and may serve to indicate the debt which modern ecological theory owes to work on insect populations. It should hardly be necessary to apologize for introducing a certain amount of elementary mathematics into this account of a subject which is now among the most highly quantitative of biological disciplines.

seem as appropriate now as the original balance was when Dr A. D. Imms' textbook was first published over fifty years ago. There are 35 new figures, all based on published illustrations, the sources of which are acknowledged in the captions. We are grateful to the authors concerned and also to Miss K. Priest of Messrs Chapman & Hall, who saved us from many errors and omissions, and to Mrs R. G. Davies for substantial help in preparing the bibliographies and checking references. London O.W.R. May 1976 R.G.D. Part I ANATOMY AND PHYSIOLOGY Chapter I INTRODUCTION Definition of the Insecta (Hexapoda) The insects are tracheate arthropods in which the body is divided into head, thorax and abdomen. A single pair of antennae (homologous with the anten nules of the Crustacea) is present and the head also bears a pair of mandibles and two pairs of maxillae, the second pair fused medially to form the labium. The thorax carries three pairs of legs and usually one or two pairs of wings. The abdomen is devoid of ambulatory appendages, and the genital opening is situated near the posterior end of the body. Postembryonic development is rarely direct and a metamorphosis usually occurs."

seem as appropriate now as the original balance was when Dr A. D. Imms' textbook was first published over fifty years ago. There are 35 new figures, all based on published illustrations, the sources of which are acknowledged in the captions. We are grateful to the authors concerned and also to Miss K. Priest of Messrs Chapman & Hall, who saved us from many errors and omissions, and to Mrs R. G. Davies for substantial help in preparing the bibliographies and checking references. O. W. R. London R. G. D. May 1976 Part III THEORDERSOFINSECTS THE CLASSIFICATION AND PHYLOGENY OFINSECTS The classification of insects has passed through many changes and with the growth of detailed knowledge an increasing number of orders has come to be recognized. Handlirsch (1908) and Wilson and Doner (1937) have reviewed the earlier attempts at classification, among which the schemes of Brauer (1885), Sharp (1899) and Borner (1904) did much to define the more distinctive recent orders. In 1908 Handlirsch published a more revolutionary system, incorporating recent and fossil forms, which gave the Collembola, Thysanura and Diplura the status of three independent Arthropodan classes and considered as separate orders such groups as the Sialoidea, Raphidioidea, Heteroptera and Homoptera. He also split up the old order Orthoptera, gave its components ordinal rank and regrouped them with some of the other orders into a subclass Orthopteroidea and another subclass Blattaeformia.