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.

In his preface to early editions of this book, the late Dr. A. D. Imms said that he intended it to be an elementary account of entomology as a branch of general biology. He had especially in mind the needs of university students of zoology and agriculture, as well as those intending later to specialize in entomology, and he suggested that the book might also interest teachers of advanced biology'in schools. These general aims and the balance between the different aspects of the subject have changed little in this and in our previous revision. We have, however, tried to bring the present edition up to date on the lines of our revised tenth edition of Imms' General Textbook of Entomology, published in 1977. The text has been entirely re-set and eleven illustrations have been replaced by new figures. The same orders . of insects are recognized as in the last edition, but the sequence in which the Endopterygote groups appear has been changed to reflect more accurately their probable evolutionary relationships. Many small changes and some addi tions have been made in the physiological sections, the chapter on the origin and phylogeny of insects has been rewritten, and a new bibliography provides a selection of modern references for the in tending specialist. It has been our object to make these alterations without materially increasing the length of the book or its level of difficulty."

In his preface to early editions of this book, the late Dr. A. D. Imms said that he intended it to be an elementary account of entomology as a branch of general biology. He had especiaHy in mind the needs of university students of zoology and agriculture, as weH as those intending later to specialize in entomology, and he suggested that the book might also interest teachers of advanced biology in schools. These general aims and the balance between the different aspects of the subject have changed linIe in this and in our previous revision. We have, however, tried to bring the present edition up to date on the lines of our revised tenth edition of Imms' General Textbook 0/ Entomology, published in 1977. The text has been entirely re-set and eleven illustrations have been replaced by new figures. The same orders of insects are recognized as in the last edition, but the sequence in which the Endopterygote groups appear has been changed to reflect more accurately their probable evolutionary relationships. Many small changes and some addi tions have been made in the physiological sections, the chapter on the origin and phylogeny of insects has been rewritten, and a new bibliography provides a selection of modern references for the in tending specialist. It has been our object to make these alterations without materially increasing the length of the book or its level of difficulty."

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.