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