years ago extensive advances have been made in all parts of the subject. Full-sized textbooks have been devoted to it; notably The Principles of Insect Physiology by the present author, the three volume Physiology of the Insecta edited by Morris Rockstein, and Insect Biochemistry by Darcy Gilmour; and articles describing the most recent advances in the physiology and biochemistry of insects appear in the Annual Review of Entomology, in Advances in Insect Physiology and elsewhere. References in this edition have therefore been confined to such textbooks and reviews, to a few recent papers which have not yet become incorporated in this way, and to a limited number of other papers which provide useful starting-points for further reading. I The Integument The key to much of the physiology of insects is to be found in the nature of their cuticle. As was first shown by Haecke1, the cuticle is the product of a single layer of epidermal cells. It is often described as being composed of non-living material; but in fact the epidermal cells give off fine filaments contained within the so-called ''pore canals'', which run through the substance of the cuticle and often come within less than a micron of the surface. Cuticle Structure As described from stained sections the cuticle consists of two primary layers, the endocuticle which makes up the greater part, and a thin refractile epicuticle on the surface, usually not more than one micron in thickness.
