ANATOMY AND PHYSIOLOGY 



73 



In many insects the tergum of the last abdominal segment" 

 forms a small suranal plate (Fig. 87, B, sp) ; this sometimes 



FIG. 87. 



Extremity of the abdomen of a grasshopper, Melanoplus differentialis. A, male; 

 B, female. The terga and sterna are numbered, c, cercus; d, dorsal valves of ovi- 

 positor; e, egg guide; p, podical plate; s, spiracle; sp, suranal plate; v, ventral valves 

 of ovipositor. 



supplements the claspers of the male in their function, as in 

 Lepidoptera (Fig. 85, A, s). 



2. INTEGUMENT 



Insects excel all other animals in respect to adaptive modi- 

 fications of the integument. No longer a simple limiting 

 membrane, the integument has become hardened into an exter- 

 nal skeleton, evaginatecl to form manifold adaptive structures 

 such as hairs and scales, and invaginated, along with the un- 

 derlying cellular layer, to make glands of various kinds. 



Chitin. The skin, or cuticula^ of an insect differs from 

 that of a worm, for example, in being thoroughly permeated 

 with a peculiar substance known as chitin the basis of the 

 arthropod skeleton. This is a substance of remarkable sta- 

 bility, for it is unaffected by almost all ordinary acids and 

 alkalies, though it is soluble in soclic or potassic hypochlorite 

 (respectively, Eau de Labarraque and Eau de Javelle) and 

 yields to boiling sulphuric acid. If kept for a year or so 

 under water, however, chitin undergoes a slow dissolution, 



1 The cuticula of an insect should be distinguished from the cuticle of 

 a vertebrate, the former being a hardened fluid, \^hile the latter consists 

 of cells themselves, in a dead and flattened condition. 



