ANATOMY AND PHYSIOLOGY TS 
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, evaginated 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 sodic 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, 
1The cuticula of an insect should be distinguished from the cuticle of 
a vertebrate, the former being a hardened fluid, while the latter consists 
of cells themselves, in a dead and flattened condition. 
