ANATOMY AND PHYSIOLOGY 



57 



has become hardened into an external skeleton, evaginated to form mani- 

 fold adaptive structures, such as hairs and scales, and invaginated, along 

 with the underlying cellular layer, to make glands of various kinds. 



s i 



FIG. 83. Extremity of abdomen of a 

 male beetle, Hydro philus, ventral aspect, g, 

 genitalia; p, penis; v l , v 2 , pairs of valves 

 enclosing the penis; 6-p, sterna of abdom- 

 inal segments. After KOLBE. 



FIG. 84. Extremity of abdomen of a 

 male May fly, Hexagenia variabilis, ventral 

 aspect, c, c, c, cerci; cl, d, claspers; i, i, 

 intromittent organs. 



Chitin. The skin, or cuticula, 1 of an insect differs from that of a 

 worm, for example, in being thoroughly permeated with a peculiar sub- 



8 



'P 



"o 



B 



FIG. 85. Genitalia of a moth, Samia cecropia. A , male; B, female; a, anus; c, c, claspers; 

 o, opening of common oviduct; p, penis; s, uncus (the doubly hooked organ); v, vestibule, 

 into which the vagina opens. The numbers refer to abdominal segments. 



stance known as chitin the basis of the arthropod skeleton. This is a 

 substance of femarkable stability, for it is unaffected by almost all ordi- 

 nary acids and alkalies, though it is soluble in sodic or potassic hypo- 



1 The 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. 



