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. 



g 



FIG. 83. Extremity of abdomen of a FIG. 84. Extremity of abdomen of a 



male beetle, Hydro phi I us, ventral aspect, g, male May fly, Hcxagciiut I'ariabilis, ventral 



genitalia; p, penis; v 1 , v' 2 , pairs of valves aspect, c, c', c, cerci; d, d, claspers; i, i, 



enclosing the penis; 6-g, sterna of abdom- intromittent organs, 

 inal segments. After KOLBE. 



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 



v 



B 



FIG. 85. Genitalia of a moth, Saw /a cccropia. A, male; B, female; a, anus; c, c, claspers; 

 o, opening of common oviduct; p, penis; s, uncus (the doubly hooked organ); i>, 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 remarkable stability, for it is unaffected by almost all ordi- 

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



x The ciiliciila 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. 



