ANATOMY AND PHYSIOLOGY 



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form of the integument, though less than half the weight of the integu- 

 ment is due to chitin. The formula of chitin is given as C 9 Hi5NOe or 

 Ci 8 Hi 5 NOi2 by Krukenberg, and many adopt the formula C^H^e^Oio; 

 though no two chemists agree as to the exact proportions of these 

 elements, owing probably to variations in the substance itself in differ- 



?9 ! 



.."P 



FIG. 89. Extremity of the abdomen of a grasshopper, Melanoplus differ entialis. 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. 



ent insects or even in the same species of insect. Iron, manganese and 

 certain pigments also enter into the composition of the integument. 



Chitin is not peculiaj' to jrthrorjods, forjt has been detected in the 

 setae and pharyngeal teeth of annelid worms, the shell of Lingula and the 

 pen of the cuttle fish (Krukenberg). 



The chitinous integument (Fig. 90) of most insects consists of two 

 layers: (i) an outer layer, homogeneous, dense, 

 without lamellae or pore canals, and being the 

 seat of the cuticular colors; (2) an inner layer, 

 " thickly pierced with pore canals, and always 

 in layers of different refractive indices and differ- 

 ent stainability." (Tower.) These two layers, 

 respectively primary and secondary cuticula, are pic section 

 radically different in chemical and physical prop- through integument of a 



,. T-II n *j .L- beetle, Chrysobothris. b r 



erties. Each layer arises as a fluid secretion baseme nt membrane; c\ 



from the hypodermis cells, the primary cuticula primary cuticula; c 2 , sec- 

 ondary cuticula; h, hypo- 



being the first to form and harden. dermis cell; n, nucleus. 



The fluid that separates the old from the new After ToWER " 

 cuticula at ecdysis is poured over the hypodermis by certain large special 

 cells, which, according to Tower, "are not true glands, but the setiger- 

 ous cells which, in early life, are chiefly concerned with the formation 

 of the hairs upon the body; but upon the loss of these, the cell takes 

 on the function of secreting the exuvial fluid, which is most copious at 



