ANATMY AM) 07 



lorm of the integument, though less than half tin- weight .t t tie integu- 

 ment is due to ehitin. The !".niuihi of ehitin is y_. CfHuNOe or 

 1 ),... l>\ Kriikenheru'. and many adopt the formula CuH^e^OioJ 

 though no two rlirmi.Ms agree aa to tl: proportion, of 

 elements. Owing probably to variations in the BUbstanCC it -elf in dit'fer- 



8g 10 11 



9 10 



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

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

 positor; e, egg guide; p, podical plate; 5, spiracle; sp, suranal plate; v, ventral valves of 



.lor. 



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 peculiar to arthropods, for it 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 p i c 

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

 erties. Each layer arises as a fluid secretion 



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



j ondary cuticula; h, hypo- 



being the first to form and harden. dermis ceil; 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 



