ELEMENTARY LESSONS ON INSECTS 



The hardening of the skin is due to 

 the formation in it of a horny sub- 

 stance called chitin. This chitinous 

 hard layer, forming an outer skele- 

 ton, segmented on body and ap- 

 pendages, is characteristic also of 

 crustaceans, spiders and myriapods. 

 These together with insects comprise 

 that great division of the animal 

 kingdom known as the Arthropoda.* 



When we look at the stoneflymore 

 closely we observe that its body 

 is divided into three principal parts, 

 head, thorax and abdomen. 



Fig. 1.— Stoneflies: an immature specimen (a nymph) in the water and a cast skin on the large 

 stone, from which the adult stonefly shown above it has issued. 



* From (arthros joint, podos=foot; joint-footed animals). ' 



