310 



ANIMAL STUDIES 



have their hind legs greatly enlarged and elon- 

 gated, and provided with strong muscles, so as 

 to make of them "leaping legs." The grubs 



PIG. 182. Head of rainbow trout (Salmo irideus) with gill cover bent back to show 

 gills, the breathing organs. 



or larvae of beetles which live as "borers" in tree-trunks 

 have mere rudiments of legs, or none at all (Fig. 186). 

 They have great, strong, biting jaws for cutting away 

 the hard wood. They move simply by wriggling along 

 in their burrows or tunnels. 

 Insects that live 



in water either come 



up to the surface to -3 

 breathe or take down j 

 air underneath their g 

 wings, or in some *? 

 other way, or have 

 gills for breathing the 

 air which is mixed 

 with the water. These 

 gills are special adap- 

 tive structures which present a great variety of form and 

 appearance. In the young of the May-flies they are deli- 

 cate plate-like flaps projecting from the sides of the body. 

 They are kept in constant motion, gently waving back and 



FIG. 183. Tree-toad (Hyla regilla). 



