ADAPTATIONS 145 
have their hind legs greatly enlarged and elon- 
gated, and provided with strong muscles, so as 
to make of them “leaping legs.” The grubs 
Fic. 82.—Head of rainbow trout (Salmo irideus) with gill cover bent back to show 
gills, the breathing organs. 
or larve of beetles which live as “borers” in tree-trunks 
have mere rudiments of legs, or none at all (Fig. 86). 
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 
breathe or take down 
air underneath their 
wings, or in some 
other way, or have 
gills for breathing the 
air which is mixed 
with the water. These Fig. 838.—Tree-toad (Hyla regilia). 
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 
11 
