276 Adjustment to Conditions of Aquatic Life 



the raembers of a few of the smaller orders live and grow 

 up in the water. These facts have been noted, group 

 by group, in Chapter IV. Here we may explain that 

 the reason for this probably lies in the greater plasticity 

 of the immature stages. All are thin-skinned on hatch- 

 ing from the egg, and a supply of oxygen may be taken 

 from the water by direct absorption thro the general 

 surface of the body. With growth gills develop; but 

 these have no relation to the structure or life of the 

 adult and are lost at the final transformation. 



Fig. 169. Adult aquatic insects: a, the 

 back swimmer (Notonecta) ; h, the water- 

 boatman (Corixa); c, a diving beetle 

 (Dytiscus) ; d, a giant water-bug (Benacus). 



Here again we find all degrees of adaptation. The 

 larvae of the long-horned leaf beetles (Donacia, etc.) 

 that live wholly submerged have solved the problem of 

 getting air by attaching themselves to plants and per- 

 forating the walls of their internal air spaces, thus 

 tapping an adequate and dependable air supply that is 

 rich in oxygen. This method is followed also by the 

 larvae of several flies and at least one mosquito. There 

 are many aquatic larvae that breathe air at the surface 

 as do adult bugs and beetles. Some of these, such as 

 the swaleflies and craneflies, (fig. 215) differ little from 

 their terrestrial relatives. Others like the mosquito 

 are specialized for swimming and breathe thro respira- 

 tory trumpets. A few like the rat-tailed maggot 



