274 Adjustment to Conditions of Aquatic Life 



and sculpturing, and (5) shortening of basal segments of 

 swimming legs, with lengthening of their oar-like tips, 

 flattening and flexing of them into the horizontal plane, 

 and limiting their range of motion to horizontal strokes 

 in line with the axis of gravity of the body. Caudal 

 propulsion does not occur with adult insects; none of 

 them has a flexible tail. Oar-like hind feet are the 

 organs of propulsion. The best swimmers among them 

 are a few of the larger beetles : Cy bister, which swims 

 like a frog with synchronous strokes of its powerful 

 hind legs, and Hydrophilus, with equally good swimming 

 legs, which, like the whale, has developed a keel for 

 keeping its body to rights. 



Adult insects, like the mammals, lack gills, and rise 

 to the surface of the water for air; but they take the 

 air not through single pairs of nostrils, but a number of 

 pairs of spiracles, and they receive it, not into lungs, 

 but into tracheal tubes that ramify throughout the 

 body. The spiracles are located at the sides of the 

 thorax and abdomen, in general a pair to each seg- 

 ment. 



In diving beetles the more important of these are 

 the ones located on the abdomen beneath the wings. 

 Access to these is between the wing tips. The beetles 

 when taking air hang at the surface head dowmward. 

 The horny, highly arched, fore wings are fitted closely 

 to the body to inclose a capacious air chamber. They 

 are opened a little at their tips for taking in a fresh air 

 supply at the surface. Then they are closed, and the 

 beetle, swimming down below, carries a store of air with 

 him. 



In other beetles there are different methods of gather- 

 ing and carrying the air. The little yellow-necked 

 beetles of the family Haliplidae, gather the air with the 

 fringed hind feet, pass it forward underneath the huge 

 ventral plates w r hich, in these beetles cover the bases 



