240 INSECT WATER TEESPASSEES. 



must be very light in proportion to its bulk, and must 

 have a very strong tendency to float. Accordingly, 

 any one who watches these insects will see that they 

 cannot keep themselves below the surface without 

 continuous exertion; and that as soon as they cease 

 from action, they rapidly float upwards. This buoyancy 

 has a use to which reference will presently be made. 



The question now presents itself, how the supply of 

 air is to be renewed below the surface of the water. This 

 is done in exactly the way to which reference was made 

 during the description of the whales namely, by carry- 

 ing a supply[of air below the water. This supply is kept 

 in the space between the wing-cases and the body; and 

 as the spiracles open into it, the insect can take with it 

 a supply of air that is sufficient to last it for some time. 



Of course, the beetle is obliged to act just as the 

 whales and seals do, i.e., come to the surface at inter- 

 vals for breath, and this it can do with great rapidity. 

 If alarmed, it just rises to the surface with its tail 

 upwards, protruding the ends of the elytra for a 

 moment, ejects the air that has been used, and takes 

 in a fresh supply. This is done with such speed, that 

 the rising to the surface and the subsequent descent 

 seem to be one and the same action. 



Perhaps the reader may have noticed when looking 

 at a still pool, that bubbles of air are almost continually 

 rising to the surface. The large bubbles, which appear 

 at rare intervals, are almost entirely due to the gases 

 caused by decomposing matter at the bottom of the 

 pool. But the very small bubbles, which are scarcely 

 perceptible, except when watched for, are mostly caused 

 by the respiration of aquatic insects. The air traverses 



