KESPIKATION OF AQUATIC INSECTS. 71 



euough to hang well over the spiracles, are sutficiently convex 

 to contain air between themselves and the body, aud fit so 

 closely to each other and to the sides, that when they are closed 

 no air can by any possibility escape. 



By reason of this structure the insect is enabled to take with 

 it a sufficient quantity of air for respiration, aud when it needs 

 a fresh supply it comes to the surface, opens its elytra slightly 

 at the tips, admits the air, and dives again in its perpetual search 

 for food. The process by which the air is passed through the 

 respiratory organs of sub-aquatic insects, has already been 

 described in my " Insects at Home," and need not be repeated 

 here. 



