POND INSECTS 71 



folied on the back while the insect is in the water. With a water 

 net collect a number of both the larger black insects (the water- 

 beetles) and the smaller gray insects (the water-bugs) and put 

 them into glass j ars of water in the school-room. 



Watch the big black beetles first. There are two kinds, both 

 looking much alike, but differing in their food habits. One kind, 

 the "predaceous diving-beetles," kill and eat other water insects; 

 while the other kind, the "water-scavenger beetles" (fig. 43), feed 

 on decaying vegetation (the they occasionally attack other insects). 

 Try to observe the feeding of the beetles in the aquarium. Observe 

 the position of the bod}', when one of the beetles is at rest, just 

 underneath the surface. If the beetle hangs head downward with 

 the tip of the abdomen at the surface, it is one of the predaceous 

 diving-beetles; but if the head end of the body is kept at the sur- 

 face, it is one of the water-scavenger beetles. 



When the beetles are resting at the surface they are breathing, 

 or rather they are collecting air which they breathe after they sink 

 beneath the surface. The water-beetles do not remain at the sur- 

 face most of the time as tlie mosquito wrigglers do, nor do they 

 remain underneath the surface all the time as the young May- 

 flies do. They do not have to come to the surface every time 

 they wish to breathe but they have no gills to enable them to take 

 up the air which is mixed with water. The way in which they 

 solve the problem of breathing under water is this: they come to 

 the surface occasionally and collect a supply of air which they 

 take down with them. The two kinds of beetles do this in dif- 

 ferent ways. The predaceous diving-beetles force the posterior 

 tip of the body above the surface and slightly lift the tips of the 

 horny black wing-covers which lie on the back. Air rushes in 

 under these wing-covers and is held there by the closing of the 

 tips. This process can be readily observed. Tie breathing pores 

 or spiracles of the beetle are situated along each side of its back 

 underneath the wing-covers so that the air held under the wing- 



