AQUATIC INSECTS 



their puckered skins remain. The shallows where tadpoles sun 

 themselves are almost sure to swarm with dytiscid larvas 

 whose appetite for tadpoles is a serious menace to the amphib- 

 ian population. Neither adult nor larva of Dytiscus hesi- 

 tates to attack an animal much larger than itself. There 

 are records of an adult Dytiscus in an aquarium, which de- 

 voured seven snails, Lymnaa stagnalis, during one afternoon 

 Whirligig beetles, Family Gyrinidae. — Whirligig beetles are 

 one of the most easily recognized of all brook insects. With 

 the first spring days they come to the surface in twos and 

 threes, from their winter hibernation, and soon gather in 

 companies on the surface of still or gently running water, 

 resting motionless or circling round and round each other 

 (PI. X). They are blue-black or bronze, oval, and flattened; 

 some are canoe-shaped on the ventral side and so smooth that 

 they cut through the water with the greatest ease. Their 

 hind legs, paddle-shaped and fringed with long hairs, are 

 used oar-like in their rapid sculling (Fig. 210, 2). The com- 

 pound eyes are divided by the sharp margin of the head so 

 that in effect there is one eye for looking up from the water 

 and one for looking down into it (Fig. 210). 



1 a. 



Fig. 210. — Smaller whirligig beetle, Gyriniis: I, 

 adult, dorsal side; 2, ventral side showing the middle 

 and hind legs which fold tightly down to the surface ; 

 3, side of head with divided eye. 



Gyrinids are more difficult to catch than would first appear. 

 Though circling all about one they can evade a net with surpris- 

 ing agility and even after they are caught in it they frequently 



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