FIELD BOOK OF PONDS AND STREAMS 



Cybister. — Cyhister (Fig. 208) is a genus of large and very 

 voracious water tigers. The fullgrown larva becomes 3 

 inclies long and will attack tadpoles, other water-beetles, and 

 small fish. 



When larvae come to the surface to breathe, they do not 

 hang head downward like Acilius and Dytiscns but the head 

 and thorax are both held near the surface. Their narrow 

 necks are also good distinguishing marks. 



<^ ^ 



Fig. 209. — Large diving beetle, Dytiscns: i, adult 

 male; 2, underside of his front tarsus (cf. Fig. 208); 

 3, larva clutching damselfly nymph. 



Adult dytiscid beetles (Fig. 209) are nearly an inch and a 

 half long and the full-grown larvas may measure three inches. 

 The adults are dark brown with a dull j^ellow stripe along the 

 sides of the thorax and wing covers. When at rest they hang 

 head downward with the tip of the body projecting through 

 the surface film. 



The Iar\'£e (Fig. 209) also hang head downward with their 

 tails thrust through the surface film for air. Much of the 

 time they clamber about through the vegetation of the shal- 

 lows and frequent the tadpole resorts; here they clutch the 

 soft side of the tadpoles, sucking the body juices until only 



270 



