ADAPTATIONS OF AQUATIC INSECTS 



153 



with powerful piercing organs. Belostoma and Benacus (Fig. 22) even 

 kill small fishes by their poisonous punctures. Some other kinds, as the 

 water-skaters (Gerridae, Fig. 226), depend on dead or disabled insects. 

 The species of Hydro philus (Fig. 227) are to some extent carnivorous as 

 larvae but phytophagous as imagines, while Dytiscidae are carnivorous 



FIG. 225. Backswimmer, Notonecta insulata, 

 natural size. 



FIG. 226. Water-skater, Gerris remigis, 

 natural size. 



throughout life. Aquatic insects eat not only other insects, but also 

 worms, crustaceans, mollusks or any other available animal matter. 



Even aquatic insects are not exempt from the attacks of parasitic 

 species. A few Hymenoptera actually enter the water to find their 

 victims, for example, the ichneumon Agriotypus, 

 which lays its eggs on the larvae of caddis flies. 



Locomotion. Excellent adaptations for aquat- 

 ic locomotion are found in the common Hydrophi- 

 lus triangularis (Fig. 227). Its general form re- 

 minds one of a boat, and its long legs resemble 

 oars. The smoothly elliptical contour and the 

 polished surface serve to lessen resistance. Owing 

 to the form of the body (Fig. 228, A) and the pres- 

 ence of a dorsal air-chamber under the elytra, the 

 back of the insect tends to remain uppermost, while 

 in Notonecta (Fig. 228, B), on the other hand, the 

 conditions are reversed, and the insect swims with 

 its back downward. The legs of Hydrophilus, ex- 

 cepting the first pair, are broad and thin (Fig. 229,^!) 



and the tarsi are fringed with long hairs. When swimming, the " stroke " 

 is made by the flat surface, aided by the spreading hairs; but on the 

 "recover," the leg is turned so as to cut the water, while the hairs fall 

 back against the tarsus from the resistance of the water, as the leg is 

 being drawn forward. The hind legs, being nearest the center of gravity, 



FIG. 227. Hydroph- 

 ilus triangularis, nat- 

 ural size. 



