ADAPTATIONS OF AQUATIC INSECTS 



i6 7 



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 



ti 



A B 



FIG. 230. Transverse sections of (A) Hydrophilus and (B) Notonecta. e, elytron; h, 

 hemelytron; I, metathoracic leg. 



legs, being nearest the center of gravity, are of most use in swimming, 

 though the second pair also are used for this purpose; indeed, a terrestrial 

 insect, finding itself in the 

 water, instinctively relies 

 upon the third pair of legs 

 for 1 ocomotion. Hydro- 

 philus uses its oar-like legs 

 alternately, in much the 

 same sequence as land 

 insects, but Cybister and 

 other Dytiscidae, which 

 are even better adapted 

 than Hydrophilus for 

 aquatic locomotion, move 

 the hind legs simultane- 

 ously, and therefore can 

 swim in a straight line, 

 without the wobbling and 

 less economical m o v e- 

 ments that characterize 

 Hydrophilus. 



Larvae of mosquitoes 



FIG. 231. Left hind legs of aquatic beetles. A, 

 Hydrophilus triangular is; B, Cybister fimbriolatus; c, 

 coxa; /, femur; s, spur; /, tarsus; ti, tibia; tr, tro- 

 chanter. 



propel themselves by 



means of lashing, or undu- 



latory, movements of the 



abdomen. A peculiar mode of locomotion is found in dragon fly 



nymphs, which project themselves by forcibly ejecting a stream of 



water from the anus. 



