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ENTOMOLOGY 



Locomotion. Excellent adaptations for aquatic locomo- 

 tion are found in the common Hydrophilus triangularis (Fig. 

 226). Its general form reminds one of a boat, and its long 

 legs resemble oars. The smoothly elliptical contour and the 

 polished surface serve to lessen friction. Owing to the form 

 of the body (Fig. 227, A) and the presence of a dorsal air- 



FIG. 227. 



A B 



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

 tron; I, metathoracic leg. 



chamber under the elytra, the back of the insect tends to re- 

 main uppermost, while in Notonecta (Fig. 227, B) , on the other 

 hand, the conditions are reversed, and the insect swims with 

 its back downward. The legs of Hydrophilus, excepting the 

 first pair, are broad and thin (Fig. 228, A) 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, are of most use in swim- 

 ming, though the second pair also are used for this purpose; 

 indeed, a terrestrial insect, finding itself in the water, instinc- 

 tively relies upon the third pair of legs for locomotion. Hy- 

 drophilus uses its oar-like legs alternately, in much the same 

 sequence as land insects, but Cybistcr and other Dytiscidse, 

 which are even better adapted than Hydrophilus for aquatic 

 locomotion, move the hind legs simultaneously, and therefore 

 -can swim in a straight line, without the wobbling and less 

 economical movements that characterize Hydrophilus. 



