154 



ENTOMOLOGY 



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, 



FIG. 228. Transverse sections of (A) Hydrophilus and (B) Notonecta. e, elytron; h, hemely- 



tron; /, meta thoracic leg. 



instinctively relies upon the third pair of legs for locomotion. Hydroph- 

 ilus 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 simultaneously, and 

 therefore can swim in a 

 straight line, without the 

 wobbling and less econom- 

 ical movements that charac- 

 terize Hydrophilus. 



Larvae of mosquitoes pro- 

 pel themselves by means of 

 lashing, or undulatory, move- 

 ments of the abdomen. A 

 peculiar mode of locomo- 

 tion is found in dragon fly 

 nymphs, which project them- 

 selves by forcibly ejecting a 

 stream of water from the 

 anus. 



On account of the large 

 amount of air that they carry 



about, most aquatic imagines are lighter than the water in which they live, 

 and therefore can rise without effort, but can descend only by exertion, and 

 can remain below only by clinging to chance stationary objects. The mos- 



FIG. 229. Left hind legs of aquatic beetles. A, 

 Hydrophilus triangularis; B, Cybister fimbriolatus; c, 

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

 chanter. 



