ADAPTATIONS OF AQUATIC INSECTS 



.87 



Larvae of mosquitoes propel themselves by means of la<li- 

 ing, or undulatory, 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. 



On account of the large amount of air that they carry about, 

 most aquatic imagines are lighter than the water in which they 



FIG. 228. 



ti 



Left hind legs of aquatic beetles. A, Hydrophilus triangularis; B, Cybister fimbrio- 

 latus; c, coxa; f, femur; s, spur; t, tarsus; ti, tibia; tr, trochanter. 



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 mosquito larva (Fig. 229, A) is often 

 heavier than water, but the pupa (Fig. 229, B) is lighter, and 

 remains clinging to the surface film. 



The tension of this surface film is sufficient to support the 

 weight of an insect up to a certain limit, provided the insect 



