ADAPRTARIONS Ob AQUATIC INSECTS Lo7 
Larvee of mosquitoes propel themselves by means of lash- 
ing, 
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 
or undulatory, movements of the abdomen. <A _ peculiar 
Left hind legs of aquatic beetles. A, Hydrophilus triangularis; B, Cybister fimbrio- 
latus; c, coxa; f, femur; s, spur; t, tarsus; ti, tibia; tv, 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, 4) 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 
