STRUCTURE OF INSECTS. 299 



hence we find flies preferring the ceiling to the 

 floor, as a place of rest. 



Insects which, like the gnat, walk much upon 

 the surface of water, have at the ends of their feet 

 a brush of fine hair, the dry points of which appear 

 to repel the fluid, and prevent the leg from being 

 wetted. If these brushes be moistened with spirit 

 of wine, this apparent repulsion no longer takes 

 place; and the insect immediately sinks and is 

 drowned. 



§ 7. Aquatic Insects. 



Although many insects are inhabitants of water 

 while in their larva state, few continue to reside in 

 that element after they have undergone all their 

 metamorphoses. When they have attained the 

 imago state, indeed, every part of their bodies 

 becomes permeated by air, which forms altogether 

 a large portion of their bulk, and gives to the 

 insect, when it is immersed in water, a strong 

 buoyant force. As the largest volume of air is con- 

 tained in the abdomen, this part is comparatively 

 lighter than either the trunk or head ; and the 

 natural position of the insect in the fluid is oblique 

 to the horizon, the head being depressed, and the 

 abdomen elevated. Any force impelling the body 

 forwards in the direction of its axis tends, there- 

 fore, to make it also descend. The effect of this 

 downward force is counteracted by the sustaining 

 pressure of the water, which is directed vertically 

 upwards : so that the real operation of the force in 

 question is to carry the body forwards nearly in a 

 horizontal direction. 



