STRUCTURE OV INSECTS. 335 



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 bo- 

 dies becomes permeated by air, which forms alto- 

 gether 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 contained 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, therefore, 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 hori- 

 zontal direction. 



In insects destined to move in water, some- 

 times all the legs, but occasionally only one 

 pair, are lengthened and expanded into broad 

 triangular surfaces, capable of acting as oars : 



