AQUATIC INSECTS. 



237 



§ 7. */2quatic Insects. 



Although many insects arc 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 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 italso 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 di- 

 rection. 



In insects destined to move in water, sometimes all the 

 legs, but occasionally only one pair, are lengthened and ex- 

 panded into broad triangular surilices, capable of acting as 



oars: and these surfaces arc farther extended by the addi- 

 tion of marginal fringes of hair, so disposed 'as to project 

 and act upon the water every time the im])ulsc is given, but 

 to bend down when the leg is again drawn up, prci)aratory 



