.300 THE MECHANICAL FUNCTIONS. 



In insects destined to move in water, sometimes 

 all the legs, but occasionally only one pair, are 

 lengthened and expanded into broad triangular 

 surfaces, capable of acting as oars ; and these 

 surfaces are farther extended by the addition of 

 marginal fringes of hair, so disposed as to project 

 and act upon the water every time the impulse is 

 given, but to bend down when the leg is again 

 drawn up, preparatory to the succeeding stroke ; 

 thus imitating the action which is called feathering 

 an oar. The impulses are given with great regu- 

 larity, all the feet striking the water at the same 

 moment. 



Of all the coleopterous insects, the Dytiscus, or 

 water-beetle (of which Fig. 156 represents the 

 upper, and Fig. 1 57 the under side), is the one best 

 constructed for swimming : its body having a flat- 

 tened form, very much resembling a boat, narrower 

 before than behind, and its surface presenting no 

 projecting parts. The upper surface in particular 

 is extremely smooth, to enable it to glide under the 

 water with the least possible friction. Its centre of 

 gravity is very near the under surface. The pos- 

 terior legs, which act as powerful oars, are attached 



