336 THE MECHANICAL FUNCTIONS. 



and these surfaces are further extended by the 

 addition of marginal fringes of hair, so dis- 

 posed 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 regularity, 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. 157 the under side), is the 

 one best constructed for swimming : its body 

 having a flattened 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 

 placed very near the under surface. The poste- 

 rior legs, which act as powerful oars, are attached 



