23S THE MECHANICAL FUNCTIONS. 



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 mo- 

 ment. 



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 present- 

 ing 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 posterior legs, which act 

 as powerful oars, are attached to very large haunches, for 

 the purpose of containing the thick muscular bands which 

 are inserted into the trochanter, and by which these joints 

 are moved with great power. As the motion of these oars 

 is to be performed in a plane nearly parallel to the axis of 

 the body, the haunches are not required to be moveable: 

 and accordingly they are firmly united to the thorax; a 

 structure which renders the motion of the other joints more 

 regular and uniform. When the Dytiscus wishes to rise, it 

 need only desist from all action, and abandon itself to the 

 buoyant force of the fluid, which quickly carries it to the 

 surface. 



The Notonecta^ or water-boatman (Fig. 15S,) is remarka- 

 ble for always swimming on its back, a peculiarity depend- 

 jrg ing on the form of its body, which is- 



semi-cylindrical, with the legs aflixed to 

 the flat surface; so that, when lying on 

 its back in the fluid, the centre of gravity 

 is below the centre of the whole figure, 

 or the metacentre, as it is termed, and 

 the equilibrium is maintained. It is evident that, under these 

 circumstances, if it were placed in the water with its legs 

 undermost, it *would unavoidably tilt over, and resume its 

 usual position. Its long legs extending at right angles to 

 the body, present a striking resemblance to the oars of a boat; 



