AQUATIC INSECTS. 337 



to very large haunches, for the purpose of con- 

 taining the thick muscular bands which are in- 

 serted 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. 158), 

 is remarkable for always swimming on its back, 

 a peculiarity depending on 

 the form of its body, which is 

 semi- cylindrical, with the legs 

 affixed 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 under- 

 most, it would unavoidably tilt over, and resume 

 its usual position. Its long legs extending at 

 right angles to the body, present a striking re- 

 VOL. i. z 



