AQUATIC INSECTS. 301 



to very large haunches, for the purpose of contain- 

 ing 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 re- 

 quired to be moveable : and accordingly they are 

 firmly united to the thorax ; a structure which ren- 

 ders 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. 



Among the Hemiptera, the Noto7iecta, or water- 

 boatman (Fig. 158,) is remarkable for always swim- 

 ming 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 equili- 

 brium 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 riglit angles to the body, present a 

 striking resemblance to the oars of a boat ; they 

 act, indeed, in the same manner, and on the same 

 principles. 



