52 NATURAL HISTORY OF AQUATIC INSECTS CM. 



On the under side of the first ring of the thorax the 

 keel-Hkc prominence is formed by a backward- 

 directed spine, which fits into a cavity in the second 

 ring. This spine contributes to the rigidity of the 

 body, like the deeply sunk head, the peculiar fixation 

 of the thighs of the hind limbs, and the fusion of the 

 first and second rings of the abdomen (see Fig. ii, 

 p. 60.) 



The wing-covers are very stout, smooth in the male, 

 and often furrowed along their length in the female. 

 Beneath them lie crumpled up the ample wings, 

 which can upon occasion easily bear the body through 

 the air. When food grows scarce, the Dytiscus takes 

 to flight, choosing twilight or dark for its journey, and 

 seeks out a fresh pond. 



The hind legs are long and specially adapted for 

 swimming. The tibia and five-jointed tarsus are 

 flattened, and furnished with a fringe of long and 

 stiff hairs along one edge. These spread out during 

 the stroke, and are depressed during the return of 

 the leg through the water. The tarsus is so articu- 

 lated that it rotates a little upon its own axis when 

 moved through the water, presenting its broad face 

 during the stroke and its edge during the return ; " in 

 other words, what rowers call feathering the oar is 

 performed by the tarsus of the Dytiscidse in a most 

 perfect manner." ^ In swimming the hind legs are 

 chiefly used for propulsion. They strike the water 

 together. The middle legs aid the movement, and 

 appear also to guide the animal's course. There is no 



1 Dr. Sharp, on " Aquatic Carnivorous Coleoptera or Dytis- 

 cidce," Trans. Roy. Dublin Society., Ser. II. Vol. II. p. 254. 



