60 NATURAL HISTORY OF AQUATIC INSECTS CH. 



FIG. ii. Under side of Dytis- 

 cus, showing the bases of the 

 middle and hind legs, c-, 

 coxa (basal joint) of middle 

 leg ; c%, coxa of hind leg, 

 very large and immovably 

 united to its fellow and to 

 the metasternum, st. 



it is. If the bases of the hind legs were free, water 



would be squeezed out between 

 them and the body with some 

 force. The progress of the 

 Insect through the water would 

 probably be impeded, especially 

 if the escaping water passed 

 out forwards. 



When the Dytiscus wishes 

 to come up to breathe, it need 

 only cease to exert itself in any 

 way. Its body, which, like that 

 of the larva, is lighter than 

 water, then rises slowly and 

 steadily to the top. The tail 



is tilted a little upwards, which shows that this end 



of the body is more buoyant than 



the rest. A quantity of air is 



always lodged beneath the wing- 

 covers. This clings to the felted 



hairs which cover the back of the 



abdomen, and forms a large flat- 

 tened bubble. When the Beetle 



reaches the surface, its tail-end 



at once pushes through the air. 



Then the wing- covers are slightly 



raised, arid the air concealed by 



them is renewed. The two last 



spiracles, which rise above the 



surface of the water when the 



Beetle is engaged in breathing, 



are unusually large. The smaller spiracles in front 



FIG. 12. Abdomen of Dy- 

 tiscus, seen from above 

 after removal of wing- 

 covers and one wing ; the 

 spiracles are seen on the 

 risrht border. 



