A CARNIVOROUS WATER-BEETLE 



35 



forms a large flattened bubble. From this bubble air can be 

 drawn into the body by a series of small spiracles which lie 

 along the back. 



The female of the Dytiscus marginalis lays her eggs in 

 March or April upon aquatic plants, making a deep incision 

 for each egg with her ovipositor. 

 In about three weeks the larva 

 hatches out. It grows fast, and 

 attains its full size in four or five 

 weieks, being then about two inches 

 lohg. 



The figure shows the shape of 

 the larva. We remark its rather 

 lohg, fringed legs,- the fringed hinder 

 segments, and the pair of fringed 

 appendages borne by the last seg- 

 ment of all. The legs and tail are 

 both effective in swimming, but the 

 tail is less important for swimming 

 than for breathing. The larva is 

 lighter than water, and can only 

 remain below by grasping solid ob- 

 jects. When it lets go, it rises to 

 the surface, tail first. The water 

 rolls off the hairy appendages in 

 a moment, and then air can be 

 sucked in by two spiracles, which 

 open on the tip of the tail, and 

 lead into the great longitudinal 

 air -tubes. The action of the 

 hairs can be explained by well-' 

 known physical principles, but we 

 must not stay to discuss the point 

 here. 



There is one curiosity of structure about the mouth-parts of 

 the larva which we must not pass over. Whoever examines 

 the larval head with a pocket-lens will be sure to notice 

 the long, sickle-shaped mandibles. It was pointed out by 

 Swammerdam, as long ago as the seventeenth century, that 

 these cruel weapons are hollow, and that when the larva preys 

 upon a red-blooded victim, the blood can be seen to flow 



Fig. 24. — Larva of Dyliscus. 



