42 NATURAL HISTORY OF AQUATIC INSECTS CH. 



their skins from time to time, like other Insects. The 

 larva when full-grown is more than two inches long. 

 It is yellow-brown in colour, and of cylindrical shape, 



tapering towards the head 

 and tail. 



The head is nearly circu- 

 lar, flat, and joined to the 

 thorax by a distinct neck. 

 The mouth-parts are those 

 of a carnivorous Insect. The 

 mandibles in particular are 

 slender, curved, and pointed. 

 On the inner or concave side 

 of each is a longitudinal 

 slit, which was long sup- 

 posed to possess a very 

 special importance as being 

 the only passage leading into 

 the mouth. 



Swammerdam says of the 

 Dytiscus larva, and his ac- 

 count is, I believe, exactly 

 true : " When it feeds, it 

 seizes its prey with the 

 mandibles, and pierces it 

 with their sharp, curved 

 points. Now the tips of the 

 mandibles are hollow, and through them the larva 

 sucks the blood in a singular fashion into the mouth, 

 into which the channels of the mandibles open. Since 

 the larva and its jaws are in some degree transparent, 

 we can see the blood flow from the mouth into the 



Fig. 4. — Larva of Dytiscus, dorsal 

 \ie\v. 



