68 



NATURAL HISTORY OF AQUATIC INSECTS CH. 1' 



" They never remain long at the bottom of the 

 water. Air is necessary for them, and this they take 

 in by the tail, which they raise from time to time to 

 the surface of the water. If they are by any means 

 prevented from passing their tails into the air they 

 become greatly agitated." [As in the larva of 

 Dytiscus, the only functional spiracles are the last 

 pair, opening at the tail.] 



" The larva," continues Lyonnet, " moults three ' 

 times after escaping from the cocoon. It is well 



known that when Insects 

 are about to moult they 

 remain for some time in- ' 

 active and without feeding. 

 At this time there is form- 

 ing within the old head, 

 the neck, and the hard en- 

 velopes of the legs, a new 

 armour, destined to replace 

 that which is about to be 

 cast off. The new armour, 

 before it has been exposed 

 to the air, is often quite 

 soft. As soon as the body 

 is freed from the old skin, 

 it becomes considerably 

 dilated. The Insect is still obliged to remain two 

 or three days without eating or exerting itself 

 actively, because many of its muscles are not as yet 

 firmly attached. Just after a change of skin the 

 head and thorax of Hydrophilus are white, and 

 one can then see distinctly on each side of the 



Fig i6. — Head of Hydrophilus larva 

 from under side. 



