90 OUR INSECT FRIENDS AND FOES 



film and adhere to it. The flaps are then 

 expanded, and in an instant the tiny "cup" is 

 filled with ain As the larva grows, it moults or 

 casts its skin, and after three or four moults it is 

 ready for pupation. The head-end now assumes 

 a very strange appearance, having increased 

 enormously in size, so that the whole of the front 

 region of the body looks as if enclosed in a kind 

 of fireman's smoke-helmet ; the body has become 

 much shortened, and now terminates in a pair 

 of oval fins which are used in swimming. 



The insect has now become an active pupa or 

 nymph, and beneath its great helmet-like mask, 

 the slender legs, the wings, antennae, and mouth- 

 parts of the perfect insect can be traced. On the 

 back of the nymph, just behind the head, are a 

 pair of tubes, the respiratory trumpets through 

 which the insect now draws its air supply, there- 

 fore there is no longer any need for it to hang 

 head downwards, as in the larval stage. The 

 encasing helmet makes it impossible for the 

 insect to obtain food. The nymph is active swim- 

 ming through the water with a jerky motion by 

 means of a pair of oval plates or fins at the end of 

 the body. It now always rises head, or rather 

 back, first to the surface, so that the respiratory 

 trumpets upon its back are brought flush with 

 the surface; the water being prevented from 

 entering the tubes by a coating of peculiar hairs 

 which line and project from their inner surface. 

 When the nymph first regains the surface, after 

 one of its excursions below, the mouths of the 

 tubes are covered with a film of water, which, 

 however, becomes rapidly thinner by evaporation, 



