THE PHRYGANID^E. 49 



out of their cases, the larvse arte often used as bait 

 for trout and other fish. Their cases are open at 

 each extremity, and when feeding or crawling 

 they expose their head and legs, which are in- 

 stantly withdrawn on the slightest alarm, and 

 their hinder parts are furnished with two hook- 

 like appendages, with which they are attached to 

 their cases, and thus can draw them along as a 

 snail does its shell. 



After having passed sufficient time in the larva 

 state to arrive at the proper degree of maturity, 

 the insect fastens its case to the stem of some 

 water plant, or other substance, and, closing it at 

 each extremity with a kind of grating, through 

 which the water necessary for respiration flows, it 

 turns to an inactive pupa, which is a further step 

 towards its perfect state, the rudiments of which, 

 indeed, now become clearly perceptible. In a 

 few days it issues from its outer case, rises to the 

 surface of the water, and, bursting its pupa skin, 

 appears in its last and perfect form of a beau- 

 tiful fly. 



The time at which this class of flies appears on 

 the water depends on the temperature of the 

 season. If it be mild, the sand fly will be seen 

 in the end of March. This fly is much more 

 abundant on some rivers than on others, and its 

 E 



