THE TRICHOPTERA. 



219 



teniue^ enclosed in their separate cases^ are laid help- 

 lessly along the sides of the body, passes through its 

 period of repose, but regains the power of motion 

 shortly before its final change. When the period 

 for this change approaches, the pupa breaks out of 

 the case which has so long sheltered it, and proceeds 

 to the surface of the water to shed its last envelope, 

 and become a free denizen of the air. For this pur- 

 pose the larger species creep up the stems of plants 

 growing in the water, until they reach a suitable 

 position, whilst many of the smaller ones undergo 

 their last transformation floating on the surface of 

 the water, when the skin which they have just quitted 

 serves them as a sort of raft on which to float until 

 their wings have attained sufiicient strength to enable 

 them to reach the shore. In this state they run 

 pretty quickly, but their flight, as already stated, is 

 heavy and awkward. Like the Ephemera, which 

 they resemble in the imperfection of their mouths, 

 the period of their aerial existence is very short, and 

 their principal business is the continuation of their 

 species ; and considering the numbers which must fall 

 a prey to those indiscriminate devourers the fishes, 

 the survivors must be endowed with no ordinary 

 fecundity to preserve their race from extinction. 



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