CADDIS-FLIES 257 



for their construction. Some use sand, small stones, 

 shells, and other mineral matter, while others build 

 with vegetable matter such as leaves, twigs, reeds, 

 and any stray particles of plants. Some, again, use 

 partly vegetable and partly mineral matter as their 

 building materials. 



The larva invariably spins a silken cylindrical 

 tubular case, on the exterior of which the stones or 

 other materials used to build the case are fixed. 



The distribution of the materials used in the con- 

 struction of the movable cases is made with great care. 

 The case must be light enough for the larva to be 

 able to drag it along, and yet heavy enough to with- 

 stand the strength of the stream, and at the same 

 time well balanced. When crawling along, the head, 

 thorax, and legs are protruding beyond the case, and 

 the abdomen inside the case is provided with two 

 hooks at the tail end with which the larva holds on to 

 the inside of the case and drags it along. As the 

 larva grows it cuts away part of the case at the smaller 

 end and constructs new segments as required of dimen- 

 sions to suit its increased size. 



The full-grown larva closes the ends of the case as 



a preparation for the rest stage 



Preparation for the during which it effects the 



change to pupa. gradual metamorphosis to the 



pupa. The closing of the ends 

 is effected by a grating ; this is differently formed by 

 different genera and species, but is invariably con- 

 structed so as to allow free ingress and egress of water 



s 



