372 



Aquatic Societies 



strikingly like a snail shell, and fastens it down closely 

 in the shallow crevices of stones on exposed surfaces. 



H* 



Fig. 221. The spirally coiled cases of the 

 caddis-worm, Helicopsyche. 



A number of other caddis- worms build portable cases 

 of sand and stones. Those of Gcera (fig. 222) are 

 heavily ballasted by means of stones attached at the 

 sides with silk. These lie down flat against the bottom 



and doubtless serve the 

 double purpose of de- 

 flecting the current and 

 preventing the case from 

 being washed away. 



The tubes of the midges 

 are here made of less 

 soft and flocculent ma- 

 terials than in still 

 waters. T any tarsus 

 makes an especially 

 tough case of a pale 

 brownish color, like dried 

 grass. It is of tapering 

 form, and easily recog- 

 nized by the three stay 

 lines that run out from 

 the open forward end. 

 p., c+ u 11 ,. a c A small greenish yellow 



ru,. 222. Stone-ballasted cases of ., .^ , J - 



caddis-worms of the genus Gcera. larva With rather long 



U% 



