266 NATURAL HISTORY OF AQUATIC INSECTS CH. 



considerable distance, and are attached to the sur- 

 rounding objects. 



" Through this trans- 

 parent case it is easy to 

 observe the larva cling- 

 ing by its feet and anal 

 hooks to the threads, 

 and to watch its move- 

 ments. Like other Cad- 



I 



dis-worms, it sets up a 

 current of water through 

 the tube by swaying its 

 body up and down. The 

 respiratory stream enters 

 at the head end, and 

 flows out behind. 



" Plectrocnemia pos- 

 sesses no gill-filaments, 

 and apparently the only 

 structures analogous to 

 gills are five finger-like 

 processes on the last 

 segment. These are cap- 

 able of being completely 

 withdrawn. The absence 

 of processes on the first 

 abdominal segment is 

 interesting. Assuming 

 that they serve to keep 

 the larva steady by pi cs- 



sing against the side of the case it is essential that 

 they should act upon a rigid support. Here, however^ 



FIG. 84. Larva of Plectrocnemia (Fam. 

 Hydropsychida;). 



