BROOK LIFE. 157 



brinks of falls and places where the slanting bed-rock 

 is washed clean by a swift flow of water, but some- 



./ 



times they cling to pieces of wood or small stones 

 that are firmly fixed in the rapids. 



When the larvae are fully grown they spin boot- 

 shaped cocoons within which the pupa state is passed ; 

 these are firmly fastened to the 

 rock upon which the larvae lived. 

 In Fig. 127 there are represented 

 a larva and a cocoon. 



1. Find a cluster of larvae and 

 take notes on the following: The 

 way in which they keep their place 

 in the swift current. (It is the tail 

 end by which they are usually 

 fastened to rock.) Their method 



of locomotion, and the action of the fan-shaped or- 

 gans attached to the head. 



2. Collect larvae and cocoons and try to keep them 

 alive in water. These insects can not be reared in 

 aquaria unless they are placed under a jet of water, 

 but they can be kept alive several hours, thus giving 

 some opportunity for watching their habits in con- 

 finement. 



3. Look for the eggs of the black-flies. These 

 are yellowish or brownish and occur in patches in 

 situations similar to those inhabited by the larvae. 



4. Look also for adults. These occur in swarms, 

 hovering over the brinks of falls and dashing back 

 and forth through the spray. Sometimes they may 

 be seen darting into the water and out again ; at such 

 times they are laying their eggs. 



Fig. 128 represents one of these flies somewhat 



