Leaf-drifts 



361 



surface and burrowing mayflies in the beds, and a con- 

 siderable variety of the lesser midges on every possible 

 support. 



We have already noted (page 86) that slack water 

 exists behind boulders and other obstructions in the 

 bed of rapid streams: but this is not stagnant water; 

 and the animals living in such shelter, if the current 

 above them be swift, are hardly ever of the same species 

 that are found in ponds. Only in slow-flowing waters, 

 where conditions merge, do lotic and lenitic forms 

 become near neighbors. 



Fig. 216. A bit of the bed of a pool in a woodland stream show- 

 ing among the forest litter the wooden cases of the larva, of the 

 caddis-fly, Halesiis guttifer. (See also fig. 104 on p. 198.) Pro- 

 tective resemblance. There are 14 cases in the picture. 



