358 



Aquatic Societies 



Moss patches On 

 the rocky beds of 

 large brooks that run 

 low but do not en- 

 tirely run dry, there 

 are frequent patches 

 of the close-growing 

 moss, Hydrohyp- 

 num. These patches 

 frequently cover the 

 vertical face of a 

 waterfall (fig. 213). 

 The little water that 

 remains in dry season 

 trickles through the 

 layer of moss, and 

 in times of flood the 

 speedier torrent 

 jumps over it. Under 

 the flattened frond- 

 like green sprays 

 there is compara- 

 tively quiet water 

 at all times; and in 

 this situation there 

 lives a peculiar as- 

 semblage of insects 

 that differ utterly 

 from the lotic forms 



dwelling in the same streams (to be discussed in a later 

 part of this chapter), tho often dwelling within a few 

 feet of them. They lack all the usual adaptations for 

 meeting the wash of currents. They are (with occa- 

 sional intermixture of a few larvas of small midges 

 and of Simulium) the following: 



FIG. 213. A moss-bed covering the face of a 

 rock ledge (in flood time, a waterfall) in 

 the bed of Williams Brook at Ithaca, 

 N. Y. The water seen on the rock above 

 trickles down through this moss. Here 

 is a restricted and peculiar animal pop- 

 ulation. 



