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 difter utterly 

 from the lotic forms 

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

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

 feet of them. The}' lack all the usual adaptations for 

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

 sional intermixture of a few larvae of small midges 

 and of Simuliuni) the following: 



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

 rock )edt;e (in flood time, a waterfall) in 

 the bed of Williams Brook at Ithaca, 

 X. Y. 'I'lie water seen on the rock above 

 trickles down through this moss. Here is 

 a restricted and peculiar animal popula- 

 tion. 



