94 ANIMAL COMMUNITIES OF STREAMS 



toward the larger swift stream (Fig. 38). While the conditions in these 

 are like those of the spring brook, the watercress is absent and there are 

 few rooted plants. Pools and riffles are well developed and the flow of 

 water is constant, but fluctuates in volume. These streams differ in 

 size, but the formation mores are practically the same, although larger 

 species commonly inhabit the larger stream. 



a) Pelagic sub-formation is very poorly developed in the smaller 

 streams and will be discussed in connection with sluggish streams. 



Fig. 38. — The permanent swift stream showing the stones in the rapids, and the 

 stiller places below (New Lenox, 111., Gaugars Station) (original). 



b) Hydropsyche or rapids formations (Stations 14, 15, 17, 19, 20, 21; 

 Tables XX, XXI, XXII). — These are usually due to the presence of 

 coarse material or an outcrop of rock. They are typical in streams with 

 large bowlders and stones of all sizes. Here current is probably the 

 controlling factor. In these streams, we find the best expression of the 

 riffle formation, which we have seen is poorly developed in the smaller 

 streams. This formation includes three ecologically equivalent modes of 

 life, each meeting the current in a different way. These are (i) clinging 



