04 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. 
Fic. 38.—The permanent swift stream showing the stones in the rapids, and the 
stiller places below (New Lenox, IIll., 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 
