BROOK, CREEK, AND RIVER 291 



carnivorous fish of the stream. The crustaceans, in turn, feed 

 upon the smaller organisms of the plancton, both plant and 

 animal. 



The first animal to appear in intermittent streams, even when 

 it is a mere trickle, is the larva of the black fly. Along with this 

 are to be found larvae of May flies, under the stones, and caddis 

 worms in their irregular tubes formed of bits of stone (Rhynco- 

 philidae). 



In the pools of such intermittent streams are found in the 

 spring crayfish (Cambarus diogenes) that dig in and live in their 

 burrows as the temporary pools dry out. They come out at 

 night to secure their food. As the pools become larger and 

 more permanent, Cambarus mrilis and C. propinquus replace the 

 burrowing crayfish. Gammarus fasci atus and Asellus are added 

 to the crustacean population. Water striders, back-swimmers, 

 and water boatmen appear. The diving beetles, Hydroporus and 

 Agabus, are found. Burrowing dragon-fly nymphs inhabit the 

 pools, and the adults fly over their surfaces (Aeschna constricta 

 and Cordulegaster obliquus). 



The first fish to appear is the horned dace (Semotilus atroma- 

 culatus). It swims away upstream to deposit its eggs, so the 

 young are common in these temporary pools and often die in 

 large numbers as they dry out. The red-bellied dace (Chrosomus 

 erythrogaster) is likely to get almost as far upstream, but breeds 

 only in pebbly and sandy bottom. It, too, is found in the tem- 

 porary pools. 



In the spring-fed brooks the characteristic vegetation is the 

 water cress. Black-fly larvae are present, as also the same 

 benders, crayfishes and beetles mentioned above. In addition, 

 the net-building caddis fly (Hydropsyche) is found. The brook 

 beetle, Elmis fastiditus, is very characteristic. The spring-fed 

 brooks in our locality are very short, usually joining some larger 

 stream quite promptly. In these springs and brooks such 

 planarians as Dendrocoelum and Planaria dorotocephala are 

 abundant. 



