90 ANIMAL COMMUNITIES OF STREAMS 



b) Temporary pool consocies. — As a young stream grows deeper it 

 often reaches some depression or marsh at its headwaters of which it 

 forms the outlet in the early spring. It is now permanent for a longer 

 period each season of normal rainfall, and small pools usually alternate 

 with the rapids just described. In these pools aquatic insects, crus- 

 taceans, and snails which belong primarily to stagnant ponds make 

 their appearance. The first resident species are the crayfishes. They 

 are found in the pools in the early spring when the water is high. The 

 drying of the stream calls forth behavior suited to the conditions, 

 and in summer their burrows are common in the stream bed. They 

 come out at night and are preyed upon by raccoons, the tracks of which 

 are commonly seen. 



c) The homed dace, or permanent pool communities. — The first per- 

 manent parts are permanent pools. In these, conditions such as current, 

 sediment, oxygen content, etc., are intermittent or spasmodic. The 

 current in the rapids is distinctly spasmodic and conditions in these 

 rapids are similar to those in the stream before even temporary pools 

 were developed. Streams with permanent pools are represented in the 

 Chicago region by many which enter the lake where high bluffs are 

 present. County Line Creek (Figs. 24, 25) has been studied as an illus- 

 tration of this type (Table XVII). 



The larger pools possess a practically permanent fauna. The char- 

 acteristic forms are the crayfishes (Cambarus virilis and propinquus). 

 The young are to be found in the pools at all seasons of the year. Water- 

 striders, back-swimmers, and water-boatmen are common. Occasionally 

 one finds dragon-fly nymphs (Aeshna constricta and Cordulegaster obli- 

 quus), dytiscid beetles (Hydroporus and Agabus), crane-fly larvae, the 

 brook amphipod (Gammarus fasciatus), and the brook mores of the sow- 

 bug {Asellus communis) (Fig. 55, p. 98). These are common among the 

 lodged leaves. They move against water current. 



The species of fish (Table XVIII) which is most commonly found 

 in the smallest streams (92) and nearest the headwaters of the larger 

 streams is the horned dace or creek chub (Semotilus atromaculatus) (Figs. 

 33, 34). It possesses certain noteworthy physiological characters. Like 

 many other species of fish, it goes farthest upstream for breeding (50). 

 Its nest is made of pebbles. Often after the breeding season is over, and 

 the adults have gone downstream, the water lowers so that young fishes 

 are left in large numbers in small drying pools. Here they swim about, 

 with their mouths at the top of the water, which is constantly being 

 stirred up by the many tails, and which often contains much blackened, 



