FIELD BOOK OF PONDS AND STREAMS 



crustacean genus, Asellus (Fig. 131), and as in the ponds, 

 there are plenty of back swimmers and water boatmen. 



Life in the rapids (Fig. 23). — In rapids the water is always 

 falling and jostling, and every plant and animal must con- 

 stantly clamber, or cling, or glue itself to some holdfast. Water 

 pushes against the flattened stones tilting them so that even 

 when it is all dried out of the stream, they still tell which way 

 the current ran. 



Fig. 24. — Collecting basket of the mayfly, CJiiro- 

 tenetes. 



The green and golden-green films of desmids and diatoms 

 upon the stones, the green tufts of such algse as Cladophora 

 (Fig. 23), and the green- black water moss Fontinalis (Fig. 55) 

 are among the scanty plant population of the rapids. Of the 

 larger animals there are a few fishes, minnows and darters, 

 and agile two-lined salamanders which skulk under the stones. 

 Immature insects predominate — midges, stonefiies, caddis 

 flies, and mayflies, along with leeches, planarians, and a few 

 snails. 



Nearly all of these animals of the rapids live upon micro- 

 scopic food. Net-building caddis worms (Hydropsyche) 

 strain their food from the water in their cup-shaped nets 

 which open upstream (PI. XVI). The mayfly Chirotenetes 

 (Fig. 24) carries its own collecting basket to catch plankton 

 from the current. Facing upstream with tail and rear legs 



22 



