LIFE IN PONDS AND STREAMS 



Its animal population is very much like that of ponds, es- 

 pecially if higher plants grow along its banks. Where the 

 bottom is sandy there are burrowing mayflies, mussels, snails, 

 and the big dragonfly nymph Cordulegaster which lies with 

 its hairy body buried in the sand. In the stretches of muddy 

 bottom are leeches, mussels, and, if the mud is firm enough, 

 little white clams of the family Sphceridce (Fig. 266). There 

 are innumerable midge larv^, a few of the never-failing 



Pj(, 23 —Diagram of the distribution of animals 

 in a waterfall. The stones A, B and C are shown 

 enlarged, with their inhabitants; A and C are right 

 side up ; B is wrong side up. i , egg masses of midges; 2, 

 water moss, Fontinalis; 3, midge larvae; 4, alga, Clado- 

 phora; 5, mayfly, Chirotenetes; 6, nets of caddis worm, 

 Hydropsyche; 7, Pupa cases of caddis worm, Helico- 

 psyche; 8, 9, mayflies, Heptagenia and Epeorus; 10, 

 planarian; 11, caddis worm; 12, water penny; 13. 

 leech; 14. stonefly nymph; 15, caddis worm; 16, 

 black fly, Simulium. 



21 



