FIELD BOOK OF PONDS AND STREAMS 



be swallowed in turn by fishes and the other larger water 

 animals. 



In a particular year many or very few individuals of a 

 species may be found but seldom does any one species entire- 

 ly die out. At one time nymphs of dragonflies may dominate 

 the water while mayflies may be hard to find ; yet by another 



Fig. 21. — Food relations in the pond where large 

 animals eat the smaller ones: i, adult fish; 2, swarm- 

 ing crustaceans; 3, young fishes; 4, frog; 5, snail; 6, 

 dragonfly nymph; 7, midge larvae; 8, mayfly nymph; 

 9, tadpole; 10, painted turtle; 11, young frog. 



season mayflies may be gathered with every scoop of the net. 

 In one way or another the balance of life is maintained. 

 Dragonfly nymphs can clutch and bite while mayfly nymphs 

 can do neither; yet mayflies make up for their lack of weapons 

 by their ability to live upon plant food which they can find 

 almost anywhere and by the great number of young which 

 they produce. Some of the pond mayflies like CallilxEtis 

 (Fig. 158) lay eggs when they are less than six weeks old, 

 so that there is time for three or four generations of them to 



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