LIFE IN PONDS AND STREAMS 



crustaceans, the females of the latter often bearing their egg 

 sacs with them. The number of different plankton organisms 

 rises and falls during the year; many species have seasons of 

 special abundance. Cyclops (Fig. 126) may predominate at 

 one time, and some other crustacean at another time, their 

 abundance paralleling the increase of one or more diatoms 

 which are their natiiral food. Plankton organisms have an 

 enormous reproductive capacity. This is especially true of 

 diatoms, which form a staple food crop of the water, compara- 

 ble to the grass crop on land. 



Larger animals of the pond. — Among the lily pads and the 

 water weeds of the shallows, lurk sunfishes, bullheads, mud 

 minnows, and young perch. All of these forage upon snails, 

 crustaceans, and insect larvae, especially the tempting mayfly 

 nymphs which they find there. Bullfrogs float with their heads 

 just out of water; of all frogs these belong most thoroughly 

 in the pond. Equally at home in it are the painted turtles, 

 and the spotted turtles often found with them (PI. XXIII). In 

 May and June stumps and floating logs usually carry a load 

 of one kind or the other. They forage in the shallows taking 

 a heavy toll of tadpoles, snails, dragonflies — a miscellaneous 

 bill-of-fare which they always eat under water. Snapping 

 turtles frequent these waters also, catching anything with- 

 in reach of the lightning-quick thrusts of their heads — fishes, 

 tadpoles, frogs, or crayfishes, as well as the smaller game of 

 insects and worms. 



Balance of life in the pond (Fig. 21). — In a pond everything 

 alive is good eating, and even dead and broken fragments do 

 not go unrelished. Swarming millions of tiny crustaceans 

 feed upon the algae; young fishes devour the crustaceans; 

 and older fishes, turtles, frogs, and even dragonfly nymphs 

 prey upon young fishes. Midge larvae and mayfly nymphs 

 consume the plant tissues, living and dead; larger insects 

 eat the little wormlike midges; and later, tiger beetles and 

 larger dragonflies feast upon their insect relatives, only to 



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