350 



ANIMALS IN INLAND WATERS 



much-diminished length of the anterior bristle, and the pond form of 

 Anuraea cochlearis 72 differs from the lake form by having a brownish 

 yellow color and a shorter posterior bristle. Daphnias are considerably 

 larger in the smaller waters than their relatives in the lakes. 73 The 

 size of Diaptomus laciniatus, 74 on the other hand, increases with the 

 size of the body of water in which it lives; it remains very small in 

 small mountain lakes in spite of a plentiful food supply, but reaches 

 its maximum length in larger lakes of the plains which have a pro- 

 nounced, though less rich, plankton area. The pond form of the gnat 

 larva, Corethra plumicornis,' 15 can also be distinguished from a lake 

 form (Fig. 102); the pond form is plumper, larger (15:11 mm.), of 

 dull coloration in contrast with the very transparent lake form, and 



Fig. 102. — a, Larva of Corethra plumicornis, with its tracheal air sacks in 

 black; b, head of pond form; c, of lake form; d, anal fan of pond form, e, 

 of lake form, a, after von Frankenberg; b-e, after Wesenberg-Lund. 



has a larger head with a short proboscis, larger eyes, plumper tracheal 

 bladders, and a larger tail fan. 



The fact that among the daphnias of pools the males appear more 

 often, and fertilized eggs are more frequently formed, than among 

 lake daphnias, depends upon the greater variations of temperature 

 in the smaller water basins or upon phenomena associated with crowd- 

 ing 70 or both. 



The animal communities of bodies of water which are not perma- 

 nent — the ephemeral water basins — show marked uniformity in spite 

 of the great variation in size and contour. These ephemeral waters 

 usually exist periodically and disappear at more or less definite times ; 

 they are less often of entirely accidental nature. From the rain pud- 

 dles and clay pits up to the widespread South African "pans," there 

 is a complete series of gradations of such rain ponds whose existence 

 depends solely and directly on precipitation ; with its absence they are 

 dry for long periods of time. Aquatic animals find similar environ- 

 ments in flood areas of rivers and rice fields, and, on the other hand, 





