CHAPTER X 

 DAPHNIA: A STUDY OF THE FOOD OF FISHES 



If one sweeps the long grass in a field with a stout, fine net, 

 one will gather in a minute or two great numbers of insects. 

 In this way one gets a notion of the vast abundance of insects 

 that there must he on all the land. Little wonder then that 

 vegetation suffers so from insect attacks or would suffer were 

 the number of insects not kept down by birds, lizards, and 

 toads which find their principal subsistence on them. 



What insects are to the land the water-fleas or Entomostraca 

 are to the water ; they are ubiquitous there. And what in- 

 sects are to the l)irds Entomostraca are to fishes. How numer- 

 ous they may become is shown when a fine silk net is drawn 

 through the water and the small Entomostraca are strained 

 out. By these means the volume of small organisms in one 

 cubic centimetre of water has been determined for various 

 localities. It appears, in general, that the organisms are more 

 numerous in lakes than in streams (Fig. 153). Thus in .studies 

 made in the middle Illinois River and adjacent lakes the 

 volume of organisms in 1 cu. metre of water was, for a 

 small stream, 0.3 of a cubic centimetre ; for a large river 

 (the Illinois) 3 cu. cm. ; for five lakes in Illinois, 2 to 23 cu. cm. 

 In one year the river (at Havana, Illinois) discharges 75,000 

 tons of microscopic organisms (Kofoid). In studies on the 

 small lakes of Wisconsin, Birge has calculated the number of 



Entomostraca in a cubic metre at different levels and at different 



153 



