Food ItcldtioH* of Fresh-Water Fishes. 503 



ora and Potamogeton; while the same number of bull-heads 

 (Aminrus nebulosus) had derived thirty-seven per cent, of 

 their food from insects, and sixty-three per cent, from mol- 

 lusks. The difference here was substantially a larger ratio of 

 mollusks for Amiurus, replacing the vegetable food of the 

 Ictalurus group. By a comparison of these differences with 

 those detected between the species at large, as explained on 

 pages 456-461, it will be seen that the former do not represent 

 the specific differences in food, but simply give evidence that 

 two species may be differently affected by the same conditions. 



Other specific differences in the same genus are shown by 

 the collections made Oct. 27, 1875, from Peoria Lake. Eight 

 examples of the wall-eyed pike(Stizostedion vitreum) had eaten 

 only soft-finned fishes, excepting one small sunfish, while 

 four of ten specimens of the related species S. canadense^ had 

 eaten spiny-finned fishes, and in only three were 4he fishes 

 recognizable as belonging to the soft-finned species. Three 

 specimens of Micropterus taken with the above had eaten cray- 

 fishes and fishes (including a catfish). 



Among my specimens of the sucker family (Catostomatidae), 

 a lot obtained at Quincy, Aug. 25, 1887, are comparable for the 

 present purpose. Four examples each of Ictiobus urus and /. 

 cyprinella presented a decided contrast with respect to the 

 elements of their food, that of /. urus consisting almost wholly 

 of Chironomus larvae, with large quantities of dirt, while three 

 of the specimens of /. cyprinella had eaten scarcely anything 

 but Algae, ninety per cent, of the food of the fourth being 

 Chironomus larvae, and the remainder, larvae of Neuroptera, 

 Hexagenia and Corydalis. 



On the other hand, two small collections of the same 

 species made at Peoria, Oct. 9, 1878 four of I. urus and five 

 of I. cyprinella exhibit similar food, composed chiefly of 

 Entomostraca, Chironomus larvae, distillery waste (meal, etc.), 

 and aquatic vegetation. The urus group alone had eaten Ento- 

 mostraca, these being replaced in the other by a larger quantity 

 of meal. 



The facts above recited are evidence that fishes are not 

 mere animated eating-machines, taking indiscriminately and 

 indifferently whatever their structures fit them to capture, to 



