72 



two or more species comparable as to size and range had been taken at 

 the same time and place to afford a tolerable average of the food under 

 local conditions, yet a sufficient ni'.ni])er of such cases was found to give a 

 considerable amount of evidence on this point. 



Thus three specimens of the marbled cat. Ainiurus marmoratti.", taken at 

 Peoria. Nov. 1, 1878, had derived nine-tenths of their food from Hexagenia 

 larvfr. the remainder consisting of leeches and a few spiders: while eight 

 specimens of the large-mouthed black bass, Mkrojfierus mlmoidex, taken at 

 the same time and place, had eaten nothing but the young gizzard shad 

 iDorosoma). 



Comparing the food of four examples of the channel cat {Ictabinis puncta- 

 tus) with seven croppies (Pomoxys). both taken at Peoria, April 10, 1878, I 

 found that aquatic insects made ninety-eight per cent, of the food of the 

 latter, seventy per cent, being Hexagenia larvte, while only sixty-two per 

 cent, of the food of the catfishes consisted of insects (ephemerid larvae 

 twenty-eight percent.), the remainder consisting of vegetation and scraps 

 of dead fishes. 



A contrast equally decided is shown by three specimens of the gizzard 

 shad (Dorosoma) and four of the rock bass {AmhlopUte.'^ rupesti-is), all ol>- 

 tained at Ottawa, July 8, 1879. The former had swallowed large quanti- 

 ties of fine mud containing about twenty per cent, of minutely divided 

 vegetable cUbris. while the latter had fed wholly upon insects, fishes, and 

 crayfishes— the first chiefly aquatic larva\ 



Even in the shallow muddy pools left behind in the retreating overflow 

 of the Mississippi in southern Illinois, fishes of the same size but differing 

 widely in alimentary structures exhibit corresponding differences in the 

 selections made from the meager food resources of their localities. Two 

 of the common blunt-jawed minnows {Hiihognathus michali.-i) had fed here 

 almost wholly upon mud mixed with Alga' and miscellaneous vegetation ; 

 while three of the little pirate perch (Aphredoderus) had eaten little but 

 Chironomus larva\ half the food of one of the specimens l)eing wholly 

 small fishes, and insignificant quantities of Entomostraca occurring in the 

 stomachs of the others. 



A small collection, made from the Little Fox river, in TVhite county. 

 in southern Illinois. Oct. 5. 1882, of four specimens each of Labidesthes 

 and Zygonectes notattis enables us to bring into comparison the food of two 

 extremely different species taken together from the same pools in a run- 

 ning stream. The Labidesthes, although predaceous in habit and feeding 

 most commonlv upon Entomostraca. was here giving its attention wholly 

 to terrestrial insects — more than two-thirds of them winged Chironomus: 

 while the Zygonectes had eaten in addition to thirty-seven per cent, of 

 terrestrial insects (scarcely any of them Chironomus imagos). about thirty 

 per cent, of aquatic vegetation, nine per cent, of Entomostraca. eleven 

 per cent, of aquatic insects, and fourteen per cent, of mollusks. These 

 differences in food have no apparent relation to the essential structural 

 differences of the species, but must be considered an illustration of the 

 various effect of like conditions wlien applied to different species. 



On the other hand, three bull-lieads (Amiurux nchidosus) and six common 

 perch (Perca) taken from Fox river, at McHenry, May n, 1880, did not 

 differ remarkably in food, both groups having eaten crayfishes, mollusks. 

 aquatic insects and vegetation. One of the catfishes had taken another 

 fish and one had eaten leeches. It is to be noted, however, that these 

 species are both bottom feeders, and that both lots of these specimens 

 had taken about the average food of their kind.* 



The above are exami)les of the food relations of fishes widely separated 

 from each other in the classification, and decidedly different in alimentary 

 structures and in feeding habits, llhistrations of the differences in food 

 apparent in species allied in classification but differing with respect to the 

 structures concerned in tlie appropriation of food are given by the follow- 

 ing exami)les. 



*See Bull. 111. St. Lab. Nut. Hist.. Vol. I.. No. 3, p. 35. 



