280 FISHES OF ILLINOIS 



Briefly described, the typical species feed on insect larvae 

 commonly abundant on the bottoms of streams, under or among 

 stones, and in other similar situations, the smaller species eating 

 mainly dipterous larvse (most commonly Chironomus) , together 

 with a smaller proportion of neuropterous larvae of the smaller 

 sizes; while the larger darters eat essentially the same kinds of 

 food, but in different ratios, the neuropterous larvae being of 

 larger average size and also making a larger part of the food. 

 The two exceptional species studied, Per etna caprodes and 

 Microperca pundulata, have deserted in great measure the usual 

 situations of the darters, and are frequently found among weeds 

 and algae in comparatively quiet water with a muddy bottom, 

 the others being much more closely confined to swift and rocky 

 shallows. Consistently with this difference, these two widely 

 unlike species agree in their choice of food, which in both consists 

 largely of Entomostraca or other small crustaceans. The larger 

 species had also eaten a few small mollusks (Ancylus) . 



Where a group of species has become assimilated by a 

 similar adaptation to a common class of situations, and has 

 thus become a definite ecological assemblage, those in which the 

 adaptive processes have gone farthest are, of course, most likely 

 to be limited to the characteristic situation — are most likely, 

 consequently, to be taken by the collector in each others' com- 

 pany. By applying this rule to an analysis of the collections 

 of darters made in Illinois, we find that the most typical species 

 obtained by us in any considerable number are the following 

 six, mentioned in the order of the relative frequency of their 

 associate occurrence in our collections: Hadropterus phoxo- 

 cephalus, Etheostoma zonale, Etheostoma flabellare, Hadropterus 

 aspro, Ajmnocrypta pellucida, and Etheostoma cceruleum. Ap- 

 parently the least stringently connected with their kind by the 

 associative relationship are Diplesion blennioides, Etheostoma 

 jessicB, Boleosoma camurum, and Boleichthys fusiformis. The 

 species of the second list will presently be seen to be those which 

 have wandered widely from the common field of the subfamily, 

 and which are consequently found most frequently in situations 

 to which the other species rarely resort. Furthermore, in 

 separating themselves from their fellows in respect to local 

 distribution, they have not, as a rule, approached each other, 

 but remain as loosely affiliated ecologically among themselves 

 as they are with the more typical members of the group. A 

 notable exception to this statement is found in Boleosoma 



