286 PISHES OF ILLINOIS 



quently also in the Rock River and its tributaries. It is com- 

 monest in northern Illinois and is least frequently found in the 

 southern part of the state. Like Per etna caprodes, it has been 

 taken by us most generally from the smaller rivers (3.39) and 

 from creeks (1.59), but only rarely from the largest rivers (.4) 

 or from lakes or sloughs (.2). It is preeminently a species of 

 swift water with a bottom of rock or sand, 94 per cent, of our 

 collections coming from the former situation and 90 per cent, 

 from the latter. 



It is further reported from Ohio, Iowa, Kansas, and Missouri 

 to Kentucky, Arkansas, and Oklahoma. 



Consistently with the relatively large size of this species, 

 larvae and pupae of May-flies were found by us to predominate 

 in its food, including one of the largest larvae of this family 

 (Hexagenia) in our streams. Larvae of dragon-flies, a small 

 percentage of Chironomus larvae, and water-bugs (Corixa), were 

 the other elements of its food. 



Females greatly distended with eggs were taken by us June 

 5, 1901. 



Fig. 68 

 HADROPTERUS ASPRO (Cope & Jordan) 



BLACK-SIDED DARTER 

 (Pi., p. 285; Map LXXXVII) 



Kirtland, 1839, Journ. Bost. Soc. Nat. Hist., 340 (Etheostoma blennioides). 



Cope & Jordan, 1877, Proc. Ac. Nat. Sci. Phila., 51 (Alvordius aspro — substitute 

 for Etheostoma blennioides of Kirtland, the name blennioides being preoccu- 

 pied in Diplesion). 



J. & G., 501 (Alvordius); M. V., 127 (Etheostoma); B., I, 59 (Percina); J. & E., I. 

 1032; N, 35 (Etheostoma blennioides); J. 39 (Alvordius maculatus); F., 65; 

 L., 27. 



Length 3 to 4 inches; bo'dy elongate, fusiform, somewhat compressed, 

 less cylindrical than in Percina; one of the most graceful and elegant in form 

 and color of all the darters; depth 5.4 to 6.8 in length; greatest width of body 

 about % of its greatest depth; depth of caudal peduncle 2.5 to 3.3 in its 



