349 



punctata, below the outlet of the lower lake at the P. P. U. Railway 

 Bridge. We have found no definite outside records of habitats of this 

 species. The conclusion is evidently to be drawn, with respect both to 

 this and the other species named, that they are of rather indifferent 

 value as an index" of the condition of muds in the Illinois River. 



MIDGE I,.\KVAE 



Larvae of midges occurred in oin- 1933 dredge-hauls from upper 

 Peoria Lake at only eight out of the total of twenty-three stations, and 

 failed altogether to appear at any of the channel stations. The total 

 number of identified kinds was the same as in the summer of 1930, 

 but three of the more pollutional species of that year were not taken 

 at all in 1923, and the single 1932 species known to be unusually toler- 

 ant (C. plumosus Linn., var.) occurred only at three extra-channel 

 upper lake stations, one each in the Chillicothe, Rome, and Spring Bay 

 cross-sections. Undetermined species of Chironomus, Procladius, and 

 Tanypus were taken at a few open lake stations well eastward of the 

 channel opposite Rome and Spring Bay; and a few specimens of Tanv- 

 pus monilis Linn., in the past reported only from cleaner situations, 

 were taken toward the west side opposite Rome, in a haul in which 

 tubificid worms exceeded 7,000 per square yard and unusually foul 

 odors "were noted. 



In the summer of 1930 all channel hauls contained midge larvae, 

 though the recognized more tolerant kinds then as in 1933 came from 

 the stations outside the channel. In the middle lake and in the lower 

 in 1922 larval Chironomidae occurred at a nuich larger projjortion of 

 all stations than in the upjjer lake, although no important increase in 

 numbers was noted at the stations southward. 



With the exception of C. plumosus or its varieties, and of two 

 other pollutional species (C. dccorus Johannscn and Tanypus dyar't 

 Coquillett) taken rather frequently in the summer of 1930, neither in 

 1933 or 1930 did numbers of chironomid larvae attain importance in 

 Peoria Lake (Table, p. 317). The first-named species does, however, 

 seem to bear an unusually distinct relation to pollution ; as has been 

 noted since some years ago in studies of its sister forms in Europe, 

 where variation in the length of its ventral blood-gills has been ob- 

 served to coincide more or less definitely with diflferences in the oxygen 

 sui^ply. The recent Illinois River and Peoria Lake specimens which 

 we have referred to this species do not seem to differ very materially 



