361 



WORMS 



Except for the irregular continuation of the decline in numbers 

 already noticed in the upper lake, and their failure to appear at all in 

 the collections opposite Al Fresco Park, the lubificid worms presented 

 little of interest in the middle lake in the summer of 1933. Substan- 

 tially the same list of species was found as in the upper lake, though 

 in all cases in much smaller average numbers and with more scattered 

 distribution. The various species of Limnodrilus, rather than Tuhifex 

 tubifcx, led as in the upper lake in frequency ; while the local form of 

 Tubifcx tubifex was found at only a single station in the channel oppo- 

 site the foot of Horshor Island. 



The absence of small annelids from the Al Fresco Park hauls is 

 not to be taken as conclusive evidence that they were not there al- 

 though noticeably improved conditions on the bottom were visible also 

 in other respects. It is more probable that if we had increased the 

 fraction of the Petersen dredge sample sieved for them at least small 

 numbers of some of the species would have been found. 



LEECHES 



Leeches were represented in the 1923 middle lake hauls by two 

 species, the same number as in the upper lake. One of them, Heloh- 

 della ncphcloidca (Graf), occurred in the wide waters of the Mossville 

 cross-section, and was also recorded in 1923 from the wide waters of 

 the upper lake above Spring Bay. The second, HelobdcUa stacpialis 

 (Linnaeus), although it occurred in the middle lake in 1933 only in 

 the lower or Al Fresco Park cross-section, was one of the two com- 

 monest leeches at the more heavily polluted stations of the Coweeset 

 just below Brockton, Massachusetts, in 1914 (Weston and Turner 

 191?). ErpobdcUa punctata (Leidy), which was taken in the channel 

 at Chillicothe in 1923 and was the other common leech at the upper 

 Coweeset hauls in 1914, was not found in middle Peoria Lake collec- 

 tions in the season of 1922. 



MIDGE L.\RVAE 



While a total of six species of immature midges were recorded 

 from the middle lake in the summer of 1922, compared with five in 

 1920, the more distinctly tolerant or pollutional kinds were reduced 

 in 1922 in the middle as in the upper lake from three to only one. The 



