373 



and upper lake figures and with the lower lake figures for 1920, which 

 for unknown reasons averaged even less than those of 1928, is shown 

 in the table on page 3-l~ preceding. 



LEECH F.S 



Four species of leeches were taken in the lower lake in the sum- 

 mer of 1922, which was more than were found in either the middle 

 or upper lake in 1922 or in the lower in 1920. These four species in- 

 cluded Erpobdella punctata and Helohdclla stagnalis, both of which 

 have been recently listed from outside of Illinois in polluted bottom; 

 Helobdclla nephcloidca. which was taken in the upper lake above 

 Spring Bay in 1922 where odors were bad and bubbles were abundant; 

 and Dina parva, of which we have no local or outside records from 

 foul bottom. Erpobdella punctata was found only in the mixture of 

 local sewage and lake water at the P. P. U. Bridge, below the foot of 

 the lake, that and the occurrence in mid-channel at Chillicothe being 

 the only records obtained anywhere in the summer of 1922. With a 

 single exception, occurrences of iiie other three species were limited 

 to the Fulton Street cross-section, where leeches were especially com- 

 mon in the suminer of 1922 both in channel and extra-channel collec- 

 tions. 



MIDGE LARVAE 



Though average abundance of midge larvae did not go as high in 

 the lower lake as in the upper and middle lakes in the summer of 

 1922, both the total number of species identified and the number known 

 to be more than usually tolerant was greater. The three pollutional 

 forms included the common one from the upper and middle lakes, 

 Chironomns pUimosus, var. as well as also typical C. plumosus, which 

 was taken once opposite Fulton Street ; and C. nmtiirus Johannsen, 

 which was taken at two hauls in the cross-section opposite Workhouse 

 Point. In addition there were upwards of five or six kinds of Chiro- 

 nomidae of more or less doubtful position, two of them apparently less 

 tolerant than the three above mentioned. These two were Cliirono- 

 mtis digitatus Malloch, which was also taken in the middle cross-sec- 

 tion in the middle lake in 1922, and which occurred in clean iriuds in 

 Lake Mendota, studied by Muttkowski in 1918; and Tanypiis uioiiilis 

 Linnaeus, found once in the Fulton Street cross-section, well to the 

 eastward, and except for its occurrence not far from mid-channel 

 opposite Rome in August 1922, not known to us to favor bad bottom. 



