401 



abundant in collections from near the lower end of the middle lake, where 

 snails and some of the other less tolerant groups were taken in variety. 

 Incompleteness of the determinations has also jiroljably included among 

 the Chironomidae, which are in the following tables all listed as at least 

 mildly tolerant, some species which are essentially cleaner water forms. 

 But if that is the case, it does not probably invalidate the comparison 

 of 1953 species lists with those of 1922 for evidence of increase in 

 cleaner-preference species, since most of the 108;5 larvae seem to have 

 counterparts in the 1!)22 (or 1920) collections. 



Study of the dates of collection of this group in the summer of 1933 

 shows that from IMossville north decidedly more Chiroiiomidae occurred 

 in July and August than in June collections; while in the lower half 

 of the middle lake the largest ntmibers were recorded in June in one 

 cross-section (opposite Long Shore Beach) and in July or August in 

 two others (opposite Towhead and Al Fresco). Cbironomus pliiiiiosus 

 var., a supposed ])ollutional species, occurred in all three months, and C. 

 dcconis. another with a similar record, occurred both in June and August. 



On the face of things to be taken as evidence of improvement in 

 the muds since 1922 would seem to be the sizable increases shown in the 

 twelve-month period in all three of the sections of Peoria Lake in both 

 the total numlier of bottom species of all kinds taken and in the number 

 of group 11* kinds (i. "e., current-loving snails, young mussels, other 

 insects than Chironomidae, and miscellaneous associated species such 

 as sponges. P>ryozoans. Planarians, etc.). Combining all collections from 

 all zones, and taking account of these gross figures only, without regard 

 to ])Ossible special or localized conditions at points of e.xact occurrence 

 of grouj) II species, it is found that in the upper lake in 1923 there 

 were 5 group II kinds taken as compared with one in 1922 ; in the middle 

 lake T as compared with none at all ; and in the lower lake, including 

 Peoria Narrows and P. P. U. Bridge in 1922 and those two stations 

 and the McKinley Bridge in 1923, 11 kinds as compared to 7. Corre- 

 spondingly, of the total number of all kinds taken, it is found that it 

 rose in the upper lake from ]S to 24-2.") in the year; in the middle lake 

 from 23 to 29-30; and in the lower lake from 21 to 24. Since the total 

 number of collections in all zones combined was very nearly the same in 

 all three lakes in 1922 and 1923 (U. L. : 1922, 23; 1923, 21; M. L. : 

 1922. 30; 1923. 30; L. L. : 1922, 18; 1923, 24). at least the changes in 

 the totals would not be expected to have been due to changes in thorough- 

 ness of collecting. Further favoring improvement, particularly in the 

 extra-chaimel zones of the upper and middle lakes, where the greatest 

 increase in group II s])ecies occurred, would appear to be the fact that 

 the increases in the number of species in those areas in both lakes took 

 jilace in the face of decreases in the number of collections between 1922 



• In text and t.ibulatlons that foUow. the No. II (troup l.s .set over aealn8t 

 group I. made up of the m<»re or less delinitely poUutfcinal or tolerant groups of 

 tubincid norms, leeches, midge larvae, and .sphaerlld or other snails which have 

 ".mood the racket" in I'eorla Lake in the last few years. 



