52: 
The extent of the increase in bulk and weight of Chironomidae in 
collections since 1915 (from a negligible quantity to 164 pounds per acre 
in the channel and from a similar figure to 45 pounds in the 4—-ft. 
zone) is greater even than the increase shown by either the channel or 
all-zone figures for the Chillicothe—Peoria district. The percentage, by 
weight, contributed by Chironomidae to average total hauls in this dis- 
trict while standing practically at zero in 1915, reached 68.3 per cent. in 
the channel and 51.7 per cent. in the 4—7-ft. zone in 1920, these ratios 
comparing with 99.5 per cent. and 98.7 per cent. for snails and less than 
2 per cent. for all other small bottom-animals during the period covered 
by the earlier valuations (table, p. 53.) 
Sludge-worms, which showed a perpendicular rise from none at all 
in this section of river in 1915, were still far under the numbers and 
weight valuations recently reached in Peoria Lake, the average number 
per square yard at channel stations, where the largest quantities were 
taken, being less than 100, and the average valuation not over three 
pounds per acre. These figures compare with over 170 pounds per acre 
in the channel of upper Peoria Lake in 1920 and with over 26 pounds per 
acre as the all-zone average of the Illinois River and its wide waters be- 
tween Chillicothe and south Peoria. The small oligochaetes taken be- 
tween Liverpool and Havana in September, 1920, apparently belonged 
without exception to the Tubificidae, the commonest form being an un- 
determined species of the genus Tubifex. 
As in the Chillicothe—Peoria reach, the miscellaneous minor groups 
of small bottom-animals all appeared in 1920 collections in this section 
in greatly reduced numbers and variety when they appeared at all. Insect 
families other than Chironomidae that were common in 1913—1915, but 
that seem to have disappeared altogether outside the 3- or 4-foot line 
since the latter year, include Corixidae, two species ;. Phryganeidae, species ° 
of Hydropsyche and other genera; Ephemeridae, species of Hexagenia, 
Heptagenia, and Caenis; Gomphidae, Libellulidae, and other families of 
Odonata; and Parnidae, as represented by adult beetles of the genus 
Stenelmis. Other small bottom-animals that have dropped out of the 
channel and 4—?-foot zone are the little amphipod Hyalella knicker- 
bockeri, Asellus aquaticus, planarians of several kinds, as well as several 
kinds each of leeches, fresh-water sponges, and Bryozoa. The average 
weight-valuation of this group in 1920, 6 pounds per acre in the channel 
and one pound per acre in the 4—7-ft. zone, stands for comparison with 
24 and 26 pounds in 1915, with the 1920 poundages largely made up of a 
single species of leech. 
