Bottom Fauna (continued)

Richardson finds the change in composition and abundance of chronomid (midges) populations from 1915 and 1920 to be more interesting even that of the mussels. Midges look like mosquitoes but do not bite and are often referred to as gnats (Figure 22). Like mosquitoes, the midge larval stage is aquatic. The adult females lay eggs in masses over open water or attached to aquatic vegetation. They hatch in several days and the young larvae drop to the bottom where they build tube-like structures of bottom debris held together by strands of silk. One interesting fact is that the blood of some midge larvae (called bloodworms) is red, unlike the blood of most other insects. It is red because it contains hemoglobin that binds tightly to oxygen. This is necessary because of the stagnant, low oxygen conditions that may be present where these larvae live.

Figure 22: Adult Midge (Gnat)

Richardson indicates that four new pollution-tolerant species of chronomids appear in the Illinois River between Chillicothe and Peoria: two are varieties of Chironomus plumosus L, C. matures, and C. frequens. In addition, four cleaner-water species cease to exist in the same section: Chironomus nigricans Joh, C. tentans F, Cricotopus sp, and Orthocladius sp.

Of greater interest are the changes in numbers of these species. In 1915, the average number of young midges taken per square yard of the substrate is 10; in 1920 this increases to 318 for the entire lake, and in the channel and a 4-7 foot zone of the upper lake the numbers jump to 1216 and 554, respectively. Table 6 provides greater detail as to the number and weight of the chronomids in this section of the river.

Table 6: Midge Larvae (Chironomidae) of Bottom Muds, Chillicothe to Foot of Peoria Lake, 1915 and 1920 (Richardson 1920)