Table 4 



Average Density of Benthic Organisms (Number Per Square Meter) 

 in the Main Channel of the Lower 80 Miles of the Illinois River 

 in 1915 and 1974 



1915 c 



Turbellaria (flatworms) 0.6 



Hirudinea (leeches) 



Oligochaeta (worms) 3.1 37,0 



Gastropoda (snails) 0.3 



Pelecypoda (clams) 



Sphaeriidae (fingernail clams) 2.4 3.5 



Corbiculidae (Asiatic clams) 9.8 



Unionidae (mussels) 0.1 



Insecta (insects) 



Odonata (dragonflies) 0.1 



Ephemeroptera (mayflies) 2.3 41.3 



Coleoptera (beetles) 



Trichoptera (caddisflies) 6.6 



Diptera (midges) 0.6 53.8 



Richardson, 1921a: 490-493. Fourteen samples were taken in August 

 between river mile 80.0 and 0.0. Averages reported for three reaches 

 of the river between mile 0.0 and 80.0 were weighted according to sam- 

 ple size and then averaged. 



Colbert et al. , 1975: G15-G20. Six samples were taken in September 

 between river mile 81.0 and 2.5. 



24 



