412 



The notably small size of the list of missing species of the old 1913- 

 1915 fauna of Peoria Lake, on the other hand, particularly when it is 

 considered that the number of collections taken in the earlier period. 

 46 in all, was more than four-fifths as many as supplied the abundantly 

 varied species lists from the reach Liverpool-Havana, seems at first prac- 

 tically unexplainable unless we assume that Peoria Lake had already 

 been injured by the sewage of Chicago and up-river towns still earlier 

 than that time. Conspicuous among the missing forms in Peoria Lake 

 collections in 1913-1015 were various aquatic insects, particularly those 

 belonging to groups other than the Diptera: as Odonata, Hemiptera. 

 Neuroptera, and Coleoptera. Though the shallower areas at that time 

 were abundantly su]3plied with aquatic plants it was noticed that results 

 even of "weed tank" collecting were extremely poor, also, as compared 

 with results obtained in the same way in the shallow lakes at Havana. 

 But though aquatic insects other than Chironomidae were rare, the rela- 

 tively sensitive Viviparidae and Amnicolidac, that have since disappeared 

 as far south as Beardstown, were still holding on. though not in nearly 

 as great numbers as near Havana. And as late as 1912-1914 Danglade 

 in work for the U. S. Fisheries Bureau found more than forty kinds 

 of mussels between Chillicothe and Peoria Lake, of which twenty kinds 

 were counted in the piles picked over at Chillicothe. Evidently, if there 

 was any injury previously to 1913-1915 it must have been principally in 

 the wide waters — the mussels are for the most part of the channel or 

 near it— and would point to certain groups of aquatic insects as even more 

 sensitive than the recently exterminated Gastropoda. 



Of the "missing" lists from the reaches distinguished below Peoria 

 Lake it may be noted that the first one, that between Wesley and the 

 Copperas Creek Dam, was very probably quite as early as 1913-1915 in- 

 fluenced somewhat unfavorably by the heavy load of Peoria and Pekin 

 wastes ; as well as by the limiting influences on the variety of small 

 bottom animals of the swifter current and harder bottoiy. Li the latter 

 respect the section of river between Havana and Beardstown was simi- 

 lar, to a large though not to ([uite so great an extent, perhaps, if the 

 slack water pool above the mouth of the Sangamon be taken into account. 

 That, and the greater distance from the Peoria and Pekin drains, favored 

 a somewhat greater variety of cleaner-preference forms ten years ago 

 than we could have expected between Peoria and the Copperas Creek 

 Dam ; though far from that which seemed to develop so easily in the lake- 

 like section of ten or so miles just above Havana. 



