382 



species of the genus Cricotopus ; and a species of the genus Ortho- 

 cladius. 



Several unidentified species of at least three families of dragon- 

 flies, whose nymphs have no green or other bright color*, are for the 

 present admitted to the list of bottom forms that have disappeared 

 from Peoria Lake' since eight or ten years ago although some members 

 of this group have recently been found to be unusually tolerant. 

 Stickney (1922) found Libellida pulchcUa Drury very tolerant to con- 

 ditions of low oxygen in recent experiments at Urbana ; and Baker 

 (1923) took the same species in the comparatively foul muds of the 

 Big Vermilion at St. Joseph, where all the mussels had been killed. 

 The writer took Chromagrion condittini Hagen and Platythcmls lydia 

 Drury in the; Kishwaukee River below De Kalb in February 1922 in 

 water polluted by acid wa.stes from steel mills to such an extent that 

 most of the normal bottom fauna had been driven out or destroyed. 

 The disappearance of nymphs of this group recently in the Peoria 

 district may have been more in the nature of an indirect effect of the 

 destruction of the vegetation, which served as headquarters for the 

 adults, than evidence in all cases of unusual sensitiveness. 



* Some green nymphs of the family Airrionidae have been excluded, as weed 

 species, from the present comparison. 



