442 



streams unpolluted by man, and the waste organic nitrogen has been more 

 or less completely mineralized. The rate of consumption of dissolved 

 oxygen is slow, and is often more than balanced by mechanical and biolog- ' 

 ical re-aeration. The water has a slightly alkaline reaction and is at low 

 stages highly transparent, unless colored by algae. A great variety of 

 both plant and animal life occurs, but characteristic saprobic forms are, 

 at least in large numbers, lacking. Members of the Characeae. very sensi- 

 tive to pollutional conditions, begin to reappear where physical conditions 

 are suitable and in their proper geographical range. The gradual increase 

 in the amount and variety of light green algal forms, replacing the blue- 

 greens, further indicates a clearing up of the water. From this region 

 Kolkwitz and Marsson record a rather extended list of insects belonging 

 to the following groups: Plecoptera (4), Ephemerida (6), .\nistoptera 

 (2), Zygoptera (2), Trichoptera (7), Hemiptera (8), Diptera fSayo- 

 myia (1), Coleoptera (6), Dytiscidae (4), Gyriiiidae (1), Hydrophilidae 

 (1). The Plecoptera and Ephemerida have in the past experience of the 

 writer and of other American authors been usually associated with the 

 best aerated and most nearly pure water in this zone. There is often some 

 invasion of the freer-ranging cleaner water species into the mososaprobic, 

 beta, or contaminate zone, especially after heavy rains, these species often 

 including higher forms of aquatic life, such as fish. 



The above remarks apply chiefly to sewage in streams. A rather sim- 

 ilar condition obtains in the disposal of organic matter in sewage-disposal 

 plants. Here the principal insect involved is the sewage-fly, Psychoda 

 altcrnata Say, which sometimes occurs in vast numbers. The gelatinous 

 or amorphous film (milk of aluminum) which is used in the filters sup- 

 ports a myriad of lower organisms and it is upon these forms that the 

 sewage-fly lives. It apparently does no harm in the filter-plant, but when 

 it becomes numerous, as happens in some of the larger eastern cities, as 

 Baltimore, the adult flies become a serious nuisance to housewives in the 

 vicinitv. This subject has been discussed in detail bv Headlee and Beck- 

 with (1918) and Headlee (1919).— see p. 460. .' 



Description of the St.ations made in the 1921 Survey 

 OF THE S.\LT Fork of the \"ermilion River 



As mentioned above, a total of thirty-three stations were established 

 during the progress of the survey. The location of these stations, with 

 lists of the insects occurring at each, is given below. 



The Salt Fork above the entrance of the Urbana and Champaign 

 sewers is a small, clean-water stream. Beyond the mouths of these 

 sewers for a distance of several miles, the streaiu has been artificially 

 straightened and deepened into what is termed the Drainage Ditch, a 

 distance of approximately thirteen miles. This ditch continues to just 

 west of the village of St. Joseph. The condition of the water in the 

 drainage ditch is very bad, with no water-breathing insects pre.sent. 

 About a mile west of St. Joseph, or a short distance above (west of) the 



