440 



pollution commonly recognized, but the results have scarcely justified any 

 sharp lines of subdivision except that between foul-water and clean- 

 water species. The rather numerous tolerant species that can dwell in 

 a varying degree of contamination occupy an intermediate classification 

 that is discussed in some detail in its proj>er place. 



When a stream becomes polluted by sewage or chemicals, the most 

 sensitive and least tolerant species of plants and animals are soon elimin- 

 ated, being gradually replaced by more tolerant forms as the amount and 

 degree of pollution increases. The destruction of the clean- water insect 

 fauna is often compensated for by a marked increase in the number of 

 species and individuals of certain unusually tolerant or foul-water forms, 

 notably species of small annelid worms (Tubificidae) and larvae of midg- 

 es (Chironomidae) which, in places, form an important source of food 

 for the few species of tolerant fish that exist in such waters. Thus in 

 certain areas of both the Illinois River and the Salt Fork, almost the 

 only water-breathing insect life consists of larvae of a few species of 

 various genera of midges which are often extremely abundant. 



Sewage Entomology 



Considerable literature has been written upon the fauna and flora 

 of polluted water-courses and sewage-disposal plants. The entomology 

 of polluted streams exhibits certain general features which may be briefly 

 outlined as follows. 



The. septic or grossly polluted portions of a stream are those in 

 which the decomposition of organic matters is progressing actively, pro- 

 ducing an abundance of carbon dioxide and the earlier and more com- 

 plex nitrogenous decomposition products to the more or less complete 

 exclusion of dissolved oxygen. The organisms of this zone are those 

 which have been termed by Kolkwitz and Marsson (1900) polvsaprobic 

 and by Forbes and Richardson (1913) septic or saprobic. The lower 

 forms of plant life, especially Schizomycetes, are abundant. Insects are 

 very rare and include no water-breathing forms. The most conspicuous 

 insects of this zone, both in America and Europe, are the so-called "rat- 

 tailed maggots" — larvae of certain syrphid flies (Eristalis and Hclophi- 

 lus). Eristalis tcnax (Linn.) was the only insect recorded bv Kolk- 

 witz and Marsson (1909), Johnson (1914), and Suter and Moore (1922) 

 from this zone. It should be understood that this larva is air-breathing, 

 obtaining its oxygen supply through the spiracles at the tip of a long, 

 extensile breathing tube, the spiracular disk being held at the surface 

 while the animal feeds at will beneath. The range of oxygen content 

 usually selected by this species of Eristalis is stated by Suter and Moore 

 (1923) to be from to 40 per cent saturation. Other characteristic oi- 

 ganisms of this region are the sludge-worms belonging t>o the family 

 Tubificidae. The mud bottoms of the saprobic zone invariablv have an 

 offensive odor and are blackened by deposits of iron sulphide which churn 

 up in dark clouds when the bottom is disturbed. 



