441 



The polluted or mesosaprobic zone represents the next step toward 

 purification of the waters. A variety of higher water-plants may exist in 

 this zone and there may be considerable amounts of dissolved oxygen 

 present. The natural physical re-aeration of the water and the oxygen 

 given oflf by the higher plants or the more tolerant phytoplanktonts per- 

 mit the existence of a much larger and more diversified fauna than 

 was fovmd in the saprobic zone. To this zone Kolkwitz and Marsson 

 gave the name mesosaprobic, and further divided it into the sub-zones 

 alpha, more strongly polluted, and beta, less strongly ])olluted. The two 

 subdivisions of this region correspond in a general way to the poUutional 

 and contaminate zones of Forbes and Richardson (1913). The fermen- 

 tations that consume oxygen have become less here and the decomposi- 

 tion of the nitrogenous products has progressed to the stage where free 

 amomnia usually predominates, though amounts of both ammonia nitro- 

 gen and dissolved oxygen are subject to change in a wide range; Kolk- 

 witz and iMarsson (1909) record eight in.secls from their alpha zone, 

 and nine from their beta zone. Of the alpha forms, all but three are air- 

 breathers, these being Stratiomyia chamaelcon L., Psychoda phalacnaidcs 

 L., P. sexpunctata Curt. {'=al'ternata Say), Ptyclwp'tera contamiuata L.. 

 and Vclia currcns Fabr. An orl-fly, Sialis liitaria L., and the two midges 

 Chironomus plnmosus L. and Tanypus monilis L. are water-breathers. 

 From the beta sub-zone all but one of the species are water-breathers. 

 The air-breathing species is Cnlcx aiinuhita F. ; the water-breathing forms 

 are caddis-worms (Trichoptera, 4) and Diptcra (1). From this same 

 zone Johnson (1911) records three species of Diptera only, these being 

 Chironomus phimosus L., Psychoda phalacnoides L., and P. sexpunctata 

 Curt. (=alternata Say), all air-breathing species. Suter and Moore de- 

 scribe the sewage-fly {Psychoda alternata Say) and the sewage crane- 

 fly (Erioptera sp.) from conditions which belong to this zone. The large, 

 brick-red larvae of the phantom crane-fly, Bittaeomorpha clavipcs 

 (^Fabr.), were found by Weston and Turner (1917) to be important 

 factors in the reduction of sewage in the Coweeset stream near Brock- 

 ton, Massachusetts, and to belong to the mesosaprobic zone. In the Salt 

 Fork, the writer found a considerable number of insects which would 

 seem to find their place chiefly in the beta mesosaprobic or weakly pol- 

 lutional zone. This list includes a variety of water-scavenger beetles (Hy- 

 drophilidac), belonging to the genera Tropistcriius (2), Hclophorus 

 (1), Enochrus (1), Paracymiis (1), Berosus (2), almost all adult air- 

 breathing beetles but including also many larvae. The occurrence of the 

 immature stages of these beetles and others listed in the fir.st section of 

 this ])aper, together with water-breathing nymphs of a May-fly, Callibactis 

 sp., in company with large numbers of sludge- worms (Tubificidae), sug- 

 gests that the insects are more than ordinarily tolerant of pollution. 



The part of a stream lying between the mesosaprobic lower limit and 

 that of the cleanest zone normal to rivers has been called by Kolkwitz and 

 Marsson oligosaprobic, and by Forbes and Richardson the zone of clean 

 water. The water here is regularly of the normal jnirity characteristic of 



