Abstract
NUISANCE often arises from the emergence of large numbers of flies from percolating filters treating sewage, situated near dwelling-houses or busy thoroughfares. Species which have caused most trouble are Anisopus fenestralis, Psychoda alternata and Psychoda severini. Various methods of control have been tried, including treating filters with creosote or bleaching powder, and flooding the filters with sewage; flooding can only be used, however, where the construction of the filter is suitable. The toxic substances so far used are non-selective in their action; the concentrations necessary to kill fly larvae also kill other members of the valuable population of scouring organisms, such as Collembola and enchytraeid worms, as well as bacteria and fungi, which form the active biological film on which the purifying efficiency of the filter depends.
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