Aspects of the biology of Psychoda alternata(Say) and P. severini parthenogeneticaTonn. (Díptera) in a percolating filter.
The following is based largely on the authors' summary. The population dynamics of two species of Psychodain a small experimental percolating filter similar to those used for sewage treatment were examined from the point of view of species balance, distribution by depth and the change in proportion of larvae and pupae over a year in south-east England. P. alternaia Say was generally the dominant species and was present throughout the year, with a peak in May. P. severini parthenogeneticaTonnoir increased considerably in May and June and declined abruptly in July. The two species occupied almost the same vertical distribution in the filter, the median depth of the population of P. s. parthenogenetica being slightly greater than that of the population of P. aliemata. The ratios of the numbers of larvae to numbers of pupae decreased in winter. The Psychodaspp. were found to be responsible for only 1% of the dissipation of carbon from the filter as carbon dioxide. However, they may assist other animals in protecting the filter against the accumulation of too much film and in increasing the settling of humus solids.