Within the subfamily Psychodinae are a number of "Drain Flies" that live in the overflow pipes of sinks, in the filter beds of sewage plants, and other such locations. Two of the most common are Telmatoscopus albipunctatus (pictured) and Psychoda alternata. The later species is sometimes referred to as Clogmia albipunctatus.
While Moth Flies in general have been neglected by researchers, Psychoda alternata in particular has been very well-studied because its larvae develop in sewage treatment plants, and can hinder the work of such plants and even endanger the health of workers there.
The Telmatoscopus albipunctatus pictured on this page was photographed in a bathroom in a college classroom building in West Virginia. The scientific name translates as "White-spotted dwarf of the pools."
In more natural settings the larvae of Drain Fly species develop in rotting hay, in rotting vegetables, in horse and cow dung, and in piles of seaweed.
Still, it is in sewage plants where Drain Flies have gained their notoriety. In a small way the larval flies may perform a service, cleaning the filtering area by eating algal scum there. On the other hand, their sheer numbers may have a clogging effect, and the sudden emergence of tens of thousands of adults can be dismaying to plant workers and to nearby residents. Some sewage plants have reported densities as great as 14,000 larvae of Psychoda alternata per liter of medium in the plant's filters.
Notes on differentiating two common drain flies, Telmatoscopus albipunctatus (pictured above) and Psychoda alternata:
Psychoda alternata is substantially smaller, at about 2 mm, and have on overall grayish color, with a few dark dots along the apical half of the wing. It is more likely to hold its wings tent-like. Telmatoscopus albipunctatus typically measures in at 3.3 to 4.6 mm, and has several dark dots primarily in the basal half of the wing, while the edges of the apical half have the white dots mentioned in the species name. It typically holds its wings out flat.
More information about Psychoda alternata:
Redborg, Hinesly, and Ziegler (1982) reared Psychoda alternata in the laboratory. They reported that eggs hatched within 24 hours of oviposition. The flies moved through four larval instars. The total developmental period from egg to adult was, for males, 11 days, and for females, 13 days. The longer development period for females is fairly common among insects.
Redborg and his co-authors attempted to induce cannibalism by pairing fourth instar larvae in a container without food, but in this experiment that larvae did not resort to cannibalism. In fact, the larvae seemed to engage in mutual care, or at least they "frequently wrapped themselves around each other in a 'pretzel-like' configuration and cleaned each other with their mouthparts."
Photo locations: Ruraldale, West Virginia (upper photo); Asunción, Paraguay (middle photo); Presqu’île de la Caravelle, Martinique (bottom photo).
American Insects site