Psychodidae, called drain flies, sink flies, filter flies, or sewer gnats is a family of true flies. Some genera have short, hairy bodies and wings giving them a “furry” moth-like appearance, hence one of their common names, moth flies. Members of the sub-family Phlebotominae which are hematophagous (feed on blood) may be called sand flies in some countries, although this term is also used for other unrelated flies.
There are more than 2,600 described species worldwide, most of them native to the humid tropics. This makes them one of the most diverse families of their order. Drain flies sometimes inhabit plumbing drains and sewage systems, where they are harmless, but may be a persistent annoyance.
The larvae of the subfamilies Psychodinae, Sycoracinae and Horaiellinae live in aquatic to semi-terrestrial or sludge-based habitats, including bathroom sinks, where they feed on bacteria and can become problematic. The larvae of the most commonly encountered species are nearly transparent with a non-retractable black head and can sometimes be seen moving along the moist edges of crevices in shower stalls or bathtubs or submerged in toilet water. The larval form of the moth flies is usually between 4 and 5 mm (0.16 and 0.20 in) long, and is shaped like a long, thin, somewhat flattened cylinder. The body lacks prolegs, but the body segments are divided into a series of rings called annuli (singular is annulus). Some of these rings will have characteristic plates on the dorsal side. The larval thorax is not significantly larger than its abdomen, giving it a more “worm-like” appearance than that of most aquatic insect larvae.
In some species, the larvae can secure themselves to surfaces of their environment using “attachment disks” on their ventral side. Like mosquito larvae, they cannot absorb oxygen through water, and instead breathe via a small dark tube (a spiracle) on their posterior end — they must regularly reach the surface to obtain oxygen. The larval stage lasts for between 9 and 15 days, depending on species, temperature, and environment. There are four instar stages.
In small numbers, the larvae are sometimes considered beneficial, as their strong jaws can cut through the hair and sludge waste in drains which might otherwise form clogs. However, unless this sludge layer is removed entirely, the adult flies will continue to find it and lay more eggs.
While the biting midges also have larvae that have no prolegs and which also have attachment disks, the larvae of the netwinged midges can be distinguished from those of the moth fly by the multiple deep lateral constrictions of the latter.
The pupal stage lasts between 20 and 40 hours. During this stage, the insect does not feed, but stays submerged near the water surface, still breathing through a spiracle, and soon metamorphoses into an adult fly, which bursts through a seam in the pupal casing and emerges onto the water’s surface.