Drain flies, or moth flies, are small, dark winged, non-biting gnats. Their wings are covered with scales so they disappear in a cloud of fine dust when swatted or mashed. These nuisance gnats can be found resting on walls or ceilings, and make short hopping flights if disturbed.
The key identifying character for the moth fly is the unique pattern of veins in its wings. The entire body and wings of the moth fly are covered with tiny hairs, giving it a moth-like appearance. To the naked eye, this tiny pest might appear to be a small fly with fat wings; the aid of a magnifying glass reveals the unmistakable moth-like appearance. This small fly is no more than 1/8 inch in length including the wings. They are usually black in color.
Moth flies lay eggs in a mass of 30 to 100. These eggs hatch in less than 48 hours. The larvae and pupae of the moth fly live in the thin film found in drains, septic tank field lines or filter stones. The larvae feed on sediment, decaying vegetation and microscopic plants and animals. The larval stage lasts from 9 to 15 days and the pupal stage lasts from 20 to 40 hours. The newly emerged adult fly is sexually mature on emergence and copulates within the first few hours of its life.
Drain flies often are a big problem. They develop in standing water. They may also breed in moist shady areas outdoors such as under potted plants, in bird feeders and baths, in moss, in clogged roof gutters, under air conditioners, in thick mulch, or on wet ground areas. The larvae feed on decaying material that collects in drains. In natural settings, moth fly larvae feed on decaying plants and animals. They breathe through a tube that helps them survive even when their environment is very wet.