Moth flies resemble tiny gray to blackish moths. They are 1/16 to 3/16 inch long with a single pair of fuzzy wings that together are heart shaped. The antennae have 12 to 16 segments and a hairy appearance. Larvae are as long as 1/8 to 3/8 inch and without eyes or legs. Tapered at each end, larvae are striated in dark brown and cream on their dorsal side. They also have an apical breathing tube, which is also dark.

Moth flies lay their eggs on the gelatinous film in drainpipes and sewage lines and on surfaces in sewage treatment plants. Larvae feed on bacteria, fungi, and protozoa that live in this gel and also pupate there. Adults are attracted to lights and will rest on walls near breeding sites until evening when they fly and hover at breeding areas. They can enter structures through standard window screens.

Moth flies are not known to be disease carriers despite their habits. They are a nuisance pest in structures but also indicators of the buildup of drain sludge. Large numbers of flies may indicate a serious sanitation issue or be a problem for the communities near a sewage treatment plant.