These flies get the common name of moth fly from their fuzzy appearance, their bodies and wings being very hairy. The drain/filter/sewage fly common names are from places or situations that represent typical breeding and developmental sites. Although usually thought of as nuisance pests, there have been cases where inhalation of their body caused bronchial asthma.
Adult drain flies are about 1/16-3/16” long and appear delicate and fuzzy. Their colours are usually pale yellowish to brownish grey to blackish, depending on the species. Adult females lay their 30-100 eggs in irregular masses on the surface of the gelatinous film which covers filter stones of sewage treatment plants or which lines the water-free portions of drain pipes.
Because of their small size, moth flies are able to penetrate ordinary screens. Moth flies are weak fliers, so indoors they are usually seen crawling on walls or other surfaces. When they do fly, it is only for short distances of a few feet and their flight is in characteristic short, jerky lines. During the day, they typically rest on vertical surfaces near drain openings indoors and in shaded areas outside. Their greatest activity is in the evening when they can be seen flying or hovering above draining openings indoors or sewage filter beds, etc. outside.