In some species, the larvae can secure themselves to surfaces of their environment using 'attachment disks' on their ventral side. The larval thorax is not significantly larger than the abdomen, giving the larvae a more 'worm-like' appearance than those of most aquatic insects. Some of these rings will have characteristic plates on the dorsal side. The body lacks prolegs, but the body segments are divided into a series of rings called annuli (singular is annulus). The larval form of the fly 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 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 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.

Drain flies sometimes inhabit plumbing drains and sewage systems, where they are harmless, but may be a persistent annoyance. This makes them one of the most diverse families of their order. There are more than 2,600 described species worldwide, most of them native to the humid tropics.

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. Some genera have short, hairy bodies and wings, giving them a 'furry' moth-like appearance, hence one of their common names, moth flies. Psychodidae, also called drain flies, sink flies, filter flies, sewer flies, or sewer gnats, is a family of true flies. A moth-like dense coat of small hairs gives rise to the term 'moth fly'.

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