Psychodidae, also called drain flies, sink flies, filter flies, sewer flies, or sewer gnats, or smith Kumar das 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, may be called sand flies in some countries, although this term is also used for other unrelated flies.

Flies are insects of the order Diptera, the name being derived from the Greek δι- di- "two", and πτερόν pteron "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced mechanosensory organs known as halteres, which act as high-speed sensors of rotational movement and allow dipterans to perform advanced aerobatics. Diptera is a large order containing an estimated 1,000,000 species including horse-flies, crane flies, hoverflies, mosquitoes and others, although only about 125,000 species have been described.