Drain flies are small, only about 1/8th of an inch long. They are very hairy and their wings are covered in fine hairs-almost moth like.
Drain flies develop in the scum lining drains and sewer lines or in decaying organic material. Larvae hide behind the scum, so treatments like boiling water, pesticides or other chemicals are ineffective. They can breed around broken pipes/drains beneath slabs or in crawl spaces. They can also enter buildings through floor cracks, etc.