Though similar in size, humpbacked flies (Phoridae) are distinct from fruit flies (Drosophilidae) and drain flies (Psychodidae). They are dark in color, and have a distinctive arched thorax and veining at the base of the wings. But the easiest way to identify them is by their tendency to scuttle erratically along surfaces before taking off, unlike drain flies and fruit flies.
Phorids can breed anywhere with chronic moisture, and don’t require the typical fermentation (fruit flies) or bio film (drain flies) to breed. Huge numbers can develop under slabs when there is a sub-slab drain leak. If the leak is repaired but the wet soil isn’t excavated, the problem will persist.
Phorids also can become a problem in humid summer weather in offices, when condensation from air-conditioning systems falls on dropped ceiling tiles. Also known as coffin flies, they can be a problem in mortuaries.
Elimination of the chronic moisture or leak is the best solution for phorid fly control. When that isn’t possible, boric acid dust applied to the moist areas (dusting, spraying or foaming — however you can get in contact with damp substrate) generally will shut down their ability to breed, and can be long-lasting as long as it isn’t washed away.