There are several species of small, dark-colored flies that may occasionally become pests in buildings. The common name for phorid flies is humpbacked fly, which reflects their distinctive humpbacked appearance.
Adult phorid flies are about 1/64 - 1/4" in size. They breed in decaying organic matter of plant or animal origin. Extremely large numbers of phorid flies on the first floor of a building may indicate a broken drain or sewer line under the structure.
Locate and remove (or dry out) the organic matter where the larvae live to eliminate the population.