They enter buildings from the outside, apparently seeking warmth and shelter. Inactive in colder weather, cluster flies may suddenly reemerge on warm spring days. They don't harm people or home furnishings. They are considered a nuisance. Description and Life History Cluster flies appear more compact than houseflies, and they are not as brightly metallic in color as blow flies, another common fly in homes.
Cluster flies are dark gray to black in color.
The top of the thorax, where the wings attach, lacks obvious striping. Young cluster flies have distinct yellow, tawny, or golden pairs on the back of the head and top of the thorax, but these become darker with age. Wings at rest overlap, giving a scissor-like appearance. Cluster flies are parasitic on earthworms. Females lay their eggs on the soil near earthworm burrows during the active season when they can be found only outside.