Cluster flies are large, black slow flies, that closely resemble house flies.
They are parasites of earthworms, and will become a nuisance where earthworms are abundant, such as around old farms and grassy areas where a good soil and moisture are abundant. In late summer, when the days lengthen and fall approaches, adult flies emerge from the soil and seek out secluded places to hibernate. They seem to be attracted to light colored siding, often congregating on the sunny sides of buildings. Gradually they move upward until they find a crack or opening to enter the house. Later on warm days in winter or when the heat is on, the flies become active again.