Welcome to cluster fly season. Cluster flies are about the size of a housefly, dark gray in color with golden hairs on their thoraxes and behind their heads, explained Jack DeAngelis, entomologist with the Oregon State University Extension Service. They have light patches on their abdomens. The cluster fly lifecycle is quite remarkable. Cluster flies normally live outdoors, not in houses or outbuildings like you might think.
Adults feed on nectar while the larvae, or maggots are internal parasites of earthworms.
The female fly lays her eggs in the soil. A few days after hatching, the larval flies locate an earthworm, penetrate its body and develop there until it is ready to complete their transformation to adult flies. As many as four generations develop each summer. So if earthworms are abundant, a very substantial fly population can build. In the fall, adult flies seek shelter where they spend the winter months, then emerge the following summer to begin the cycle again.