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. The fly maggot isn't associated with garbage or other kinds of food. Maggots feed inside an earthworm host for two to three weeks, after which they leave to pupate in the soil. Egg to adult development takes four to six weeks. In Delaware, they have two or three generations a year.