Order: Diptera

Family: Calliphoridae

Cluster flies get their common name from their habit of gathering together to form tight clusters of hibernating individuals in wall voids and attics. Cluster flies are found in Europe, Canada and most regions of the United States.

Description: Adult Cluster flies are about 3/8 inch long. Their dark grey, non-metallic body lacks stripes. However, these flies are easy to identify because when at rest, they fold their wings one on top of the other, covering the body in a scissor fashion. Most other flies fold their wings forming an upside down "V".

Biology: Adults over-winter in sheltered places such as wall voids and attics. They emerge from hibernation in the spring to mate. Eggs are subsequently laid in the ground and hatch within a few days. The larva then becomes a parasite, using an earthworm as a host as it develops. Developmental time from egg to adult is approximately one month.

Habits: Cluster flies are annoying and bothersome because of their slow flying, sluggish crawling on walls and windows, and the sweet-sticky odor they emit. They will enter a home singly, but gather together in the walls or attics on the warmer sides of the structure. They can be stimulated from hibernation on warm, sunny days. Although they do not breed within structures, they will re-enter the same structure for hibernation year after year.