Similar to the house fly, the cluster fly is 3/8 inch in length and dark nonmetallic gray. The thorax (the middle body segment where the wings and legs are attached) has yellow or golden hairs.
The name reflects its behavior of forming clusters before hibernating. They are bothersome because they often enter attics and wall voids of structures to over winter, seeking shelter to avoid the cooling temperatures. When temperatures rise, they become active, either leaving the structure, or entering the interior.
Adults feed on flowers, the young feed on earthworms.
After mating in the spring, the female lays her eggs in cracks in the soil. Hatching in about 3 days, the maggot will burrow into the body of an earthworm to develop. The developmental time from egg to adult is 27 – 39 days. Cluster flies generally produce 4 generations a year.