Cluster flies (Pollenia rudis) are about the same length as house flies (5/16-inch) but are more robust in body structure. They are close relatives to blowflies, but are non-metallic gray. They lack stripes on the thorax (segments with the wings and legs attached) but they do have yellow or golden hairs on the back, behind the head, and around the base of the wings. Cluster flies appear narrow when at rest because their wings completely overlap over their backs. The larvae are typical spindle-shaped maggots but are seldom seen because they develop as parasites in earthworms.