Cluster flies, Pollenia rudis, are close relatives of blow flies and are similar in size to house flies (3/8-inch) but are more robust in body structure. They are nonmetallic gray, lack stripes on the thorax (segments with the wings and legs attached), and 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 never seen because they develop as parasites in earthworms.