This general fly species thanks its name to its habit to overwinter in buildings in large numbers. The cluster fly looks like a very big house-fly with a lot of golden hairs on its thorax (breast plate).
The adult females lay their eggs on a moist floor, under rotting leafs etc. After about a week, the larvae hatch and start actively looking for rain worms, which they then hang onto and drill a hole in. The maggot develops inside the body of the rain worm. If the rain worm is dead or nearly dead, the maggot drills a way out and pupates in the soil. The life cycle of the cluster fly strongly depends on the weather conditions, two generations per year is normal. However, in hot summers there can be four generations per year. The adult cluster fly feeds off of the nectar of flowers.
During the summer and the beginning of the fall, cluster flies hardly ever cause nuisance. When the weather turns cooler, they seek shelter in nooks and crannies in houses and other buildings. As the temperature drops further, they look for better shelter and often form clusters of thousands of flies. It is not uncommon for the same building to be used year after year for purposes of overwintering. Cluster flies do not cause any damage, but due to their great numbers, they are very annoying.