These are dark greyish flies about 8mm long with yellowish hairs on the back and with overlapping wings. In autumn they congregate in large numbers in upper rooms or roof spaces of houses to hibernate. They will then reemerge in Spring to seek out egg laying sites outside. A mass of cluster flies has a characteristic smell. They are sluggish in flight and are a nuisance in the house. The larvae of one species are parasitic upon certain earthworms, so this species is more common in rural areas.