The common name reflects this groups’ habit of forming compact clusters of overwintering individuals, typically in wall voids or attics. They are widely distributed in Europe, Canada, and throughout the United States.
Adults are about 3/8” long and robust. Their colouring is usually dark grey and non-metallic. Adults overwinter in sheltered places, emerging in the spring to mate. Eggs are laid in soil cracks and hatch in about 3 days. As days shorten and the weather cools, cluster flies often enter structures to overwinter, sometimes travelling more than a mile to do so. They usually occupy attics and/or between-wall voids in the walls which receive the most sunlight, usually the south and possibly east or west walls. Typically, they use the same structure year after year. Cluster flies can usually be found at windows crawling on the panes or frames, or around lamps or lights. They are sluggish in their movements in comparison to house flies. They usually give off a buckwheat honey odour and leave a greasy spot when crushed.