The Polleniidae are a family of flies in the order Diptera, commonly known as cluster flies. These flies are found worldwide, with several species living in the UK and Europe. They are most often noticed in autumn and winter, when adults gather in large numbers inside houses and other buildings to hibernate, a behaviour that gives them their common name.

Cluster flies are usually dark grey or black in colour, often with a slightly golden sheen caused by fine, pale hairs on the body. They are larger and slower than the common house fly and tend to move lazily when disturbed. Their wings overlap flatly over the body when at rest, which helps distinguish them from other similar flies.

Unlike house flies and blowflies, cluster flies do not breed in rubbish or decaying matter. The larvae are parasites of earthworms, developing inside them before pupating in the soil. Adults feed on nectar and plant juices and are harmless to humans, though their habit of clustering indoors can make them a nuisance.

In the UK, the most common species is the Cluster Fly (Pollenia rudis), which often appears on sunny windows in spring when it awakens from hibernation. Other related species may be found in the countryside, woodlands, and meadows.

Although sometimes unwelcome in homes, cluster flies are a natural and beneficial part of the ecosystem, helping to control earthworm populations and pollinating flowers during their active months.