Cluster fly

These flies do not present a health hazard because they do not lay eggs in human food. They are strictly parasitic on earthworms; the females lay their eggs near earthworm burrows, and the larvae then infest the worms. When the adults emerge in the late summer or autumn, they enter houses to hibernate, often in large numbers.
They are difficult to eradicate because they favour inaccessible spaces such as roof and wall cavities. They are often seen on windows of little-used rooms. They are also sometimes known as attic flies.