Cluster flies or attic flies are seen two times of the year, when it gets cold and in the spring as it heats up again. Unlike more familiar blow flies, such as the bluebottles, they do not present a health hazard because they do not lay eggs in human food.
fly infestation, fly in the house, fly killer.
They are strictly parasitic on earthworms; the females lay their eggs near earthworm burrows, and the larvae then infest the worms. However, the flies are a nuisance; 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 but they can treated to ease the problem if done yearly. They are often seen on windows of rooms and can be seen with ladybirds in the window frames.