Nature Notebook – Cluster Flies
Apparently the large fly lumbering about the building did not get the memo that insects are inactive during winter. Although there are always exceptions to nature’s “rules,” this cluster fly is actually obeying the winter rule.
It was hibernating…inside the building. When the furnace heated the building, it also warmed the fly, who now thinks it is spring. Time to emerge, leave the hibernation area and procreate.
However, flies have a lousy sense of direction. They can’t find their entrance cracks so they fly about looking for other ways out. The warming rate differs slightly for each individual so the cluster of hibernating flies doesn’t emerge all at once. Just enough to convince homeowners that the flies are laying eggs all over their house.
They are not. Cluster fly eggs are deposited on soil. The maggots feed on earthworms by burrowing into the soft bodies. Adults feed on nectar. So cluster flies in a building are not eating. They also do not carry disease.
They’re just an interesting winter nuisance.