The Cluster Fly is 10 mm in width (including wingspan) and 6 mm in length. Its eyes are large and reddish. The thorax is covered with yellow-gold hairs, whilst the abdomen is yellow and black.
The adult female lays eggs loosely on and around damp soil and beneath dead and rotting leaves. After a week the larvae hatch from the eggs and actively seek earthworms to which they cling and than bore through the body wall. The conventionally shaped fly maggots develop inside the earthworm. At the end of the earthworms death the larva bores its way out again and pupates in the soil.
Since this is a free-living “field” insect, the life cycle is very dependent on the weather. In Britain it is common that two generations per year are common and in hot summers up to four generations per year might be possible.
The adult flies, resplendent with their thorax clothed in golden hair feed on the nectar of garden and wild flowers.
The larvae feed off earthworms, whilst the adult fly feed on the nectar of garden and wild flowers.
Why are Cluster Flies a problem?
During the summer months and early autumn these flies are of no consequence. As the season cools they seek shelter in nooks and crannies in houses and other buildings. As temperatures drop they search for more protection and frequently form vast clusters in roof spaces and lofts. It has been observed that the same houses or buildings will be chosen year after year. Adult cluster flies are generally found on the south facing side the buildings this is because they will be exposed to more sun and in turn will keep the adult cluster flies warmer.
Such large aggregations of flies do produce a rather sickly smell. When the areas are warmed up accidentally or artificially (during their hibernation) the adult cluster flies emerge rather lazily and create some consternation among the people using the building. This occurs commonly in church halls and domestic bedrooms.