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Cluster Flies - During the summer months these flies live and breed in grassy fields and are seldom noticed. In the autumn they would, under natural conditions, hibernate in dry sheltered areas such as under loose bark or in hollow trees. The survivors of the winter would then emerge the following spring to return to the grassy fields to continue their life cycle. However, with the encroachment of housing onto green field sites bordering pasture land it is quite common in the autumn for these flies to collect in large numbers on the outside of buildings, especially on sunny, light coloured walls. As the night time temperatures begin to fall they tend to crawl into crevices for shelter, sometimes through ill fitting windows and frames, but mainly under the eaves/fascia boards into the roof space. The following days they may emerge and sun themselves but finally they remain inside until the process is reversed the following spring. These flies may then be discovered when you enter your attic (to retrieve your Christmas decorations?). Your attic light is enough to make some think spring has arrived and they will sleepily buzz around.

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The Cluster Fly is named so due to its habit of clustering and hibernating in large numbers in buildings and loft spaces. This fly is commonly found throughout Europe and the UK. These flies are not classed as a pest species, but often require treatment due to their large numbers. During the summer 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 clustering masses in roof spaces and lofts, with several thousand flies clustered together. It has been observed that a single house or building in a row of similar buildings will be chosen year after year for this clustering phenomenon. Such large numbers of these flies can produce a sickly smell. If warmed up either accidentally or artificially they may emerge from hibernation and become a nuisance to the people using the building.

The adult cluster fly is a fairly large species at around 10mm in length and a wingspan of around 20mm; it has large reddish compound eyes. the thorax (body) has a distinctive dark greyish/ olive colour and is covered in golden hairs. When resting the wings are held tight on top of the abdomen.

The adult female lays her eggs loosely on and around damp soil, beneath dead and rotting leaves etc. after about a week the larvae hatch from the eggs and actively seek earthworms to which they cling and bore through the body wall. The conventionally shaped fly maggots develop within the earthworm. The larvae have been observed to push their hind ends back through the earthworm's body wall to allow their breathing spiracles to gain access to free air. At or near to the end of the earthworms life the larva will bore its way out and pupates in the soil. Since this is a free-living field insect, the life cycle is very dependent on weather conditions. In Britain it seems that two generations per year are common, but in hot summers up to four generations are possible.