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Cluster fly Yellow swarming fly Autumn fly (Pollenia rudis) (Thaumatomyia notata) (Musca autumnalis)

These flies have the habit of coming indoors in late autumn to hibernate during the cold winter months, and they are often found in groups or clusters of many hundreds or even thousands of flies.
During the summer they live out-of-doors and can often be seen sunning themselves on fences, walls and tree trunks. Indoors, they congregate in roof spaces and dark corners of attics and little-used rooms. The flies also enter sash window boxes, cracks round window frames and unused Venetian blinds or curtains.

Occasional warm days bring the cluster flies out of their hiding places  and they crawl or fall into rooms, often covering window sills with buzzing flies spinning round and round in uncoordinated attempts to fly. In the following spring, the flies start moving again, and the problem recurs. The appearance of these lethargic flies causes considerable annoyance, although they are quite harmless.
Although we know why clustering flies seek shelter in this way, we do not know why they enter certain buildings in preference to other adjacent or nearby properties. However, once a building has been invaded, it is almost certain that more flies will return in subsequent years.

Biology:
Cluster fly
The larvae live as parasites inside earthworms. Eggs are laid in the soil and hatch in about a week. The young larvae bore into worms where they stay throughout the winter. In spring the larvae develop quickly, usually killing the worms, then pupate in the surrounding soil. The adult flies emerge in summer. These are large flies (10 mm long), with a shifting pattern of silver and grey-brown markings on the abdomen, and crinkly yellow hairs on the thorax.
Yellow swarming fly
Is a predacious species, the larvae feeding on aphids living on the roots of grasses. There are two generations a year, the later one usually over wintering as pupae in the soil. However, a warm spell in late autumn may cause the adults to emerge prematurely, and they then invade buildings for shelter when the weather becomes colder. The adult flies are very small (2.5 – 3 mm) and are bright yellow with black markings.
The Autumn fly
Breeds in animal dung in fields. Adult females are almost identical in appearance to common house flies, but males are distinctive in having an orange abdomen with a black mark down the centre.