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The "Cluster Fly" or "Attic Fly" is a common sight in the autumn and winter in the UK, in our houses - very often in little-used rooms such as attics and basements.
I found this hibernating on the internal surface of the large windows in Anna's parent's flat - typical behaviour for this species.
As August ends (generally, although probably later these days), these Blowflies of the Pollenia sp, literally crawl into houses and attics, sometimes in numbers (hence their name) seeking warmth in which to pass away the winter months.
One can recognise a Cluster Fly in a number of ways. Firstly - it will be the most commonly encountered Fly by far inside, in winter. Secondly it is slightly larger than a common Housefly, grey(ish) in colour and has a very distinctive golden-haired thorax (see my photographs). Lastly, it holds its wings flat along its body at rest, rather than flat and across its body, like most Blowflies.
It represents NO health hazard to us, as it lays its eggs not on rotting meat for example, but in the ground (in cracks and under leaves etc...) near Earthworm burrows (Allolobophora sp). The eggs hatch - and the Cluster Fly maggots crawl into the host earthworms body, eating it from the inside. When the earthworm is nearly dead, the maggots break free and pupate in the ground. There can be up to four generations of Cluster Flies during the year, but often 2 or 3 are more common.
Cluster Flies are very sluggish during the winter, and can quite easily be hoovered up, rather than squashed or swatted, which will result in a very greasy splodge on a wall or upholstery!