Bluebottle
Bluebottle , Blow Fly, Calliphor erythrocephala 
Blow flies and bottle flies are important scavengers in nature as they are one of the first insects to reach a dead animal. These flies are part of the decomposition process that recycles nutrients back into the soil. The maggots of these flies may be used by forensic entomologists to place the time of death in murder cases. They also breed in garbage containers, dumpsters, and decaying vegetative matter (e.g., compost piles).
These flies are extremely common and can be found one to a few at a time in homes or businesses during the warm summer months. They are attracted to buildings by food odors and also warm/cool air currents emitting through cracks around doors and windows or through doors propped open for ventilation purposes. The sudden appearance of dozens of blow flies or bottle flies within a building signals a potential dead rodent, bird, or other animal in the wall, ceiling, attic, or crawl space.
The Bluebottle is a member of a group of flies commonly referred to as ‘blowflies’ on account of their habit of ‘blowing’ or depositing their eggs on exposed meat. They are a pest of buildings where meat is to be found: slaughter houses, canning factories, meat processors and, of course, houses. Outdoors they are associated with decaying animal matter and rubbish tips.
The female fly will enter houses with a loud buzzing noise, searching for flesh for depositing eggs on, or for food.