The adult cluster fly is slightly larger than the common house fly, about 3/8 inch (8mm) long. Its robust body is dark gray, nonmetallic, and covered with short golden hairs. Often confused with the common House Fly, Cluster Flies are roughly the same size.  Some characteristics that differentiate the Cluster Fly: they fly somewhat more slowly than the House  Fly, they almost always fly toward windows on the warm side of a structure, their wings overlap almost completely when at rest. Cluster Flies breed in the ground outside of buildings during the warm weather (late Spring into early Summer) using earthworms as a food source for the immature larva (maggots).   The flies later pupate (go into the cocoon stage) and then later hatch as adult flies.  In temperate areas, often in late August, these flies begins to migrate indoors finding any small cracks or crevices that permit entry into structures.  These may include areas around window frames, door frames or eaves.  Entry tends to be on the same, warm, sunny side (often the southern or western exposure) of the structure as the flies later emerge from. During the Fall, Winter or Spring months, these flies may emerge - particularly on warm, sunny days.  The flies appear at windows buzzing and "clustering" around those areas to the dismay of the occupants.  This fly can become a problem in virtually any structure. Though they usually exit the building, their presence in large numbers inside is often annoying to the building's occupants.