The adult is about 6mm long and has distinctive golden hairs on the thorax (chest) area. The flies live outdoors and the adults feed on nectar from flowers. The fly lays eggs in damp soil and rotting leaves and when the eggs hatch, the larvae hunt and destroy earthworms before hatching into an adult fly. They do not breed indoors and they are not thought to carry any diseases. However, in the autumn, the flies look for shelter and will hibernate over winter. They will shelter in lofts and sometimes will invade the living space of a house.
The big problem is that they tend to invade in large numbers. Another problem is that although they may go unnoticed in autumn, if their "lodgings" warm up during the winter, the flies may emerge and cause a sudden problem. The other bad news is that the flies tend to return to the same property, year after year. The good news is that although you may have invasions of cluster flies every year, control is easy for a pest control technician through the use of smoke generators, powdered fog generators or by using a low pressure spray.