Cluster flies look a lot like houseflies, but are slightly larger, darker and fuzzier. However, if you have difficulty identifying them by sight, try listening instead: The loud buzzing sound they make is a dead give-away. Another helpful clue for distinguishing between the two species is the time of year. Cluster flies are usually the ones you see (or hear!) indoors each fall, just when you thought fly season was over. They enter homes when the leaves start turning in order to escape the cold and hibernate through the winter. Since cluster flies don't feed during hibernation, they pose much less of a sanitation problem inside the home than do active species such as houseflies and moth flies.