Although closely related to blow flies, cluster flies are not believed to be a health hazard. They are slightly larger than a house fly, 3-6mm and they tend to fly clumsily in comparison. They are a grey/black in colour and the wings overlap at rest. Little golden hairs are visible near the head and base of wings, they are more prominent when they are young. The females lay eggs 4 times per year, 130 eggs each time. From being an egg to their death they live 1-3 months. They gather inside walls, voids, attics, window frames and other secluded areas. From these areas, with the arrival of spring/fall the cluster fly will emerge by the thousands on warm days to bask in the sun on the outside walls, or on the inside of windows.