The bloodsucking behaviour of female black flies and mosquitoes does much to explain their low popularity. These insects are surprisingly common in some areas of the Arctic, even where potential blood donors are rare. There is a simple explanation for this fact. While southern biting flies need a blood meal before laying eggs, northern flies do not. Although blood may be a rare treat, northern mosquitoes and black flies have retained their appetite for it, as evidenced by the swarms of flies which surround Arctic birds and mammals on any warm day.