A number of different kinds of flies feed on blood, including mosquitoes (family Culicidae), black flies (family Simuliidae), no-see-ums or biting gnats (family Ceratopogonidae), sandflies (family Psychodidae), snipe flies (family Rhagionidae), horse and deer flies (family Tabanidae) and stable flies (family Muscidae). Biting flies all have free-living larvae that feed on decaying plant matter or microorganisms. None of these insects live in the skin. Their feeding bites cause round, itchy, raised papules or welts. Mosquitoes are known to transmit a wide variety of disease-causing pathogens. In North America they are only known to transmit a number of encephalitis viruses including West Nile Virus and dog heartworm.

There are two groups of flies that do have larval stages that live on or in human skin. One, the human bot fly (Dermatobia hominis, family Oestridae), has large, 1/2 - 1 inch long, larvae that live and feed in the skin. These flies are not found in North America, only in Central and South America. The other flies, with larvae that feed on humans, are screw worms (genus Cochliomyia, family Muscidae). The larvae of these flies feed on dead or decaying tissue. They are sometimes used to clean wounds. The primary screw worm feeds on living tissue but is only found in Central and South America. These flies are not known to transmit any disease causing pathogens.