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), sand flies (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 can cause round, itchy, raised papules or welts. Mosquitoes are known to transmit a wide variety of disease-causing pathogens, but in North America they are only known to transmit a number of encephalitis viruses, such as West Nile Virus, Western and Eastern Equine Encephalitis, dengue fever and dog heartworm.

Biting Gnat - Leptoconops Ceratopogonidae Biting Gnat - Leptoconops

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 cm (½-1 inch) long, larvae that live and feed in the skin. These flies are not found in North America, only in Central and South America. However, there are rare cases of rodent bots infesting humans in California. The other flies, with larvae that feed on humans, are screw worms and blowflies (family Calliphoridae). The larvae of these flies feed on dead or decaying tissue, and less commonly living tissue. The primary screw worm feeds on living tissue but is currently only found from Mexico to South America. These flies are not known to transmit any disease-causing pathogens.

The blowflies, Lucilia sericataPhanecia regina and some species of flesh fly (family Sarcophagidae) may infest lesions or wounds in immobilized persons in rest homes, advanced care facilities and surgical wards. However, these infestations are short-lived, as they are easy to treat and remove. Indeed, such infestations may have positive medical outcomes. Some of these species are used by physicians to debreed dead tissue in severe burns and wounds.