Several types of flies bite humans, and they range from large, painful horse flies to tiny midges so small you can barely see them. The most common biting flies are horse flies, deer flies, stable flies, black flies, biting midges (no-see-ums), and sand flies. Each one looks different, lives in different habitats, and leaves a distinct mark.
Horse Flies and Deer Flies
Horse flies and deer flies belong to the same family (Tabanidae) and are among the most painful biters you’ll encounter. Horse flies are the larger of the two, sometimes over an inch long. Some are entirely black, while others, called “greenheads,” are light brown with shiny green eyes. Deer flies are closer to house fly size, roughly a quarter inch, and tend to be yellow-brown to black with dark bands on their wings. Both species can have striking iridescent green eyes.
These flies use blade-like mouthparts to slice the skin open and lap up blood, which is why their bites hurt immediately and often produce a raised, swollen welt. Horse fly bites in particular can take a long time to heal and sometimes become infected. Some people also experience hives, dizziness, weakness, or wheezing after a bite.
Both species breed in moist to wet soils, with some larvae developing directly in water. You’re most likely to encounter them near ponds, marshes, and wooded areas during warm months. One frustrating reality: no repellent works particularly well against horse flies, deer flies, or greenheads. DEET, the gold standard for mosquitoes, offers little protection against these larger biting flies.
Stable Flies
Stable flies are the sneaky ones. At about a quarter inch long and gray with four dark stripes behind the head, they look almost identical to a common house fly. The giveaway is a pointed, needle-like mouthpart that sticks forward from beneath the head. If something that looks like a house fly just bit you, it was almost certainly a stable fly.
These flies prefer biting the lower legs and ankles of humans and livestock, though they’ll bite elsewhere on the body when their numbers are high. Their activity depends heavily on temperature. They’re most active between roughly 59°F and 86°F (15–30°C), feeding once a day in cooler weather and twice a day when it’s warmer. Outside that range, biting drops off significantly.
Stable flies breed in moist, decaying organic matter, like rotting hay, compost piles, and muddy areas around livestock feeding stations. If you live near a farm or keep horses, you’re far more likely to deal with them.
Black Flies
Black flies are small, no more than an eighth of an inch, with broad wings and a distinctive humpbacked shape. Despite their size, their bites pack a punch. They tend to swarm around the head and face, and the bites often bleed freely because, like horse flies, they cut the skin rather than pierce it.
These flies need clean, flowing water to reproduce. Their larvae develop in streams, creeks, and rivers, so black fly problems are concentrated near moving water in open or wooded areas. They’re a particular nuisance in northern climates during late spring and early summer. DEET does work reasonably well against black flies, as does oil of lemon eucalyptus (PMD).
Biting Midges (No-See-Ums)
Biting midges earn their nickname “no-see-ums” honestly. At roughly 1/32 of an inch, they’re far smaller than a mosquito and nearly invisible. You’ll often feel the bite before you ever spot the insect. They’re also called punkies or gnats depending on where you live.
Their larvae develop in moist environments: the shores of streams and ponds, muddy soils, and even water-filled tree holes. Adults tend to bite in large numbers, leaving clusters of small, intensely itchy welts. Beyond the irritation, biting midges can transmit a virus called Oropouche, which normally causes fever and body aches that resolve on their own. In rare cases, it can lead to more serious illness, and recent cases in Brazil have been associated with stillbirths and birth defects in people who were infected during pregnancy.
DEET and picaridin both repel biting midges effectively. Products containing IR3535 (found in some Avon products) are also labeled for midges and no-see-ums.
Sand Flies
Sand flies are tiny, long-legged, and hairy, typically brown to gray and no more than an eighth of an inch long. Their wings form a distinctive V shape when resting. On average, sand flies that transmit disease are only about one-fourth the size of a mosquito, sometimes even smaller. Their bites are often painless, and they fly silently, so many people never notice being bitten.
Sand flies are most active from dusk to dawn. Their primary health risk is leishmaniasis, a parasitic disease spread by infected female sand flies. Leishmaniasis occurs in parts of Central and South America, the Middle East, Africa, and South Asia. Anyone traveling to or living in these regions who gets bitten by an infected sand fly is at risk.
How Fly Bites Differ From Mosquito Bites
Most biting flies produce a more painful, more visible bite than a mosquito. Mosquitoes pierce the skin with a thin needle-like mouthpart, leaving a small, round, itchy bump. Larger flies like horse flies and black flies slash the skin open, which causes immediate sharp pain, more bleeding, and a larger swollen area. The welts from horse fly bites can persist for days and are more prone to secondary infection than mosquito bites.
Smaller biters like midges and sand flies produce bites closer in appearance to mosquito bites, small and itchy, but they often come in clusters because these flies attack in groups. Sand fly bites in particular may not even be noticeable at first.
Treating a Fly Bite
For any biting fly, the basics are the same: wash the bite with soap and water, apply a cloth-wrapped ice pack for at least 20 minutes to reduce swelling, and keep the area elevated if possible. An over-the-counter antihistamine helps with itching, and a pain reliever like ibuprofen can manage soreness. The most important thing is to avoid scratching, which dramatically increases infection risk.
Watch for signs that a bite has become infected: increasing redness or red streaking around the bite, warmth in the surrounding skin, pus or fluid-filled blisters, swollen lymph nodes near the bite, or fever and chills. These symptoms suggest cellulitis, a bacterial skin infection that needs treatment. A severe allergic reaction, though uncommon, can cause swelling of the lips, tongue, or throat, difficulty breathing, dizziness, or stomach pain with vomiting. This is a medical emergency.
Which Repellents Actually Work
Repellent effectiveness varies dramatically depending on the fly. DEET works well against black flies, biting midges, and fleas, but it does very little against horse flies, deer flies, or greenheads. Picaridin is effective against mosquitoes, biting midges, and chiggers. Oil of lemon eucalyptus covers black flies, gnats, mosquitoes, and no-see-ums. Products with IR3535 are labeled for biting midges, black flies, sand flies, and mosquitoes.
For the larger biting flies like horse flies and deer flies, physical barriers are your best defense. Long sleeves, long pants, and hats do more than any spray. Light-colored clothing also helps, since these flies are attracted to dark colors and movement. If you’re in an area with heavy stable fly activity, covering your lower legs is particularly effective since that’s where they prefer to feed.