Several types of flies bite humans, and they all do it for the same reason: females need blood to produce eggs. (Stable flies are the exception, where both males and females bite.) The most common biting flies are horse flies, deer flies, black flies, stable flies, biting midges, and sand flies. Each has a distinct size, habitat, and bite pattern, and knowing which one got you can help you treat the bite and avoid the next one.
Horse Flies and Deer Flies
Horse flies and deer flies are the largest and most painful biters you’re likely to encounter. Horse flies measure 14 to 19 mm long with clear wings and a grayish-brown body. Deer flies are smaller, 10 to 13 mm, with patterned or smoky-tinted wings and a greenish-yellow body with dark stripes. Both are common along hiking trails, woodland edges, flood plains, and near ponds, streams, and coastal marshes.
These flies use scissor-like mouthparts to slice the skin open rather than piercing it like a mosquito, which is why their bites hurt immediately and often bleed. They’re active during the day and are most common in summer. One deer fly species in the United States can transmit tularemia, a bacterial infection, though this is relatively rare.
A frustrating detail for hikers: DEET, the most widely used insect repellent, doesn’t work well against horse flies or deer flies. Picaridin-based repellents are labeled for biting flies and may offer better protection, though no repellent is fully reliable against these larger species. Wearing light-colored clothing and a hat can help, since both flies are attracted to dark colors and movement.
Black Flies
Black flies, also called buffalo gnats, are small, humpbacked flies found near creeks and rivers where their larvae develop on submerged rocks. They’re most active in late spring and early summer, biting primarily during sunny, warm days with peak attacks in mid-morning and again in the evening before dusk. Overcast or stormy weather makes them worse: biting can intensify sharply at the onset of a storm and persist all day under cloud cover.
Their bites produce a burning sensation followed by swelling, redness, and intense itching that can last for days. In large swarms, the sheer number of bites can cause a serious reaction in both people and livestock. Black flies in the United States don’t transmit diseases to humans, but in parts of Africa and Central and South America, certain species spread river blindness.
Stable Flies
Stable flies look almost identical to common house flies, about a quarter inch long and gray with four dark stripes on the thorax. The key difference is visible up close: stable flies have a sharp, bayonet-like mouthpart that projects forward from the head, while house flies have a soft, sponge-like mouth. If a “house fly” lands on you and bites, it’s a stable fly.
Both males and females feed on blood, which is unusual among biting flies. They land with their head pointed upward, puncture the skin quickly, and leave. They prefer the lower legs, ankles, and feet on humans, and they tend to bite during early morning and late afternoon. Stable flies are most abundant in late summer and fall, especially around farms, barns, and areas with decaying vegetation or manure. They’ll fly several miles from breeding sites to find a host.
Biting Midges (No-See-Ums)
Biting midges go by many names: no-see-ums, punkies, or simply gnats. They’re tiny, often less than 3 mm, which means you frequently feel the bite before you ever see the insect. They’re common along the shores of oceans, lakes, ponds, and rivers, and they bite during the day or at night.
Most species bite heavily at dusk, with a secondary peak at dawn. Where conditions are favorable, they can remain active and breed for months at a time. Their bites cause red, itchy welts that are disproportionately irritating given the fly’s size. In parts of the Americas, biting midges can spread Oropouche virus, an emerging infection that the CDC is actively monitoring.
DEET does work against biting midges, as do repellents containing oil of lemon eucalyptus or IR3535. Fine-mesh screens on windows and tents also help, since standard window screening has holes large enough for midges to pass through.
Sand Flies
Sand flies are small, hairy flies found in tropical and subtropical regions. In the United States, they’re mainly a concern in southern Texas and parts of the Southeast. Globally, they’re significant disease carriers: certain species transmit cutaneous leishmaniasis, a parasitic disease that causes skin ulcers and scarring. Sand flies tend to bite at night and are weak fliers, so even a light breeze or a fan can reduce exposure.
How to Identify a Bite by Its Appearance
Most biting fly species produce similar-looking welts, which makes identification tricky based on the bite alone. A few patterns help narrow it down. Horse fly and deer fly bites tend to be larger, may continue to bleed after the fly leaves, and often swell more than other insect bites. Black fly bites frequently produce a small puncture wound surrounded by a wide area of redness and swelling that develops over hours. Biting midge bites appear as clusters of tiny red dots, often on exposed skin like the arms, neck, and ankles.
The timing and location of the bite are often more useful clues than the bite’s appearance. If you were bitten on the lower legs during late afternoon near a farm, a stable fly is the likely culprit. If it happened at dusk near a lake and you never saw the insect, midges are the best guess. A painful bite along a wooded trail in summer points to a horse fly or deer fly.
Treating Biting Fly Bites
The treatment approach is the same regardless of which fly bit you. Wash the bite with soap and water as soon as possible to reduce infection risk. Apply a cloth-wrapped ice pack for at least 20 minutes to bring down swelling. An over-the-counter antihistamine helps with itching, and an anti-inflammatory painkiller can take care of soreness. Keep the area elevated if it’s on a limb, and resist the urge to scratch, which can break the skin and invite a secondary bacterial infection.
Most bites resolve within a few days. Some people develop a larger local reaction with swelling that spreads several inches from the bite site. This is an exaggerated immune response to proteins in the fly’s saliva and, while uncomfortable, usually isn’t dangerous.
Rarely, a fly bite can trigger a systemic allergic reaction. Warning signs include hives spreading beyond the bite area, swelling of the lips or throat, difficulty breathing, dizziness, or stomach pain. These symptoms indicate anaphylaxis and require emergency treatment. People who know they’ve had severe reactions to insect bites in the past should carry an epinephrine auto-injector when spending time outdoors during peak fly season.