Several types of flies bite humans, and they all do it for the same reason: the females need blood to produce eggs. The most common biting flies you’re likely to encounter include horse flies, deer flies, black flies, stable flies, sand flies, and biting midges (no-see-ums). Each looks and behaves differently, and knowing which one got you can help you understand the reaction on your skin and how to avoid future bites.
How Biting Flies Differ From House Flies
Regular house flies can’t bite you. They have soft, sponge-like mouthparts designed to soak up liquified food. Biting flies, on the other hand, have blade-like or needle-like mouthparts that slice or puncture your skin to reach blood vessels underneath. These mouthparts contain serrated tips that anchor into your skin, which is why biting fly bites tend to hurt immediately rather than going unnoticed like a mosquito bite.
All biting flies use a two-channel system: one channel pumps blood up into the fly’s gut, while the other injects saliva into your skin. That saliva contains compounds that prevent your blood from clotting, and it’s the main reason bites swell, itch, and turn red. The more saliva a fly injects, the stronger your reaction tends to be.
Horse Flies and Deer Flies
Horse flies are the largest biting flies you’ll encounter, reaching up to 3.5 centimeters (about 1.4 inches) long. They’re heavy-bodied with large, iridescent eyes and powerful wings. Many species are brightly colored. Deer flies are their smaller relatives, identifiable by a dark band running across each wing and a yellowish-brown abdomen with black patches.
Both breed near open water. Females lay eggs close to lakes, ponds, and streams, and the larvae develop in muddy, waterlogged environments. You’re most likely to encounter them in summer near rural waterways, marshes, or wooded areas. Their bites are among the most painful of any fly because they use a scissor-like cutting motion rather than a clean puncture, literally tearing a small wound in your skin. A horse fly bite typically produces a raised, red bump that may bleed slightly and remain sensitive to the touch for hours.
Black Flies
Black flies are small (2 to 5 millimeters), dark, humpbacked flies that breed in flowing water. Nearly all streams with any amount of current can support black fly populations, but they especially thrive near rapids, waterfalls, and pond spillways. Adults are active through spring, summer, and fall, and in warmer climates they can even appear during mild winter days.
Black flies tend to swarm around your head and bite exposed skin on your face, neck, and ears. Their bites often produce a reaction that’s disproportionate to the fly’s size: a swollen, intensely itchy welt that can last for days. In parts of Africa and Latin America, certain black fly species transmit onchocerciasis, also known as river blindness. This isn’t a concern in North America, but the bites themselves can still be miserable.
Stable Flies
Stable flies look almost identical to common house flies, which makes them easy to misidentify. The key difference is a checkerboard pattern on the abdomen and a visible, bayonet-like mouthpart that sticks straight out from the head. They’re about a quarter-inch long.
Both male and female stable flies bite, which is unusual since most biting fly species rely on only the females for blood-feeding. They prefer livestock but will readily bite humans when animals aren’t available. This makes them a particular nuisance at beaches, outdoor restaurants, and recreational areas near agricultural land. In coastal Florida, stable flies have been documented disrupting beach activities in large numbers. They tend to bite the lower legs and ankles, and their bite is a sharp, immediate sting.
Sand Flies and Biting Midges
Sand flies are tiny (1.5 to 3.5 millimeters), moth-like flies with hairy wings that breed in soil, leaf litter, and animal burrows. They’re most active at dawn and dusk. In tropical and subtropical regions, sand flies can transmit leishmaniasis, a parasitic disease that causes skin sores or, in severe cases, damages internal organs. Sand fly fever, a viral illness causing flu-like symptoms, is another risk in certain parts of the Middle East, Asia, and the Mediterranean.
Biting midges, commonly called no-see-ums, are even smaller. At 1 to 3 millimeters, they can pass through standard window screens. They’re active during the daytime and evening and are notorious near coastal marshes, mangroves, and lakeshores. Their bites produce tiny red welts that itch intensely, often in clusters. Because the flies are nearly invisible, people sometimes don’t realize they’re being bitten until the welts appear.
Tsetse Flies
Tsetse flies are found only in sub-Saharan Africa, so most readers won’t encounter them at home. But if you’re traveling to rural parts of central or eastern Africa, they’re worth knowing about. They’re medium-sized, tan or brown flies with a distinctive folded-wing resting posture. Both sexes bite. Tsetse flies transmit African trypanosomiasis, commonly called sleeping sickness, a parasitic disease that can be fatal without treatment.
Treating Biting Fly Reactions
Most biting fly reactions follow the same pattern: immediate pain at the bite site, followed by a raised red or discolored bump that swells and itches over the next several hours. The severity depends on the fly species and your individual sensitivity. Some people develop large welts from black fly or midge bites while others barely react.
Basic care works for the vast majority of bites. Wash the area with soap and water to reduce infection risk. Apply a cloth-wrapped ice pack for at least 20 minutes to bring down swelling. An over-the-counter antihistamine can help with itching, and a standard anti-inflammatory painkiller can address soreness. Keep the area elevated if possible, and resist the urge to scratch, since breaking the skin opens the door to secondary infection. Most minor allergic reactions resolve on their own within a few days.
Preventing Bites
EPA-registered repellents containing DEET, picaridin, oil of lemon eucalyptus (OLE), IR3535, or 2-undecanone all provide meaningful protection against biting flies. Higher concentrations last longer: products with less than 10% active ingredient typically protect for only one to two hours. For extended time outdoors near water or in wooded areas, choose a product with a higher concentration and reapply as directed.
Repellents labeled specifically for “flies,” “biting flies,” or “no-see-ums” are your best bet for midges and sand flies, which don’t always respond to products formulated only for mosquitoes. Clothing matters too. Long sleeves, long pants, and light-colored fabrics make it harder for flies to reach your skin. Horse flies and deer flies are visually attracted to dark colors and movement, so wearing lighter shades near water can reduce how many come after you. Fine-mesh head nets are effective in areas with heavy black fly or midge activity, since these flies target the head and neck.