The flying insects that bite humans are almost exclusively females feeding on blood to produce eggs. The most common culprits are mosquitoes, horse flies, deer flies, black flies, biting midges (no-see-ums), stable flies, sand flies, and tsetse flies. Each one bites differently, leaves distinct marks, and shows up in different environments, so identifying which one got you comes down to where you were, what time it was, and what the bite looks and feels like.
Why They Bite in the First Place
In nearly every species of biting fly, only the females bite. Males feed on nectar and plant sap. Females need the protein in vertebrate blood to develop mature eggs. After a blood meal, proteins are broken down into amino acids, which the female’s body converts into yolk proteins, lipids, and carbohydrates that get packed into each egg. Without that meal, the eggs don’t develop. A single blood feeding triggers the entire chain of egg production, and many species feed multiple times across their lifespan, laying a new batch of eggs after each meal.
Mosquitoes
Mosquitoes are the most familiar biting fliers and the most dangerous. They use a thin, needle-like mouthpart to pierce skin and draw blood, often without you noticing until afterward. Bites typically produce small raised lumps that itch for a day or two. People with higher sensitivity can develop fluid-filled blisters or larger welts around the bite site.
The real concern with mosquitoes is disease. Globally, mosquito-transmitted malaria alone caused an estimated 282 million cases and 610,000 deaths in 2024, according to the World Health Organization. Mosquitoes also spread dengue, Zika, West Nile virus, and several other infections depending on your region. They breed in standing water, from marshes down to a bottle cap’s worth of rainwater, and most species are most active at dawn and dusk.
Horse Flies and Deer Flies
Horse flies and deer flies belong to the same family and share a brutal feeding method. Instead of piercing skin with a needle, they use blade-like mouthparts that work like scissors to slice the skin open, then lap up the pooling blood. You feel it immediately. The bite is sharp and painful, and it often leaves a visible welt or raised swollen mark that can bruise.
Horse flies are large, sometimes over an inch long, with big iridescent eyes. Deer flies are smaller, roughly the size of a housefly, with patterned wings. Both are most active on warm, sunny days near water or wooded areas. They’re persistent biters and tend to circle back repeatedly if you swat them away. They don’t transmit major diseases to humans in most regions, but the wounds can become infected if not kept clean because the cut is relatively large compared to a mosquito bite.
Black Flies
Black flies are small, humpbacked flies that breed in fast-moving rivers and streams. Their larvae and pupae develop in running water, so the adults are most common near rivers, streams, and forested areas close to waterways. They tend to bite exposed skin on the head, arms, and legs, often in large numbers at once.
Their bites produce itchy, raised papules or small blisters, sometimes filled with clear fluid. In people who are sensitive or who receive many bites simultaneously, the reaction can be severe: extensive red, inflamed patches on the skin lasting up to four weeks, along with burning sensations, hot skin, and significant itching. Systemic symptoms like headaches, low-grade fever, and swollen lymph nodes have been documented in heavily bitten patients. Some bites develop secondary bacterial infections, especially when scratched. In tropical regions of Africa and Central and South America, black flies can also transmit the parasite that causes river blindness.
Biting Midges (No-See-Ums)
Biting midges, often called no-see-ums or sand gnats, are tiny flies measuring just 1 to 2.5 millimeters long. They’re so small they can pass through standard window screens. Their bites are disproportionately painful and itchy for an insect you can barely see. The bite marks look similar to mosquito bites: small raised lumps, sometimes with surrounding redness.
Their larvae develop in a wide range of wet habitats, from coastal mud flats to damp soil and the edges of ponds. Adults are most active during dawn and dusk and on calm, humid days. Wind is actually your best natural defense, since they’re weak fliers and even a light breeze keeps them grounded. Biting midges are a seasonal nuisance in many coastal, marshy, and tropical areas worldwide.
Sand Flies
Sand flies are small, hairy-winged flies found primarily in tropical and subtropical regions. They’re the only known carriers of Leishmaniasis, a parasitic disease with several forms. The skin form causes sores and is most common in the Americas, the Mediterranean, the Middle East, and Central Asia. The more dangerous internal form, which affects organs, is concentrated in Brazil, East Africa, and India. Over 90 sand fly species are known to transmit the parasite.
Sand flies are most active from dusk to dawn. Their bites are small and can be painless at first, but they often develop into itchy red bumps over the following hours. If you’re traveling to endemic regions, fine-mesh bed nets and insect repellent are your primary defenses, since sand flies are small enough to slip through standard mosquito netting.
Stable Flies
Stable flies look almost identical to common house flies, which is why people are sometimes startled when a “house fly” suddenly bites them. The key difference is in the mouthparts: stable flies have a long, bayonet-like probe that extends forward from the head, visible if you look closely. House flies have soft, sponge-like mouthparts and cannot bite at all. Stable flies also have a checkerboard pattern of seven dark spots on their abdomen, while house flies have a plain abdomen.
Stable flies typically bite ankles and lower legs. The bite is a quick, sharp pinch. They breed in decaying organic matter like compost, wet hay, and animal waste, so they’re most common around farms, stables, and beaches with seaweed accumulation. They don’t transmit diseases to humans but can be a persistent annoyance in the right conditions.
Tsetse Flies
Tsetse flies are found only in sub-Saharan Africa, primarily in rural and wooded areas. Unlike most biting flies, both males and females feed on blood. They’re known for transmitting the parasite that causes sleeping sickness (Human African Trypanosomiasis), though aggressive control programs have reduced human cases to near zero in some countries like Uganda. The flies remain widespread, however, and continue to cause significant disease in livestock across the region.
How to Tell What Bit You
The bite itself gives you the first clue. A painless bite you notice later as an itchy bump is likely a mosquito or midge. A sharp, immediately painful bite points to a horse fly, deer fly, stable fly, or black fly. Multiple bites clustered on exposed skin near a river suggest black flies. Bites on the lower legs near a farm or beach point to stable flies.
Location and timing matter just as much. Biting midges and mosquitoes peak at dawn and dusk. Horse flies and deer flies hunt during the brightest, warmest part of the day. Black flies swarm near fast-moving water. Sand flies come out after dark in tropical areas. If you were bitten through a screen or in a closed room, biting midges are small enough to be the likely suspect.
Protecting Yourself
EPA-registered skin repellents containing DEET, picaridin, or oil of lemon eucalyptus provide protection against most biting flies. Protection times vary by product concentration and formulation, so check the label for specific duration. Higher concentrations of DEET generally protect longer, but the relationship isn’t linear: 30% DEET doesn’t last twice as long as 15%.
Physical barriers are equally important. Long sleeves and pants in light colors (dark colors attract horse flies and tsetse flies) reduce exposed skin. Fine-mesh screens and bed nets help with midges and sand flies. Avoiding peak activity times for the species in your area, and staying away from their preferred habitats when possible, cuts your exposure significantly. For horse flies and deer flies, which are visual hunters attracted to movement and dark shapes, walking calmly and wearing lighter clothing can reduce the number of approaches.