Dozens of fly species bite, but the ones you are most likely to encounter fall into a handful of well-defined groups: horse flies and deer flies, black flies, biting midges (no-see-ums), sand flies, stable flies, and tsetse flies. Each group has a distinctive size, habitat preference, and style of bite, so knowing what to look for makes it much easier to figure out what just got you. The differences matter beyond curiosity, too, because some of these flies transmit serious diseases while others are simply painful nuisances.
Horse Flies and Deer Flies
Horse flies (family Tabanidae) are the bruisers of the biting-fly world. They are large, often as big as a bumblebee, and the females use scissor-like mouthparts to slice open skin and lap up the pooling blood. The bite is immediately painful, produces a noticeable welt, and can bleed for a short time after the fly leaves. Deer flies are close relatives, somewhat smaller and typically marked with patterned wings and green or gold eyes. Both are most active on warm, sunny days near water or livestock, and they tend to target the head, neck, and shoulders.
One reason horse flies are so persistent is that their visual system is finely tuned for finding hosts. Research shows that horse flies use polarized light to pick out dark-bodied animals against a cluttered visual background, and that sunlit hosts are easier for them to detect than shaded ones because sunlit surfaces reflect light with a higher degree of polarization.1PubMed Central. Why do horseflies need polarization vision for host detection? Polarization helps tabanid flies to select sunlit dark host animals from the dark patches of the visual environment Experiments with controlled-spectrum lures have also shown that removing UV and blue wavelengths from a lure’s reflected light made it unattractive to horse flies, while removing the green component actually increased attractiveness.2PubMed Central. Horsefly object-directed polarotaxis is mediated by a stochastically distributed ommatidial subtype in the ventral retina In practical terms, this means dark clothing on a bright day is a magnet for horse flies, and light-colored or white clothing genuinely helps.
Black Flies
Black flies (family Simuliidae) are small, humpbacked flies that breed in flowing water. They can appear in staggering numbers near rivers and streams during late spring and early summer, and in some regions their swarms are intense enough to drive people indoors. Like horse flies, female black flies are pool feeders, slicing the skin and drinking from the wound. The bite itself may not hurt much at first because the fly injects a local anesthetic along with anticoagulant saliva, but within minutes or hours the site swells, itches fiercely, and can stay inflamed for days.
Black fly species vary widely in habitat preference. Some are generalists that breed in everything from narrow streams to enormous rivers hundreds of meters wide, while others are specialists found only in large river systems.3PubMed Central. Discovery of the Larvae and Pupae of the Black Fly Simulium (Gomphostilbia) khelangense and Breeding Habitats of Potential Pest Species of the S. (G.) chumpornense Subgroup (Simuliidae) This is why black fly problems tend to be hyper-local: a campsite a few hundred meters from a fast-flowing stream might be miserable, while a site on a still lake is fly-free.
Biting Midges, Also Known as No-See-Ums
Biting midges in the genus Culicoides (family Ceratopogonidae) are the tiniest biting flies most people will encounter, often small enough to pass through standard window screens. Their common name, “no-see-ums,” comes directly from that experience: you feel a sharp, burning bite but cannot find the culprit. They are most active during early morning and evening hours, especially on calm, overcast days, and they target anyone outdoors, from campers and gardeners to beachgoers.4EDIS. Biting Midges, No-See-Ums Culicoides spp. (Insecta: Diptera: Ceratopogonidae)
Despite their size, biting midge bites pack a disproportionate punch. The lesions can be intensely itchy and, for some people, remain painful for days. Because the flies are so small, physical barriers like fine-mesh screening or head nets need a tighter weave than what works for mosquitoes. Wind is one of the most reliable natural deterrents; even a light breeze can ground midges because they are weak fliers.
Sand Flies
Sand flies (subfamily Phlebotominae) are another tiny biting fly, roughly the size of a biting midge, but with a distinctly hairy, moth-like appearance and a weak, hopping flight pattern. They tend to bite at dusk and after dark, and their bites leave small, red, intensely itchy papules. Sand flies are most common in tropical and subtropical regions, and they are medically important because certain species transmit leishmaniasis, a parasitic disease that can range from disfiguring skin ulcers to a life-threatening systemic infection.
Sand fly host preferences depend on what is available. In eastern Sudan, where visceral leishmaniasis is a serious concern, the local vector species preferentially feeds on cattle and donkeys but also takes human blood meals in a meaningful fraction of cases.5PubMed. Host preference and human blood index of Phlebotomus orientalis, an exophilic sand fly vector of visceral leishmaniasis in eastern Sudan That dual feeding pattern is what makes sand flies effective disease vectors: they bridge the gap between animal reservoirs and people.
Stable Flies
Stable flies (Stomoxys calcitrans) look almost exactly like house flies, which is why people are sometimes startled when what appears to be an ordinary fly suddenly delivers a sharp, piercing bite. The key visual difference is the bayonet-like proboscis that points forward from the head. Stable flies are associated with livestock operations and decaying organic matter such as old hay or manure, but they readily move into suburban and coastal areas and will bite humans on the lower legs and ankles.
Unlike flies that transmit pathogens biologically (where the pathogen reproduces inside the fly), stable flies act primarily as mechanical vectors. If a stable fly’s blood meal is interrupted, it will move to a new host and resume feeding, potentially carrying infected blood on its mouthparts. Some pathogens can even survive temporarily in the fly’s crop and be regurgitated during the next meal.6PubMed Central. Transmission of pathogens by Stomoxys flies (Diptera, Muscidae): a review Experimental work has confirmed that stable flies can mechanically transfer viruses between animals within days of exposure.7Scientific Reports. Experimental evidence of mechanical lumpy skin disease virus transmission by Stomoxys calcitrans biting flies and Haematopota spp. horseflies
Tsetse Flies
Tsetse flies (genus Glossina) are confined to sub-Saharan Africa, but they deserve mention in any discussion of biting flies because of the severity of the disease they carry: African trypanosomiasis, commonly known as sleeping sickness in humans and nagana in livestock. Tsetse flies are unusual even among blood feeders. Both males and females bite, they feed exclusively on blood throughout their entire lives, and they reproduce by a process more like mammals than insects, nourishing a single larva internally with milk-like secretions before giving birth to it live.8PubMed Central. Viviparity and obligate blood feeding: tsetse flies as a unique research system to study climate change The bite itself is painful and leaves a red, swollen mark.
Less Common Biters
A few other fly groups bite humans occasionally but do not get the same attention because encounters are rarer. Snipe flies in the genus Symphoromyia (family Rhagionidae) are found across the Northern Hemisphere and are known to bite hikers and outdoor workers, though they receive far less study than the major pest species.9The Canadian Entomologist. A REVISION OF THE GENUS SYMPHOROMYIA FRAUENFELD (DIPTERA: RHAGIONIDAE): I. INTRODUCTION. SUBGENERA AND SPECIES-GROUPS. REVIEW OF BIOLOGY Bat flies (families Nycteribiidae and Streblidae) are obligate blood feeders on bats and look almost nothing like typical flies; most are wingless and spider-like. They do not bite people under normal circumstances, but they are biologically fascinating as extreme examples of host-specific blood feeding.
The sheep ked (Melophagus ovinus) is another wingless, blood-feeding fly that lives in sheep wool and only rarely contacts humans. And in some tropical regions, the tumbu fly and the human botfly lay eggs that result in parasitic larvae burrowing into skin, though these are not biting flies in the usual sense since it is the larva, not the adult, that causes the problem.
How Biting Flies Find You
All biting flies share a basic host-seeking sequence, though the details vary by species. At a distance, carbon dioxide from your breath is the primary long-range attractant, triggering the fly to switch from random searching to active orientation toward the source. Visual cues take over at closer range: movement, contrast against the background, and body shape all draw the fly in. Specific body odors then fine-tune the approach at short range.10International Journal of Tropical Insect Science. Distance orientation of biting flies to their hosts Some flies also respond to particular volatile compounds from skin bacteria. Research on one cattle-biting species found that butyric, propionic, and acetic acids all acted as attractants.11Veterinary Parasitology. Host finding and feeding in Hydrotaea irritans (diptera, muscidae): The role of chemical senses
Color plays a surprisingly specific role. Biting flies across multiple groups are attracted to blue and dark objects. Neural network modeling of fly visual systems found that, from the fly’s perspective, blue objects share key visual features with potential animal hosts, which is probably why the flies misidentify blue-colored traps and clothing as something worth landing on.12PubMed Central. Why are biting flies attracted to blue objects? This finding explains why blue and black fabrics attract more horse flies and tsetse flies than white or yellow ones, and it is the reason many commercial fly traps use blue or dark panels.
Two Different Ways to Draw Blood
Not all biting flies attack the same way. Some are pool feeders (a process called telmophagy) and others are capillary feeders (solenophagy), and the distinction shapes what the bite feels and looks like. Pool feeders, including horse flies, deer flies, and black flies, use their mouthparts to tear or cut the skin, creating a small wound that fills with blood. They then lap or sponge up the pooled blood. These bites tend to be more immediately painful and may bleed afterward. Capillary feeders, like sand flies and some biting midges, insert their mouthparts into a capillary vessel and draw blood directly, more like a mosquito. The clinical reactions differ because of this mechanical difference and because of the different cocktails of salivary compounds each approach requires.13Allergie et Immunologie. Solenophagy and telmophagy: biting mechanisms among various hematophagous insects
Fly saliva is itself a remarkable pharmacological toolkit. Sand fly saliva, for instance, contains compounds that prevent clotting, dilate blood vessels, and suppress local immune responses, all to keep blood flowing freely during the meal.14PubMed Central. What’s behind a sand fly bite? The profound effect of sand fly saliva on host hemostasis, inflammation and immunity When the fly finishes and the immune system encounters the foreign salivary proteins left behind, the result is the itchy, swollen reaction you feel. Repeated exposure to the same fly species can either sensitize you, making reactions worse, or in some cases lead to partial tolerance over time.
Disease Risks by Fly Type
The disease threat from biting flies varies enormously depending on which fly is responsible and where you are in the world. Here is a practical breakdown:
- Sand flies: Transmit leishmaniasis (cutaneous and visceral) and sandfly fever viruses. Risk is highest in the tropics and subtropics, including parts of Central and South America, the Mediterranean, the Middle East, and East Africa.
- Tsetse flies: Transmit African trypanosomiasis (sleeping sickness). Risk is limited to sub-Saharan Africa.
- Black flies: Transmit the parasitic worm that causes river blindness (onchocerciasis), primarily in equatorial Africa and small foci in Central and South America.
- Biting midges: Transmit several livestock viruses (bluetongue, Schmallenberg) and in some regions, filarial parasites. Direct disease risk to humans from midges is low in most settings.
- Horse flies and stable flies: Act primarily as mechanical vectors. They can carry pathogens between hosts on contaminated mouthparts rather than through a biological cycle inside the fly.
The distinction between mechanical and biological transmission matters. A biological vector like a tsetse fly harbors the parasite inside its body, where it develops into an infective stage. A mechanical vector like a stable fly simply transfers contaminated blood from one host to the next. Mechanical transmission is less efficient and usually requires the fly to feed on an infected host and then bite another host within a short time window.
Protecting Yourself From Biting Flies
Prevention strategies depend on which fly is the problem, but a few principles are broadly useful. Physical barriers remain the most reliable line of defense. Long sleeves, pants, and hats reduce exposed skin. Fine-mesh head nets and bed nets are especially important for sand flies and biting midges, which are small enough to pass through standard mosquito netting.
For chemical repellents, DEET is the most widely known option, but it is not always the best performer against every biting fly group. For ticks, biting midges, sand flies, and black flies specifically, repellents containing IR3535, picaridin, or oil of lemon eucalyptus (the refined compound known as PMD) may offer better topical protection than DEET alone.15PubMed. Chemical and Plant-Based Insect Repellents: Efficacy, Safety, and Toxicity Permethrin-treated clothing adds another layer, particularly for people spending extended time in areas with high fly pressure.
For horse flies around properties, trapping can be effective. Blue and dark-colored panel traps exploit the horse fly’s visual attraction to those wavelengths. Studies comparing trap designs have found large differences in catch efficiency. In one comparison in a floodplain forest, box-style traps caught roughly twelve times as many horse flies as canopy traps, and Nzi traps (a large, tent-like design) caught roughly five times as many.16European Journal of Entomology. Comparative efficiency of traps for horse fly (Diptera: Tabanidae) survey in riparian oak-ash forests in Danube floodplain For homeowners near wetlands or livestock, placing a couple of large dark traps in sunny, open areas can make a noticeable dent in local horse fly numbers.
Animals have their own defense strategies against biting flies. Livestock and wild mammals use tail swishing, ear flicking, head shaking, skin twitching, and bunching together in groups to reduce the per-individual attack rate. Grouping behavior, in particular, dilutes the fly pressure across more animals so that each individual is bitten less often.17Parasitology. Behavioural defense against parasites: interaction with parasite invasiveness Zebra stripes, interestingly, may serve a related function: there is active research into whether the striped pattern disrupts the visual landing cues that biting flies rely on, though that work is still debated.
Why Blood-Feeding Flies Rely on Internal Bacteria
Blood is a nutritionally lopsided meal. It is rich in protein and iron but nearly devoid of certain B vitamins that insects need to grow and reproduce. Flies that feed exclusively on blood have solved this problem by partnering with symbiotic bacteria that live inside their cells and manufacture the missing nutrients. These bacterial partners produce B vitamins, especially biotin, riboflavin, and folate, that the fly cannot get from blood alone.18Frontiers in Microbiology. Genome analysis of “Candidatus Aschnera chinzeii,” the bacterial endosymbiont of the blood-sucking bat fly Penicillidia jenynsii (Insecta: Diptera: Nycteribiidae) Genome analysis of these bacteria shows they have shed almost everything else from their DNA, retaining little beyond the vitamin-production pathways their host requires.
This dependence is not unique to one fly lineage. Tsetse flies, bat flies, and other obligate blood feeders across distantly related insect groups all harbor functionally similar microbiomes dominated by vitamin-producing symbionts. The convergence is striking: evolution has independently arrived at the same bacterial solution to the same dietary shortfall in group after group. For researchers, that dependence also represents a potential vulnerability. If you could disrupt the symbiotic bacteria, the fly might lose its ability to reproduce, offering a possible future control strategy that sidesteps traditional insecticides entirely.