Plenty of flies bite, and the ones that do are not just annoying but can transmit disease, cause allergic reactions, and cost livestock industries billions of dollars a year. Out of roughly 150,000 known fly species in the order Diptera, several thousand are obligate blood-feeders, meaning they need a blood meal to survive or reproduce. The list of biters includes horse flies, stable flies, deer flies, black flies, sand flies, tsetse flies, and biting midges, each with its own feeding strategy and set of risks for humans and animals.
Which Flies Bite and Why
The common housefly that lands on your sandwich does not bite. It feeds by vomiting digestive enzymes onto food and slurping up the liquid, which is disgusting but painless. Biting flies are a different story. In almost every biting species, the females are the ones drawing blood. They need proteins from blood to develop their eggs. Male flies of these same species typically feed on nectar or plant sugars and never bite anyone.
Stable flies (Stomoxys calcitrans) are one notable exception: both males and females feed on blood. But for most biting flies, it comes down to reproduction. Research on stable fly colonies has shown that ovarian development does not begin until after feeding, and females deprived of blood experience significant delays in egg maturation.1PubMed. Effects of two blood-feeding regimes on mortality and female reproduction in a laboratory colony of stable flies, Stomoxys calcitrans Without blood, many of these species simply cannot reproduce.
Here are the main groups of flies that bite people:
- Horse flies and deer flies (Tabanidae): Large, fast, and aggressive. Their bites are immediately painful because they slash the skin open rather than piercing it neatly.
- Stable flies (Muscidae): Look almost identical to houseflies but have a bayonet-like proboscis that points forward. Common around livestock but will readily bite people.
- Black flies (Simuliidae): Tiny flies that breed in running water and bite during the day, often around the head and ears.
- Sand flies (Psychodidae): Very small, hairy flies most active at dusk and dawn. Vectors of leishmaniasis and several viral diseases.
- Biting midges (Ceratopogonidae): Often called “no-see-ums” because they are so small. Their bites are disproportionately painful for the size of the insect.
- Tsetse flies (Glossinidae): Found in sub-Saharan Africa, both sexes feed on blood. Vectors of African trypanosomiasis (sleeping sickness).
How the Bite Actually Works
Not all biting flies bite the same way. There are two fundamentally different strategies. Some flies are vessel feeders: they insert fine, needle-like mouthparts directly into a tiny blood vessel and drink from it, somewhat like a mosquito. Mosquitoes (family Culicidae) are the classic example within the fly order. Other flies are pool feeders: they use blade-like mouthparts to slice open the skin, creating a small wound that fills with blood, and then lap it up. Horse flies, black flies, biting midges, and sand flies all use this pool-feeding approach.2PubMed Central. An approach to telmophagous Nematocera (Ceratopogonidae, Psychodidae, and Simuliidae) of Spain, with emphasis on its medical and veterinary importance
Pool feeding is the reason horse fly bites hurt so much more than mosquito bites. The fly is literally cutting you. And because a larger surface area of the fly’s mouthparts contacts the blood pool, pool feeders are also more efficient at mechanically transmitting pathogens between hosts, since more contaminated blood clings to those mouthparts between meals.
To keep the blood flowing while they feed, biting flies inject saliva packed with pharmacologically active compounds. Horsefly salivary glands, for instance, contain an arsenal of anti-clotting tools: enzymes that break down fibrin (the protein that forms blood clots), proteins that block platelet aggregation, thrombin inhibitors, and vasodilators that widen blood vessels near the wound.3PubMed Central. Anti-thrombosis repertoire of blood-feeding horsefly salivary glands Different horsefly species have evolved somewhat different anti-clotting strategies, with some relying heavily on antithrombin activity while others lean more on fibrin-degrading enzymes.4PubMed. Anticoagulant activities in salivary glands of tabanid flies These saliva proteins are also what triggers the itching, swelling, and sometimes allergic reactions that follow a bite.
How Biting Flies Find You
Biting flies do not randomly stumble onto hosts. They use a layered set of sensory cues, starting from a distance and getting more precise as they close in.
Carbon dioxide is the long-range beacon. Your exhaled breath creates a COâ‚‚ plume that biting flies can detect from tens of meters away. But COâ‚‚ alone does not explain host preference. Field experiments with sand flies demonstrated that anthropophilic species are attracted to human body odors in addition to COâ‚‚, suggesting that chemical compounds in human sweat and skin help flies zero in on a specific host type.5PubMed. Phlebotomine sandfly responses to carbon dioxide and human odour in the field
At closer range, vision and heat take over. Horse flies are strongly attracted to dark colors and warm surfaces. Studies on tabanid behavior found that female horse flies prefer sunlit, warm, shiny dark targets over cooler ones with identical optical properties.6PubMed Central. Attractiveness of thermally different, uniformly black targets to horseflies: Tabanus tergestinus prefers sunlit warm shiny dark targets One explanation is that a warmer microenvironment allows the fly’s wing muscles and nervous system to work faster, helping her escape when the host swats. Male and female horse flies also respond differently to visual cues: females seeking blood are attracted to dark objects that reflect polarized light, reading them as potential hosts, while males may interpret similar cues as water surfaces.7PubMed Central. Horsefly reactions to black surfaces: attractiveness to male and female tabanids versus surface tilt angle and temperature
Color preference goes beyond just dark versus light. Research on why biting flies are drawn to blue objects found that a blue preference is functionally useful for identifying hosts and is not simply a side effect of the flies’ polarization-sensing system, as had been previously suggested.8PubMed Central. Why are biting flies attracted to blue objects? This has practical implications for trap design and clothing choices, which we will get to shortly.
Diseases Biting Flies Transmit
The medical importance of biting flies goes well beyond the bite itself. Sand flies are the primary vectors of leishmaniasis, a parasitic disease that affects millions of people worldwide and can cause skin ulcers, organ damage, or death depending on the species of Leishmania involved.9PubMed Central. What’s behind a sand fly bite? The profound effect of sand fly saliva on host hemostasis, inflammation and immunity Sand flies also transmit sandfly fever viruses, Chandipura virus encephalitis, and bartonellosis (Carrión’s disease).10PubMed. Phlebotomine sandflies and the spreading of leishmaniases and other diseases of public health concern
Tsetse flies transmit trypanosomes that cause sleeping sickness in humans and nagana in cattle across much of sub-Saharan Africa. Recent research has also linked repeated tsetse fly bites to alpha-gal syndrome (AGS), a delayed allergic reaction to red meat. Case studies documented individuals developing AGS symptoms after multiple tsetse bites, and laboratory analysis suggested that tsetse flies may produce alpha-gal modifications that sensitize the immune system.11PubMed Central. Allergic reactions associated with tsetse fly bites This is still an emerging area of research, but it parallels findings with tick bites and AGS.
Black flies transmit the parasitic worm that causes river blindness (onchocerciasis), a major cause of preventable blindness in parts of Africa and Latin America. Horse flies and deer flies are primarily mechanical transmitters, meaning they physically carry pathogens on their mouthparts from one host to the next rather than serving as biological incubators for a pathogen. They have been implicated in transmitting anthrax, tularemia, and certain blood parasites of livestock.
When Biting Flies Are Most Active
Knowing when biting flies are out can help you avoid them. Different species follow very different schedules, and those schedules are driven by a combination of internal clocks and environmental conditions.
Sand flies in tropical forests bite mostly at dawn and dusk. A year-long study tracking seven sand fly species in Panama found that about 46% of biting activity occurred during crepuscular periods (dawn and dusk), about 39% was nocturnal, and only about 15% happened during full daylight at ground level. Up in the forest canopy, the pattern shifted even more toward nighttime: nearly 64% nocturnal and less than 1% diurnal.12Journal of Medical Entomology. Daily and Seasonal Man-Biting Activity of Phlebotomine Sandflies in Panama The practical takeaway is that wearing repellent and long sleeves is most important during twilight hours if sand flies are a concern.
Tsetse flies show a characteristic V-shaped activity pattern through the day, with peaks in the morning and late afternoon and a lull during midday heat. Interestingly, about 80% of this pattern is driven by an internal circadian clock rather than by temperature directly, with temperature accounting for only about 20% of the variation.13Physiological Entomology. The control of circadian activity rhythms in tsetse flies: environment or physiological clock? So even on an unusually cool or warm day, tsetse flies maintain roughly the same biting schedule.
Horse flies and deer flies, by contrast, are daytime feeders that are most active in warm, sunny, calm conditions. They tend to be worst from late morning through mid-afternoon. Black flies are also daytime biters, with activity peaking around sunrise and again before sunset. Biting midges often peak just after sunset, though some species are daytime biters near coastal areas.
The Economic Toll on Livestock
For most people, a fly bite is a brief, irritating event. For livestock producers, biting flies are a genuine economic disaster. Stable flies alone cost the U.S. cattle industry an estimated $2.2 billion per year, based on reduced milk production in dairy cows and lost weight gain in beef cattle.14PubMed. Economic impact of stable flies (Diptera: Muscidae) on dairy and beef cattle production Updated to 2018 cattle inventories and prices, that figure rises to roughly $2.7 billion.15Journal of Integrated Pest Management. Stable Fly (Diptera: Muscidae)—Biology, Management, and Research Needs
The damage is not mainly from blood loss. When stable flies attack, cattle respond by bunching together to protect their legs, which is where stable flies prefer to feed. Bunching reduces airflow around the animals, increasing heat stress. Research found that about 72% of the reduction in weight gain caused by stable flies was attributable to bunching behavior and heat stress, while only about 29% was due to blood loss and the energy spent fighting the flies.16Journal of Economic Entomology. Effects of Stable Flies (Diptera: Muscidae) and Heat Stress on Weight Gain and Feed Efficiency of Feeder Cattle Feed efficiency dropped by 11–12% regardless of the type of ration cattle were given. During severe infestations, livestock mortality has been documented.
How to Prevent Bites
Prevention is mostly about creating barriers between you and the flies, whether chemical, physical, or visual.
Repellents That Work
DEET remains the gold standard for personal repellent against biting flies. In controlled bioassays testing DEET, picaridin, and a compound called SS220 against mosquitoes and sand flies, all three effectively deterred and repelled the insects. In half-treated test cells, 98–100% of bites occurred on untreated skin, with the insects never making physical contact with treated surfaces. The flies detected a vapor-phase chemical gradient and avoided the repellent entirely.17Journal of Medical Entomology. Repellent and Deterrent Effects of SS220, Picaridin, and Deet Suppress Human Blood Feeding by Aedes aegypti, Anopheles stephensi, and Phlebotomus papatasi Picaridin is a good alternative for people who dislike the feel of DEET; it performed comparably in these tests. For horse flies and stable flies, permethrin-treated clothing adds an extra layer of protection that topical repellents alone sometimes cannot provide, since larger tabanids are more persistent feeders.
Clothing Color and Visual Cues
What you wear matters more than you might think. Since horse flies and other tabanids are strongly attracted to dark, warm surfaces, wearing light-colored clothing reduces your visual profile as a target. Blue and black are particularly attractive to biting flies, while white and light yellow are less so. Trap studies in India confirmed that blue sticky traps caught nearly three times as many flies as yellow ones.18PubMed Central. Comparing trap designs and methods for assessing density of synanthropic flies in Odisha, India If you are spending time outdoors in horse fly country, choosing a white hat and light khaki pants over a dark blue shirt gives you a meaningful advantage.
Traps and Area-Level Control
For people managing property, traps can reduce local biting fly populations. Blue or black targets coated with adhesive film are remarkably effective for tsetse flies; in standardized trials, these flat targets captured 4–10 times more tsetse than traditional biconical or monoconical traps, depending on the species.19PLoS Neglected Tropical Diseases. Standardizing Visual Control Devices for Tsetse Flies: West African Species Glossina tachinoides, G. palpalis gambiensis and G. morsitans submorsitans For horse flies, dark sphere traps hung at animal height exploit the flies’ attraction to dark, warm objects. Commercial horse fly traps typically use a black ball that heats up in the sun, mimicking a large dark animal, with a funnel above that captures flies as they spiral upward after landing.
Eliminating breeding habitat also helps. Stable flies breed in decaying organic matter like old hay, manure mixed with straw, and wet feed residue. Keeping barns clean and managing compost piles reduces stable fly emergence. Black flies breed in flowing water, so they are harder to control at the source, though larviciding programs using Bacillus thuringiensis israelensis (Bti) in streams have been effective for black fly management in some regions.
The Zebra Stripe Effect
One of the more fascinating recent findings in biting fly research involves zebra stripes. Studies have shown that biting flies strongly prefer landing on uniformly colored surfaces over striped ones. When researchers draped zebra skins and impala skins over frames, flies strongly preferred impala skin. And when flies did land on zebra skin, they chose black stripes about 75% of the time, suggesting the white stripes are specifically disruptive.20Scientific Reports. Zebras of all stripes repel biting flies at close range
Further experiments found that flies avoided landing on both striped and checked pattern rugs compared to uniform grey rugs, flew faster near them, and did not approach as closely. The fact that checked patterns worked too rules out the idea that the effect depends on parallel lines disrupting the fly’s optic flow in some specific way; the contrast pattern itself seems to confuse the fly’s landing approach.21PubMed Central. Zebra stripes, tabanid biting flies and the aperture effect
This has been tested in a livestock setting with striking results. Japanese researchers painted cows with black-and-white zebra-like stripes and compared them to unpainted controls and cows painted with only black stripes. The striped cows had roughly half the number of biting flies on their bodies and showed about 20% fewer fly-repelling behaviors like tail swishing and leg stamping.22PLOS ONE. Cows painted with zebra-like striping can avoid biting fly attack Painting cattle is not yet a commercially scalable solution, but the research points toward pesticide-free approaches for protecting livestock, which matters increasingly as insecticide resistance grows.
How Blood-Feeding Evolved in Flies
Blood-feeding did not evolve just once in the fly lineage. It appears to have arisen independently multiple times across different fly families, which is why the mouthparts and feeding strategies of horse flies, mosquitoes, and tsetse flies look so different from one another. Evolutionary analysis of the broader group that includes mosquitoes (Culicomorpha) suggests that blood-feeding likely evolved from ancestral plant-feeding, or possibly from a combination of plant-feeding and predatory insect-feeding.23PubMed. Sensory mechanisms for the shift from phytophagy to haematophagy in mosquitoes Evidence from salivary gland proteins and life-history traits supports plant-feeding as the ancestral diet, with blood-feeding evolving later as a way to obtain the concentrated protein needed for egg production.
This evolutionary history explains why even blood-feeding flies often still drink nectar or plant juices for their daily energy needs. Blood is not a general fuel source for these insects; it is specifically a reproductive resource. A female horse fly may spend weeks feeding on flower nectar between blood meals, only seeking a host when she needs to mature a batch of eggs. Males of most species never make the switch at all, spending their entire lives on plant sugars. The evolutionary push toward blood-feeding was strong enough to reshape mouthparts, sensory systems, and salivary biochemistry, but it only needed to do so in the sex that produces eggs.