Horse flies or clegs belong to the fly family Tabanidae. They are large, robust biting flies that are active during the summer months and are well known for their painful bites. While they are primarily a nuisance to livestock such as horses and cattle, they also bite humans and can cause significant discomfort.
This article explains where horse flies and deer flies occur, how they feed, how their bites affect humans and animals, and what can be done to avoid and manage bites under UK conditions.
Occurrence and Activity
When and where are horse flies found?
There are 30 species of horse flies recorded in the UK. All species share several key traits:
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Active mainly during the summer months
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Seen almost exclusively during daylight hours
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Most active on warm, sunny days
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Particularly common near wetlands, lakes, rivers, marshes and damp meadows
Horse flies are rarely active in cool, windy or overcast weather.
Feeding Behaviour
What do horse flies feed on?
Adult horse flies feed primarily on:
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Nectar and plant juices (both males and females)
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Blood (females only, required for egg development)
Female horse flies usually take blood from living hosts, but some species will also feed on blood from carcasses. Although mammals such as horses, cattle, deer and humans are the main hosts, horse flies have also been recorded feeding on birds and reptiles.
Prevention and Control
Can horse flies be controlled?
In practice, horse flies are extremely difficult to control. Unlike mosquitoes, they are not usually targeted by large-scale control programmes, and no specific insecticides are available for effective population reduction.
The most realistic approach is bite prevention, primarily through the use of insect repellents.
Avoiding horse fly bites
The only reliable way to avoid horse fly bites is to use high-quality insect repellents that deter biting insects. Not all repellents are equally effective, so choosing a proven product is important.
Repellents designed for mosquitoes generally provide good protection against horse flies when applied correctly.
Treating Horse Fly Bites
What can be done after a bite?
There is no specific treatment for horse fly bites, only symptom relief. The same products used for mosquito bites are typically effective.
Recommended measures include:
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Immediate care: Clean the bite with soap and clean water
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Cooling: Apply a cold compress to reduce pain and swelling
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Anti-itch treatment: Use bite relief creams or gels to reduce itching
Traditional remedies such as applying saliva to the bite may offer mild, short-term relief but are not necessary if proper products are available.
If symptoms are severe, worsen over time, or show signs of infection, medical advice should be sought.
How Do Horse Fly Bites Feel?
Horse fly bites are painful and typically cause:
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Immediate sharp pain
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Redness and swelling
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Localised inflammation
Most people describe horse fly bites as more painful than mosquito bites and often more noticeable than wasp or bee stings at the moment of biting.
Severe reactions
In some cases, bites may cause:
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Large or spreading swelling
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Oozing fluid from the bite site
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Hives, nausea or dizziness
These reactions are usually systemic responses. Severe allergic reactions, including anaphylactic shock, are extremely rare and typically occur only in individuals with a specific genetic predisposition.
In most cases, symptoms resolve within a few days to a week.
Bite, Not Sting
Do horse flies bite or sting?
Horse flies bite, they do not sting.
Unlike mosquitoes, which pierce the skin with a slender proboscis, horse flies:
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Cut a triangular wound in the skin using blade-like mouthparts
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Secrete saliva containing anticoagulants to prevent blood clotting
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Lap up the blood with a sponge-like mouth structure
Because the bite creates an open wound, bleeding may continue for a short time after the fly leaves.
Preferred Biting Sites
Different groups of horse flies tend to bite different areas of the body:
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Rain horse flies (Haematopota) often fly silently and bite hands or legs
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Horse flies and cattle flies (Tabanus) buzz loudly and often target ankles, calves and the backs of knees
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Deer flies (Chrysops) fly higher and frequently bite the neck, shoulders and arms
Hosts often brush flies away before feeding is complete, so females may bite multiple hosts to obtain a full blood meal.
Disease Transmission
Because horse flies feed on blood from multiple hosts, they can act as mechanical vectors for disease-causing organisms, including bacteria, viruses, protozoa and parasitic worms.
In practice:
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Blood loss and stress in livestock (horses and cattle) are the most common problems
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Disease transmission to humans is rare
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Fatal outcomes in humans are extremely uncommon
What Is a Horse Fly?
General characteristics
Horse flies are large, powerful flies with distinctive features:
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Size: Many species are large and robust; the largest species can reach 2.5–3 cm in length
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Wingspan: Up to 6 cm in the largest species
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Eyes: Very large eyes with excellent vision, often showing colourful patterns when alive
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Flight: Some species buzz loudly, while others fly almost silently
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Landing: Extremely light and often unnoticed until the bite occurs
Species in the genus Tabanus are particularly fast and agile flyers. Some species have even been observed performing complex aerial manoeuvres.
Life Cycle
How do horse flies develop?
Horse flies have a complete life cycle consisting of four stages.
Eggs
Eggs are laid in clusters (up to 1,000) on vegetation or stones near water. They are initially pale but darken over time and hatch after about six days.
Larvae
Larvae fall into water or moist soil after hatching. They are long, cylindrical and predatory, feeding on small organisms such as worms and insect larvae. Some species are cannibalistic. Larvae moult several times over the course of a year.
Pupae
After completing larval development, the insect pupates. The adult fly becomes visible through the pupal case as development progresses.
Adults
Adults emerge fully developed and mate in swarms, often around hilltops or prominent landscape features. The exact timing and location vary by species.
Common Species
Some of the most frequently encountered horse flies and deer flies in Northern Europe include:
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Rain horse fly (Haematopota pluvialis)
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Cattle fly (Tabanus bovinus)
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Giant horse fly (Tabanus sudeticus)
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Deer fly (Chrysops relictus)
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Water horse fly (Heptatoma pellucens)
Horse flies are found across most of the world, with the exception of polar regions and some isolated islands.
Natural Enemies
Predators and parasites
Horse flies have several natural enemies:
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Eggs are often parasitised by tiny wasps
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Larvae are eaten by birds and attacked by parasitic flies, fungi and nematodes
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Adult flies are preyed upon mainly by birds
In some regions, specialised predators exist that feed primarily on horse flies.
Although these natural enemies help regulate populations, they are usually insufficient to prevent nuisance levels during warm summer periods.