The common horse fly, also known in the UK as the notch-horned cleg fly (Haematopota pluvialis), is one of the most troublesome biting flies encountered during the British summer. It is fast, persistent and well known for its painful bites, which can be highly irritating for both people and animals, particularly in warm and humid weather.
This article explains what cleg flies are, where and when they occur in the UK, how their bites affect the body, and what you can do to avoid and treat them.
Occurrence in the UK
Where and when are cleg flies found?
The common horse fly is widespread across the UK, especially in damp and semi-natural landscapes. It is strongly associated with moisture and is most abundant in areas with wet or marshy ground.
Typical habitats include:
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Marshland and wetlands
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Damp meadows and pastures
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Woodland edges and forests
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Areas close to lakes, rivers and ditches
In the UK, cleg flies are active only during the summer, usually from June to August, with peak activity in warm, humid conditions. They are strictly day-active insects and are most aggressive on warm, still days, often before or after rainfall.
They are inactive at night and avoid cool, shaded conditions.
Feeding Behaviour
What do cleg flies feed on?
Adult cleg flies primarily feed on plant nectar. Their larvae, which live in damp soil or mud, are predatory, feeding on small soil-dwelling organisms and occasionally showing cannibalistic behaviour.
Only the adult female bites humans and animals. A blood meal is essential for egg development.
Common hosts include:
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Humans
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Horses and ponies
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Cattle and sheep
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Deer and other large mammals
Control and Prevention
Can cleg flies be controlled?
Cleg flies are very difficult to control. There are no targeted control products available for domestic or agricultural use in the UK.
Although researchers sometimes use specialised traps (often baited with animal odours such as urine), these are not practical or effective solutions for private individuals.
The most effective strategy is therefore bite prevention.
How to avoid cleg fly bites
The only reliable way to reduce the risk of being bitten is to use effective insect repellents on exposed skin and clothing. While repellents are generally more effective against mosquitoes, many also provide partial protection against cleg flies.
Additional measures include:
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Wearing long sleeves and trousers in high-risk areas
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Avoiding wetlands and damp pastures on hot days
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Taking breaks in shaded or breezy areas
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Reducing outdoor activity during peak daytime heat
For horses and livestock
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Use approved insect repellents and fly rugs
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Stable horses during peak activity periods
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Improve drainage in fields to reduce wet areas
About Cleg Fly Bites
When do bites occur?
Cleg flies are most active in sunny, warm and humid conditions. The risk of bites decreases rapidly in cooler weather, shade or wind.
A well-known Scottish saying reflects this behaviour:
“When the sun is out, the clegs bite; when the sun goes in, the midges bite.”
In some parts of the UK, particularly in Scotland, it is possible to encounter both cleg flies during the day and biting midges later in the evening.
The Bite Itself
How does a cleg fly bite?
Cleg flies do not sting – they bite.
Their mouthparts are sharp and blade-like, allowing them to cut a small triangular wound in the skin. Unlike mosquito bites, this creates an open wound rather than a puncture.
Their saliva contains anticoagulant substances that keep the blood flowing, which increases bleeding, irritation and swelling.
Typical symptoms
Most bites cause:
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Immediate pain
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Redness and swelling
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Local inflammation
Cleg fly bites are generally described as more painful than mosquito, wasp or bee stings, although reactions vary between individuals.
In some cases, stronger reactions may occur, including:
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Large swelling
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Fluid discharge from the bite
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Widespread itching or hives
Very rarely, severe allergic reactions (including anaphylaxis) have been reported, usually only in genetically predisposed individuals.
Behaviour Around Hosts
Cleg flies are often described as persistent or “clingy”:
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They do not always fly away when swatted
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They may survive being hit
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They often continue circling a host repeatedly
This behaviour has led to colloquial names such as “blind flies”, despite the fact that cleg flies have excellent vision.
In areas with high populations, several flies may attack the same person or animal simultaneously.
Treating Cleg Fly Bites
What should you do after a bite?
Although cleg fly bites cannot be “treated” in the medical sense, symptoms can be relieved.
Recommended steps:
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Clean the wound promptly with soap and water
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Apply soothing or anti-itch products
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Use a cold compress to reduce swelling
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Avoid scratching to prevent infection
If swelling is severe, fluid leaks from the wound, or signs of infection develop, medical advice should be sought.
Identification
What does the common horse fly look like?
The common horse fly is smaller than the giant horse flies but still distinctive.
Key features include:
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Length: Typically 6–10 mm (females up to 12 mm)
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Colour: Grey to brown
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Wings: Distinctive marbled pattern with dark patches
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Eyes: Large, with subtle metallic patterns when alive
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Body: Compact and relatively flat compared with larger Tabanus species
Only females bite; males feed exclusively on nectar and plant sap.
Distribution
Where are cleg flies found?
The common horse fly occurs across:
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Most of the UK
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Much of Europe
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Parts of western and eastern Asia
In the UK, they are most common:
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In July–September
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After rainfall
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Near wetlands, moors, forests and damp grassland
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Around grazing animals and humans
They are attracted to heat, movement and carbon dioxide, which makes people and animals easy targets.
Eggs and Larvae
Female cleg flies lay their eggs at the base of grasses in moist but not flooded soil. Larvae develop in damp ground that is usually undisturbed by ploughing or fertilisation.
Larvae are predatory, feeding on small soil organisms, and may exhibit cannibalism.
Males and Females
As with many blood-feeding insects:
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Only females bite
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Males are rarely noticed because they do not approach humans
Male cleg flies tend to remain in tall grass near water, where they sit on vegetation and make short flights. Even when actively searched for, males are rarely found in large numbers.
Name and Classification
The common horse fly belongs to:
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Species: Haematopota pluvialis
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Genus: Haematopota
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Family: Horse flies (Tabanidae)
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Order: True flies (Diptera)
The genus name Haematopota comes from Greek and means “blood drinker”, while pluvialis means “of rain” – a reference to the species’ strong association with humid, rainy conditions.
Understanding the behaviour and ecology of the common horse fly makes it easier to avoid bites and manage exposure. While complete prevention is not possible, informed timing, protective clothing and effective repellents can greatly reduce the impact of this aggressive summer biter in the UK.