Common Horse Fly / Notch-Horned Cleg Fly (Haematopota pluvialis)

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The common horse fly, also known as the notch-horned cleg fly (Haematopota pluvialis), is a small but aggressive biting fly in the family Tabanidae. While this exact species is native to Europe and not established in the United States, it is closely related to several North American Haematopota and small Tabanus species. These U.S. relatives behave in almost identical ways and cause similar problems for people and animals.

This article explains how notch-horned cleg flies behave, where similar flies occur in the U.S., how their bites affect humans and animals, and what you can do to avoid and treat bites.

Occurrence and Activity

In Europe, Haematopota pluvialis is active only during the summer months. Comparable cleg and small horse fly species in the United States follow the same pattern.

Typical activity patterns:

  • Active from late spring through summer

  • Peak activity on warm, humid days

  • Strictly day-active

  • Inactive at night and during cool or overcast weather

  • Avoid shaded and windy areas

They are most abundant near wet or marshy ground, which is essential for larval development.

Habitat Preferences

Cleg flies thrive in moist environments, especially:

  • Wet meadows and pastures

  • Marshes, bogs, and floodplains

  • Forest edges near water

  • Areas with saturated or muddy soil

In the U.S., similar biting flies are common around:

  • Streams, ponds, lakes, and wetlands

  • Irrigated fields and low-lying pastureland

  • Wooded areas adjacent to open grassland

Feeding Behavior

Adult cleg flies feed primarily on nectar and plant fluids.
Only females bite, because a blood meal is required to produce eggs.

  • Females bite humans, horses, cattle, deer, and other mammals

  • Males do not bite and are rarely noticed

  • Larvae are predatory, living in moist soil or mud and feeding on small invertebrates; cannibalism can occur

What Is a Cleg Fly?

The notch-horned cleg fly (Haematopota pluvialis) is:

  • Smaller than most large horse flies

  • Extremely fast and persistent

  • Known for painful bites

  • Often active in humid weather, sometimes just before rain

In everyday language, “cleg fly” or “rain fly” is sometimes used incorrectly to describe various biting insects. This article focuses on true cleg flies (family Tabanidae), which are responsible for the painful daytime bites described here.

How to Avoid Cleg Fly Bites

There is no practical way to eliminate cleg flies from an area. Avoidance is the only reliable strategy.

For people

  • Use insect repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus

  • Apply repellent to both skin and clothing

  • Wear long sleeves and pants when possible

  • Avoid wetlands and open sunny areas during peak activity hours

For horses and livestock

  • Use fly repellents approved for animals

  • Fly sheets, masks, and leg protection can reduce bites

  • Provide access to dry, shaded, or breezy areas

  • Improve drainage in paddocks and pastures when feasible

About Cleg Fly Bites

Cleg flies bite — they do not sting.

Their mouthparts:

  • Cut a triangular wound in the skin

  • Inject saliva containing anticoagulants

  • Allow blood to flow freely while feeding

Symptoms

  • Immediate sharp pain

  • Redness and swelling

  • Itching during healing

Bites are usually described as more painful than mosquito bites. Symptoms vary between individuals.

In rare cases, stronger reactions may occur, including:

  • Large localized swelling

  • Fluid leakage from the bite site

  • Generalized hives or systemic reactions

Severe allergic reactions are extremely rare and require medical attention.

Treating Cleg Fly Bites

Treatment focuses on symptom relief and infection prevention.

Recommended steps:

  • Clean the bite promptly with soap and water

  • Apply cold compresses to reduce swelling

  • Use anti-itch or soothing products (e.g. hydrocortisone cream)

  • Avoid scratching

Seek medical care if:

  • Swelling is severe or spreading

  • The bite becomes infected

  • Unusual symptoms develop

Identification

Adult notch-horned cleg flies typically measure:

  • 6–10 mm, with females occasionally reaching 12 mm

Key features:

  • Gray-brown body with banded abdomen

  • Relatively flat, compact build

  • Distinctively mottled wings with dark patches

  • Large eyes with metallic patterns when alive

  • Black legs with yellow-brown rings

They are smaller and lower-profile than the large Tabanus horse flies but often more persistent.

Eggs and Larvae

Females lay eggs:

  • Near grass roots

  • On moist but not flooded soil

  • In muddy or marshy environments

Larvae develop in undisturbed, moist soil and feed on small organisms. They do not develop in dry or heavily cultivated ground.

Males vs. Females

As with other blood-feeding flies:

  • Only females bite

  • Males feed on nectar and plant sap

Males are rarely seen because they remain in vegetation, often:

  • In tall grass near water

  • Perched on grass tips or leaf edges

  • Flying short distances within the vegetation

Distribution and Relevance to the United States

Haematopota pluvialis occurs throughout most of Europe and parts of Asia. While it is not established in the U.S., closely related species fill the same ecological niche in North America.

For U.S. residents, the key takeaways are:

  • Small horse flies and clegs are day-active, humidity-loving biters

  • They are most problematic near wetlands

  • Elimination is unrealistic

  • Repellents, protective clothing, and habitat avoidance are the most effective defenses

Understanding the biology of the European notch-horned cleg fly helps explain why similar biting flies in the United States are so persistent, painful, and difficult to manage during summer months.