**Horse Flies** (family Tabanidae) are large, stout, true flies notorious for their painful bites. They are globally distributed and are significant pests of livestock (horses, cattle) and humans. Only the **female horse fly bites**—she uses her large, razor-sharp mouthparts to slice the skin and feed on the blood necessary for egg production. Their aggressive biting, strong flight, and ability to transfer disease agents make them a major nuisance in recreational areas and a source of economic loss in agriculture.
Taxonomy and Classification
Horse Flies belong to the order Diptera (true flies), family Tabanidae. This family includes the large horse flies (*Tabanus* spp.) and the smaller **Deer Flies** (*Chrysops* spp.), which share the same biting behavior and medical significance. They undergo complete metamorphosis. Unlike mosquitoes, their mouthparts are not needle-like; instead, they are composed of six blade-like structures designed to slash the skin and create a pool of blood (telmophagy).
Physical Description
Adult Horse Flies are large, robust flies, typically $1/2$ to $1$ inch long.
- **Appearance:** Gray, black, or brown, often with large, brilliant, iridescent eyes that are frequently banded or patterned, especially in the smaller Deer Flies.
- **Mouthparts:** The prominent, dagger-like mouthparts are clearly visible.
- **Larvae:** Found in semi-aquatic or moist environments (mud, decaying organic matter) and are predatory, feeding on insect larvae, worms, and snails.
- **Behavior:** They are diurnal (active during the day), strong, fast fliers, and often pursue their targets persistently, attracted by movement, carbon dioxide, and dark colors.
Distribution and Habitat
Horse Flies are distributed globally but are most common in warm, humid regions with abundant standing water or marshy areas necessary for larval development. Their adult habitat is open pastures, woodlands, and the immediate vicinity of their hosts. They are often most active during mid-summer.
Behavior and Conflict
Only the female fly feeds on blood (hematophagy); males feed primarily on nectar and plant sap.
- **Painful Bite:** The bite is immediately painful due to the laceration of the skin, causing a large, localized welt and bleeding. This pain causes extreme distress in livestock, leading to reduced grazing, bunching, and sometimes injury.
- **Disease Transmission:** Horse Flies are mechanical vectors, meaning they can transfer pathogens on their mouthparts. They are known to transmit diseases such as **Equine Infectious Anemia (Swamp Fever)** in horses, **Tularemia** in humans, and trypanosomes in cattle.
- **Interrupted Feeding:** They are easily disturbed while feeding, often flying immediately to another host to resume feeding. This behavior greatly increases their efficiency as mechanical disease vectors.
Management and Prevention
Control is difficult due to the fly’s size, mobility, and the broad, wet areas required for larval development.
- **Trapping (Recommended):** Use specific, non-chemical traps (e.g., sticky balls, box traps, or the **Manitoba trap**) that attract the flies visually (often dark colors and movement) and physically capture them.
- **Repellents:** Traditional insect repellents (containing DEET) offer only limited protection against the determined bite of a horse fly. Products specifically formulated for livestock containing permethrin or pyrethrins may provide temporary relief.
- **Exclusion:** Keeping animals stabled during peak daytime activity can reduce biting, especially for high-value animals.
- **Habitat Management:** Reducing or draining excessive wet or marshy areas can minimize breeding sites, though this is often impractical.
Conservation and Research
Horse Flies are managed as veterinary and public health pests. Research focuses on optimizing the design and placement of non-chemical traps, and understanding the chemical ecology (pheromones) that drives their host-seeking behavior to develop better, species-specific lures.