Horse flies are large, biting insects common in outdoor environments, especially during warmer months. Known for their painful bites, they can be a significant nuisance to humans and animals.
Identifying Horse Flies
Horse flies are robust insects, typically 0.5 to 1.25 inches (14 to 30 mm) long. Their stout bodies are often dark, black or gray, and covered in fine hairs.
A distinguishing feature is their large, often iridescent compound eyes, which can display brilliant green, purple, or horizontal stripes. In many species, the eyes of males almost touch, while those of females are more widely separated.
Their wings are typically clear or uniformly cloudy, though some species have dark wings. Horse flies possess short antennae, shorter than their head, and distinctive mouthparts. These characteristics make them visually distinct.
The Biting Behavior of Horse Flies
Only female horse flies bite, as they require a blood meal for egg development. Male horse flies, in contrast, feed on nectar and plant exudates.
Female horse flies have specialized, blade-like mouthparts that function like scissors. Unlike mosquitoes, which pierce skin with a needle-like proboscis, horse flies cut into flesh, lacerating tissues and blood vessels. This cutting action causes blood to pool, which the fly then laps up using sponge-like mouthparts. They also inject saliva containing anticoagulants to prevent clotting.
This feeding method results in a painful bite, causing localized swelling and bleeding. Horse flies locate hosts using chemical cues, such as carbon dioxide exhaled by warm-blooded animals, and visual cues like motion, size, shape, and dark colors. They are persistent biters, often pursuing a host until they obtain a full blood meal.
Where Horse Flies Live
Horse flies are found globally, with the exception of some islands and polar regions. They prefer moist environments for breeding and development. Common habitats include wetlands, marshes, forests, and areas near bodies of water such as rivers, streams, ponds, and lakes. Larvae develop in wet soil, mud, or aquatic margins, feeding on small invertebrates.
Adult horse flies are most active during warmer months, typically from late spring through summer. Activity peaks on warm, sunny, windless days. While active throughout the day, they show increased activity around sunrise and in the late afternoon. They are less active at night and in shaded areas, preferring sunlight for hunting.
Distinguishing Horse Flies from Similar Insects
Horse flies are often confused with other biting flies, but distinct features can help differentiate them.
Deer flies, also members of the Tabanidae family, are generally smaller than horse flies, typically measuring 0.4 to 0.5 inches (10 to 13 mm) in length. A key difference lies in their wings: deer flies often have dark bands or patterned wings, while horse flies typically have clear or uniformly cloudy wings. Deer flies also have greenish-yellow thoraxes with dark stripes and may prefer biting around the head and neck, unlike horse flies which often target legs and bodies.
Stable flies are another biting insect that can be mistaken for horse flies, but they are considerably smaller, about 0.25 inches (6-7 mm) long, and have a more slender build. Unlike horse flies, stable flies possess a forward-projecting, bayonet-like proboscis they use to pierce skin, commonly biting the lower legs of animals.
House flies, a common nuisance, do not bite at all. They are smaller than horse flies, usually around 0.25 inches long, and have sponging mouthparts for lapping up liquids, not for cutting skin. House flies are typically dull gray with four dark stripes on their thorax, lacking the iridescent eyes seen in many horse fly species.