Horse flies, members of the insect family Tabanidae, are large, robust, biting insects found across the globe, excluding only the polar regions and a few isolated islands. Their size and persistent nature make them a noticeable presence in warmer months. The distinctive name—horse fly—is a direct reference to the insects’ historical and biological interactions with large mammals, particularly domesticated livestock. This powerful flier and blood-feeder earned its common name through centuries of agricultural nuisance.
The Reason Behind the “Horse” Designation
The common name “horse fly” originates from the centuries-old relationship between these insects and the large, domesticated animals they relentlessly target. Historically, horses and oxen were the primary draft animals on farms, and the flies’ aggressive pursuit of them was a constant and significant problem. The name, which has been in use since at least the late 14th century, is simply a descriptive label for the fly that preys upon the horse.
These insects are attracted to large, dark, moving objects that produce carbon dioxide, which fits horses, cattle, and other livestock perfectly. When horse flies congregate in large numbers, their overwhelming attacks cause significant distress, leading to reduced grazing time and energy expenditure from constant avoidance behavior. Heavy infestations can result in a measurable reduction in weight gain and milk output in cattle. The volume of blood taken by a swarm of flies, sometimes several ounces daily from a single animal, highlights the serious economic and physical harm they inflict on livestock.
The Unique Mechanics of the Horse Fly Bite
The pain associated with a horse fly interaction is due to the unique, specialized mouthparts possessed only by the female fly. Unlike the fine, needle-like stylet of a mosquito, the female horse fly uses a stout, stabbing organ equipped with two pairs of blade-like appendages: the mandibles and maxillae. These sharp structures work like miniature scissors, lacerating the host’s skin and cutting through the capillaries rather than simply piercing them.
This cutting action creates a small, open wound where blood pools, which the fly then laps up with a sponge-like part of its mouth. This “pool feeding” method is why the bite is immediately painful and often results in a noticeable flow of blood. The female requires a blood meal to obtain the necessary protein to mature and lay her eggs. This feeding behavior, where the fly is interrupted and moves quickly from one host to another, can mechanically transmit blood-borne diseases such as equine infectious anemia among livestock.
Identifying Horse Flies
Horse flies are generally characterized by their considerable size, often measuring between 10 to 25 millimeters in length, making them much larger and more robust than a common housefly. They possess prominent, compound eyes that are frequently iridescent or brightly colored with patterns of green or purple in living specimens. In the male fly, these large eyes meet at the top of the head, while in the female, they are distinctly separated.
Their body is broad and stout, and they are known for being strong, agile fliers that produce a loud, distinctive buzzing sound during flight. This audible approach, combined with their large size, often makes them identifiable before they land. All horse flies belong to the family Tabanidae, a group that includes both the large horse flies (genus Tabanus) and the slightly smaller deer flies (genus Chrysops).