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The Biting Truth about Horse Flies

Horse fly face
(Image: VPM/What's Bugging You)

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Anyone who has spent time outdoors during the summer has likely endured the painful bites of horse flies. Just the threat of attack by these notoriously persistent blood-suckers causes great anguish, not only in people, but also among domestic and wild animals alike. Join entomologist Dr. Art Evans and VPM radio producer Steve Clark as they explore the fascinating natural history and habits of horse flies.

Horse flies and deer flies are both in the Tabanidae, a family of flies that includes about 4,000 species distributed nearly worldwide. Tabanids, along with some other unrelated biting flies, are also known as clegs and gadflies. Interestingly, the term ‘gadfly’ is more likely to be used today in reference to an annoying individual who constantly criticizes.

Chunky, but agile, horse flies are fast-flying insects capable of amazing bursts of speed. For example, a male of one North American species was clocked at 90 mph as it pursued a female! Adults generally prefer open, sunny habitats and feed on nectar. The males have contiguous compound eyes and only weakly developed mouthparts, thus are incapable of biting. Female horse flies have distinctly separated compound eyes and well-developed mouthparts.

Most females are capable of biting as they require proteins from blood meals to stimulate egg production. They use their blade-like mandibles to cut through the skin and capillaries, then employ a sponge-like labella to sop up the blood that wells up into the wound. Species divvy up resources by attacking different parts of the body. Deer flies in the genus Chrysopa tend to go for the head, while other tabanid species prefer the legs. Recent studies have shown that the stripes of zebras impact the visual acuity of tabanids, thus making it difficult for them to land a bite.

Females lay their egg masses on wetland vegetation. In about a week, the hatching larvae drop into the water or mud along moist shorelines. Their long, spindle-shaped, and legless bodies lack a distinct head and are ringed with bands of tubercles and short, stiff bristles that help them gain purchase as they hunt for invertebrates.

The most beautiful horse fly in North America is the green horse fly, Chlorotabanus crepuscularis. It is distinguished from all other tabanids on the continent by its distinctly greenish hue. They are active primarily in the spring and summer. Adult females take their blood meals at night and locate their victims by tracking carbon dioxide in their exhalant.

To avoid being beset by horse and deer flies and their painful bites, avoid marshlands and beaches or other open areas adjacent to woodlands on windless days. Wear light-colored long pants and long-sleeved shirts, as well as a hat. Applying insect repellents on bare skin that contain DEET, picaridin, or lemon-eucalyptus helps, too.

Here’s another quirky story from WBY about flies found in the Richmond Courthouse.