Horse Flies
Species
Horse Fly Facts
- Nicknames: Clegs, breeze flies and gadflies. Deer flies are smaller relatives in the same family.
- Size: Most are between about 1/2 inch and 1 inch long, and a few large species run a little over an inch.
- Appearance: Thick bodies, very large and often colorful eyes, and wings that range from clear to smoky.
Horse Fly Behavior
Horse flies live on nearly every continent, and they are disliked almost everywhere they occur. Females bite people, pets and livestock to get the blood they need for their eggs, and they will chase a target for some distance. They can pass certain diseases between animals, though they are rarely a disease concern for people in Florida.
Identifying Horse Flies and Their Relatives
Horse flies are stout, heavy-bodied flies with wide heads that are mostly eye. In many species the eyes shine green, gold or purple, sometimes with stripes or bands. You can even tell the sexes apart: in males the eyes meet at the top of the head, while females have a gap between them. Short, stubby antennae and a single pair of wings separate a horse fly from the bees it can resemble.
Florida has many species, from the large, all-black black horse fly to smaller striped kinds. Their close cousins, deer flies, are smaller, closer to house fly size, and usually have dark patches or bands across the wings, while many horse flies have clear or lightly tinted wings. Stable flies, another common biter, are gray, house fly-sized, and tend to go after ankles and legs.
- Diet: only females bite, because they need blood to produce eggs. Males feed on nectar and plant juices.
- Life cycle: eggs are laid on plants or objects above water or wet ground. Larvae live in mud, wet soil and marsh edges, where they prey on other small creatures.
- Activity: they fly by day, mostly on hot, sunny days, and are most common from late spring through summer in Florida. They find hosts by sight, warmth and the carbon dioxide in breath, and they favor dark, moving targets.
They’re common throughout Florida near marshes, swamps, ponds and wooded edges. For what to expect from a bite, see horse fly health concerns.
Like mosquitoes, female horse flies need a blood meal before they can develop eggs. Males never bite. They feed at flowers and play a small part in pollination. Size varies widely across the family, from flies close to a house fly to species bigger than a honeybee.
The bite hurts enough that the animal or person usually reacts right away, and the fly is often driven off before it is done. It then moves to the next host to keep feeding. That habit is how horse flies can carry blood-borne diseases from one animal to another on their mouthparts. On farms, heavy fly pressure keeps cattle and horses stamping and bunching up instead of grazing, which can cut weight gain and milk output. Numbers change from year to year, often following how wet the season has been, since the larvae need mud and soaked soil. In a rainy Florida summer, they can be thick near marshes and pastures.
Fly Species
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