Horse and Deer Flies

image of Horse Fly on tree trunk
Scientific Name
Tabanus, Chrysops, and related genera
Tabanidae (horse and deer flies) in the order Diptera (flies)

Horse and deer flies belong to the same family, and there are about 350 species of them in North America north of Mexico. Like other true flies, they have only one pair of wings, short antennae, and large compound eyes. Distinguishing them from other kinds of flies, horse and deer flies are stout and usually medium to large. Overall, most are drab browns, grays, and blacks, but many species have bright, iridescent, or rainbow-colored eyes, sometimes with spots or stripes. Horse and deer flies are notorious for drinking blood from cuts they make into their host’s skin.

Deer flies are usually smaller than horse flies, and they often have spotted eyes and a dark-spotted pattern on the wings. Horse flies are larger and usually have solid or striped eyes. In both, there is a space between the eyes in the females (when viewed from above), while the eyes of males nearly touch.

The larvae are fairly straight, segmented, wormlike maggots that are tan, whitish, or brownish. They are aquatic or live in mud, and most are predaceous.

Length: ½ to 1 inch; horse flies are usually larger than deer flies (varies with species).
Where To Find
image of Horse and Deer Flies Distribution Map
Adults, being strong fliers, can be found nearly anywhere. They are most common near streams and wetlands, where the females will lay eggs. Females are also common around cattle, horses, deer, and other large mammals, from which they obtain the blood needed in order to make eggs. Most larvae are aquatic, living in streams, on the edges of ponds, or in wetlands, where they prey on insects and other small animals or (in some species) feed on detritus.
Adults eat nectar and pollen from flowers. Females (but not males) also drink blood. They are attracted to large, dark, moving objects. They land softly on a vertebrate (such as a cow or a person), then use mouthparts that are like a pair of scalpels, which they slide back and forth like scissors to make an incision into the skin. They lap up the blood that seeps from the wound. Their saliva contains anticoagulants to keep the blood flowing. The larvae are voracious predators of other small animals, including insects, small fish, and more.
Common. Horse and deer flies are pests of horses, cattle, and other livestock, not only for the pain and frustration they cause for the animals, but also for the diseases they occasionally transmit. Because the wounds can keep bleeding long after the fly’s had its blood meal, an animal that’s been attacked multiple times can experience significant blood loss.
Life Cycle
These, like all other members of the fly family, go through complete metamorphosis, starting out as eggs, which hatch into grublike or caterpillar-like larvae, which pupate and then emerge as winged adults capable of reproducing. In horse and deer flies, mating swarms result in fertilized females, which lay eggs on plants or other objects overhanging water. The larvae live — sometimes for years — in water, then pupate and become creatures of the air.
Tularemia is just one disease that can be transmitted by these flies, though the most common problems they cause for people and livestock are painful, itchy welts. Because they are such determined attackers, they often don’t leave when swatted at.
The larvae help control populations of the many small animals they eat. The adults of some species are important pollinators. Several predators, including insectivorous birds, eat them, and certain wasps parasitize them. The horse guard wasp stings horse flies and uses them to feed its larvae.
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Similar Species
About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.