Horse and Deer Flies

Tabanus, Chrysops, and related genera

horse_fly_2012.jpg

image of Horse Fly on tree trunk
Donna Brunet
Family

Tabanidae (horse and deer flies) in the order Diptera (flies)

Description

Horse and deer flies belong to the same family. 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.

Key Identifiers

Adults:

  • Stout flies, usually medium to large.
  • Overall color usually drab brown, gray, or black.
  • Only one pair of wings.
  • Antennae short.
  • Eyes large, compound, sometimes bright, iridescent, or rainbow-colored, sometimes with spots or stripes.
  • Females drink blood from cuts they make in their host’s skin.
  • Deer flies: usually smaller than horse flies; often have spotted eyes and a dark-spotted pattern on the wings.
  • Horse flies: usually larger than deer flies; usually have solid or striped eyes.
Size

Length: ½ to 1 inch; horse flies are usually larger than deer flies (varies with species).

deerfly_brunet2012_7602.jpg

Photo of a deer fly.
Deer Fly
Deer flies are usually smaller than horse flies, and they often have spotted eyes and a dark-spotted pattern on the wings.

Deer_Fly_Male_2-22-16.jpg

Photo of a male deer fly, probably Chrysops reicherti, on a leaf.
Deer Fly Male
In male deer and horse flies, viewed from above, the large compound eyes are so close together they almost touch. In females, there is a noticeable space between the eyes.

Deer_Fly_Female_Biting_Skin_2-22-16.jpg

Photo of a female deer fly biting human skin.
Deer Fly Female
Female deer flies and horse flies drink the blood of vertebrate animals, including that of horses, deer, cattle, and people.

Deer_Fly_Female_C_cincticornis_nigropterus_2-22-16.jpg

Photo of a female deer fly, probably Chrysops cincticornis nigropterus, on a leaf.
Deer Fly Female
Female deer flies and horse flies are often found near water.

Deer_Fly_Female_C_pikei_2-23-16.jpg

Photo of a female deer fly, probably Chrysops pikei, perched on a green stem.
Deer Fly Female
Although the bodies of deer and horse fly adults can be rather drab, their eyes can be strikingly colored and patterened.

Deerfly_Larva_2-23-16.jpg

Photo of a deer fly larva, probably in a petri dish, on a gray background.
Deer Fly Larva
Deer fly larvae have wormlike, cylindrical bodies that taper at both ends. There are no true legs.

Deerfly_Larva_2-22-16.jpg

Photo of a deer fly larva, probably in a petri dish, on a gray background.
Deer Fly Larva
A thin, pointed breathing tube (at right) extends from the hind end of a relaxed deer fly larva.

Deer_Fly_Eggs_2-22-16.jpg

Photo of deer fly egg mass affixed to underside of a green leaf.
Deer Fly Eggs
Fertilized female deer flies and horse flies lay eggs on plants or other objects overhanging water. Upon hatching, the larvae drop into the water.
Habitat and conservation

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.

Foods

Adults eat nectar and pollen from flowers. Females (but not males) also drink blood. 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.

image of Horse and Deer Flies Distribution Map
Distribution in Missouri

Statewide.

Status

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.

Human connections

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.

Ecosystem connections

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.