Horse and Deer Fly Larvae

Photo of a deer fly larva, probably in a petri dish, on a gray background.
Scientific Name
Tabanus, Chrysops, and related genera
Family
Tabanidae (horse and deer flies) in the order Diptera (flies)
Description

The larvae of horse and deer flies are fairly straight, segmented, wormlike maggots that are tan, whitish, or brownish. Several fleshy rings circle the body. They are robust, circular in cross-section, and taper at both ends. There are no true legs, although fleshy, knobby pseudopods or prolegs are present. In relaxed specimens, a thin, pointed breathing tube extends from the hind end to protrude above the water surface.

Horse and deer flies belong to the same family. Like other true flies, adult horse and deer flies 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. Adult female horse and deer flies are notorious for drinking blood from cuts they make into their host’s skin.

Key identifiers of larvae:

  • Robust, wormlike
  • Tan, whitish, or brownish
  • Fairly straight, circular in cross-section, tapering or conical at both ends
  • Segmented, with several fleshy rings circling the body
  • Lacks true legs
  • When relaxed, a breathing tube extends from the hind end and protrudes above the water surface
  • Live in aquatic environments or in damp soil
Other Common Names
Tabanids
Size

Larva length: ¾ to 1½ inches. Deer fly larvae are usually smaller than horse fly larvae.

Where To Find
image of Horse and Deer Flies Distribution Map

Statewide.

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 are strong fliers and can be found nearly anywhere. They are most common near streams and wetlands, where the females 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.

In some species, the larvae are voracious predators of other small animals, including insects, small fish, and more. Others eat detritus.

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 scissor-like mouthparts, 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.

Common.

Adult 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 has finished its blood meal, an animal that’s been attacked multiple times can experience significant blood loss.

Like all other members of the fly family, these insects go through complete metamorphosis, starting out as eggs, which hatch into grublike larvae (maggots), which grow, then pupate, 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.

As troublesome as the adults can be, the aquatic larvae play important roles in water habitats as predators of small animals that, in turn, become food for other, larger predators, including fish, which many people appreciate.

The larvae help control populations of the many small animals they eat. Several predators, including medium and fairly large fish, eat the larvae.

More insectivorous animals, including birds, eat the adults, and certain wasps parasitize them. The horse guard wasp, for example, stings horse flies and uses them to feed its larvae.

The adults of some species are important pollinators.

Media Gallery
Photo of a deer fly larva, probably in a petri dish, on a gray background.
Deer fly larvae have wormlike, cylindrical bodies that taper at both ends. There are no true legs.
Jim Rathert
Photo by Jim Rathert, courtesy Missouri Department of Conservation
1/8
Photo of a deer fly larva, probably in a petri dish, on a gray background.
A thin, pointed breathing tube (at right) extends from the hind end of a relaxed deer fly larva.
Jim Rathert
Photo by Jim Rathert, courtesy Missouri Department of Conservation
2/8
Photo of deer fly egg mass affixed to underside of a green leaf.
Fertilized female deer flies and horse flies lay eggs on plants or other objects overhanging water. Upon hatching, the larvae drop into the water.
Sturgis McKeever, Georgia Southern University, Bugwood.org
Use of this image is restricted to MDC only
3/8
Photo of a deer fly larva, probably Chrysops vittatus, on a sandy substrate.
In some species, deer fly larvae eat other small animals, including insects, small fish, and more. Others eat detritus.
Sturgis McKeever, Georgia Southern University, Bugwood.org
Use of this image is restricted to MDC only
4/8
Photo of a deer fly pupa, probably Chrysops flavidus, on a sandy substrate.
Like butterflies, flies undergo complete metamorphosis. They transform from a maggot into a very different-looking winged adult. The transformative stage in between is called a pupa.
Sturgis McKeever, Georgia Southern University, Bugwood.org
Use of this image is restricted to MDC only
5/8
Photo of a deer fly pupa, probably Chrysops vittatus, lying on a sandy substrate.
In deer flies as in other insects with complete metamorphosis, the pupa stage seems outwardly to be inactive. But inside, the insect is changing from a wormy creature into an animal capable of flight.
Sturgis McKeever, Georgia Southern University, Bugwood.org
Use of this image is restricted to MDC only
6/8
image of Horse Fly on tree trunk
Horse flies (Tabanus spp.) are larger than deer flies and usually have solid or striped eyes. In both horse and deer flies, females have a space between the eyes (when viewed from above); the eyes of males nearly touch. Only the females drink blood.
Donna Brunet
Use of this image is restricted to MDC only
7/8
Photo of a deer fly.
Deer flies (Chrysops spp.) are usually smaller than horse flies, and they often have spotted eyes and a dark-spotted pattern on the wings.
Donna Brunet
Use of this image is restricted to MDC only on this page
8/8
Photo of a deer fly larva, probably in a petri dish, on a gray background.
Photo of a deer fly larva, probably in a petri dish, on a gray background.
Photo of deer fly egg mass affixed to underside of a green leaf.
Photo of a deer fly larva, probably Chrysops vittatus, on a sandy substrate.
Photo of a deer fly pupa, probably Chrysops flavidus, on a sandy substrate.
Photo of a deer fly pupa, probably Chrysops vittatus, lying on a sandy substrate.
image of Horse Fly on tree trunk
Photo of a deer fly.
Similar Species
About Aquatic Invertebrates in Missouri
Missouri's streams, lakes, and other aquatic habitats hold thousands of kinds of invertebrates — worms, freshwater mussels, snails, crayfish, insects, and other animals without backbones. These creatures are vital links in the aquatic food chain, and their presence and numbers tell us a lot about water quality.
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