Horse and Deer Fly Larvae

Tabanus, Chrysops, and related genera

Deerfly_Larva_2-23-16.jpg

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
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, nobby 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, the adults 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.

Key Identifiers
  • 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.
Size

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

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.

Deer_Fly_Larva_C_vittatus_2-22-16.jpg

Photo of a deer fly larva, probably Chrysops vittatus, on a sandy substrate.
Deer Fly Larva
In some species, deer fly larvae eat other small animals, including insects, small fish, and more. Others eat detritus.

Deer_Fly_Pupa_C_flavidus_2-22-16.jpg

Photo of a deer fly pupa, probably Chrysops flavidus, on a sandy substrate.
Deer Fly Pupa
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.

Deer_Fly_Pupa_C_vittatus_2-22-16.jpg

Photo of a deer fly pupa, probably Chrysops vittatus, lying on a sandy substrate.
Deer Fly Pupa
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.

horse_fly_2012.jpg

image of Horse Fly on tree trunk
Horse Fly

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.
Habitat and conservation

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.

Foods

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.

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

Like all other members of the fly family, these 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.

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.