Tabanus atratus

Geographic Range

Tabanus atratus is primarily found in the eastern United States, although it has been collected throughout the entire continental US. (Roberts and Janovy Jr., 2000)


This horsefly is able to survive in a wide range of habitats although it cannot survive in extreme climates such as mountain tops or deserts. It requires moist environments in which to lay eggs, and mammals to feed on. ("Welcome to the World of the Horseflies", 2000)

Physical Description

Adults of this species reach 20-25 mm in length. The flies can be entirely black, including the wings, or dark brown to blackish purple. Tabanus atratus have large compound eyes, which are dichoptic (separated) in females, and holoptic (continuous) in males. They have prominent mouthparts, which are easily distinguishable: The fascicle is made of six piercing organs. Starting from the outside, there are 2 flattened, bladelike mandibles with tooth like serrations used for cutting. Two narrow maxillae also serrated used to pierce the tissue and blood vessels of the host, a median hypopharynx and a median labrum-epipharynx. The hypopharynx and labrum-epipharynx make up the food canal, while the labrum is a large sponge like organ used to lap up blood (Roberts and Janovy, 2000). Short stout hairs cover the body, and wing venation is pronounced.

Larva of Tabanus atratus are similar to the larvae of all other horseflies. Larvae can be white to tan, while possessing a slender, cylindrical body, which tapers at the head. (Strother, 2000) Like other tabanids, the larva of the black horsefly has twelve segments and a retractable tracheal siphon used for respiration. The retractable head of the larva has two sharp mandibles that can cause a painful bite (Roberts and Janovy, 2000). The pupae have a series of spines sticking out from the abdominal segments, and usually exhibit little movement (Hutchinson, 1999). (Hays, 1956; Hutchinson, 1999; Roberts and Janovy Jr., 2000; Strother, 2000)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range length
    20 to 25 mm
    0.79 to 0.98 in


Horse flies pass through the following holometabolous life stages: egg, larva, pupa, and adult. After the eggs hatch, the larvae will drop down into water or burrow into a moist environment where it then matures through six to nine instars before pupation (Hutchinson, 1999). In colder climates, larvae will overwinter, then move to drier soil to in order to pupate (Strother, 2000). The pupal stage lasts from one to three weeks. (Hays, 1956; Hutchinson, 1999; Strother, 2000)


Tabanus atratus generally breed near aquatic environments. When the adult fly emerges, mating occurs. Females then search for a blood meal, while males feed on nectar. (Hutchinson, 1999; Strother, 2000)

Each female will lay three to four masses of one hundred to a thousand eggs each, in layers near water's edge or somewhere quite close to water. ("Welcome to the World of the Horseflies", 2000; Hutchinson, 1999)

  • Range eggs per season
    300 to 4000

After searching for and laying eggs in a suitable environment for larvae to grow and develop, there is no further parental involvement.

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female


Horseflies are active during the daytime, and once a host is found, the horsefly will attack relentlessly in order to feed (Strother, 2000). Adult flies can fly over 65 kilometers, but will generally not disperse more than 2 miles away from where they emerged. The flies rest on paths and roads, especially in wooded areas. Whenever the wind picks up or temperature drops, a significant decrease in flies can be observed. Tabanus atratus is also attracted to light, commonly congregating at light sources during the night (Greenvalley, 2000). ("Welcome to the World of the Horseflies", 2000; Strother, 2000)

Food Habits

The females of T. atratus feed on mammalian blood, while males, which lack mandibles, feed on nectar and plant juices (Hutchinson, 1999). A blood meal is necessary in order for females to nourish their developing eggs (Strother, 2000). Horseflies are diurnal, usually feed during the day (Greenvalley, 2000). The mouthparts are made of six piercing organs: two mandibles; two maxillae; hypopharynx; and a labrum-epipharynx. The mandibles and the maxillae have sharp serrated teeth on them, which are used for puncturing the skin and rupturing blood vessels (Roberts and Janovy, 2000). The labrum then functions to lap up the pool of blood that is formed from the bite, otherwise known as telmophagy (Hutchinson, 1999). When searching for a host, females are attracted to large, dark, moving objects and to CO2. Larvae feed voraciously on other insect larvae, other invertebrates and small vertebrates (Strother, 2000). ("Welcome to the World of the Horseflies", 2000; Hutchinson, 1999; Roberts and Janovy Jr., 2000; Strother, 2000)

  • Animal Foods
  • blood

Economic Importance for Humans: Positive

This species provides no major economic benefit to humans.

Economic Importance for Humans: Negative

Although Tabanus atratus do not often bite humans, when it does happen it leaves painful memories. This fly can also transmit bacterial, viral, and other diseases such as surra and anthrax, to both humans and other animals through its bite.

The effect of T. atratus on livestock can be a serious problem. Blood loss and irritation from the flies can severely affect beef and milk production, as well as grazing. Livestock usually have no way of avoiding the painful bites, and millions of dollars have been spent trying to control these pests. (Roberts and Janovy Jr., 2000; Strother, 2000)

Conservation Status

This species requires no special conservation status.


Sara Diamond (editor), Animal Diversity Web.

Wilson Long (author), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

  1. active during the day, 2. lasting for one day.

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an animal that mainly eats blood


reproduction that includes combining the genetic contribution of two individuals, a male and a female


living in residential areas on the outskirts of large cities or towns.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


2000. "Welcome to the World of the Horseflies" (On-line). Accessed March 2, 2001 at

Hays, K. 1956. A Synopsis of the Tabanidae (Diptera) of Michigan. Ann Arbor: University of Michigan. Museum of Zoology.

Hutchinson, R. 1999. "Horse Flies" (On-line). Accessed March 3, 2001 at

Roberts, L., J. Janovy Jr.. 2000. Foundations of Parasitology. Boston: The McGraw-Hill Companies, Inc..

Strother, S. 2000. Tabanids (horseflies). Dermatology Online Journal, 5(2): 6.