Members of the family Tabanidae are not only significant pests of livestock, possibly causing direct economic losses due to their painful and persistent biting behavior, they are also important vectors of a number of disease agents, including Trypanosoma evansi.

General Aspects

Taxonomy

The family Tabanidae includes approximately 4300 species and subspecies in 133 genera worldwide. Of these, 335 species in 25 genera are found in the Nearctic Region (Burger, 1995). The family Tabanidae is divided into three subfamilies: Chrysopinae, Tabaninae and Pangoniinae (Fairchild, 1969; Mackerras, 1954). The former two of these subfamilies contain most of the economically important tabanids. Tabanids in the subfamily Chrysopinae are called deer flies, with nearly all being members of the genus Chrysops. The subfamily Tabaninae includes the horse flies, represented by the genera Tabanus and Hybomitra.

Table 1: Morphological characteristics used to differentiate adult horse and deer flies (Mullens, 2002)

Distribution

Tabanids are present on every continent except Antarctica and have managed to colonize remote islands such as the Galapagos and the Melanesian Archipelago. Large seasonal populations of some species occur as far as 60° N latitude, but they disappear above tree line. Diversity within the family is greatest in the tropics, but moist temperate regions have a rich fauna as well (Mullens, 2002).

Veterinary and Medical Importance

Besides being significant pests of livestock, causing reductions in weight gains, milk yield and feed-utilization efficiencies, owing to their painful, persistent biting behavior, they are also vectors of a number of disease agents of veterinary and medical importance. Of high importance among these is also the protozoan Trypanosoma evansi, which can affect camels and equids causing Surra, but which can also cause trypanosomosis in dogs. The transmission of the protozoan is occurring mechanically by species of the genera Tabanus, Haematopota and Chrysops.

General Morphology

Overview

Tabanid adults are stout-bodied flies. They generally can be distinguished as horse flies or deer flies based on several morphological characteristics (see Table 1). The antennae are prominent and extent anteriorly. In general, they possess fairly large size, striking appearance and diurnal habits. The eyes of many species are brilliantly patterned with shades of green, yellow, orange, and violet. They often consist of large ommatidial facets dorsally and smaller facets ventrally. For further details on morphology see e.g., Mullens (2002).

The developmental cycle includes the following stages: Egg, 1-3 mm long, which are deposited in masses; larvae (predaceous and cannibalistic), performing 6-13 larval molts with larvae overwintering; a pupal stage and the adult stage. Some tabanids can develop from egg to adult in as little as 6 weeks, whereas in other species 2 to 3 years may be spent as larvae, depending on environmental conditions etc. Tabanid mating occurs in flight, especially in the morning.

Feeding Behavior

Activity Dynamics

Most temperate species have very distinctive seasonal flight periods which vary little from year to year. Tabanids are strong fliers and readily disperse several kilometers in short-term flights. But it was suggested, based on mark-release-recapture studies, that local populations tend to remain in a given area, with dispersal occurring in a series of short flights. Males do not feed on blood, whereas the blood meal size in females varies from 20-25 mg for many Chrysops species to almost 700 mg for Tabanus atratus.

The transmission of T. evansi is mechanical in contrast to other Trypanosoma species in e.g., tsetse flies.

Host Spectrum

Most species feed on large mammals such as cattle, horses and deer. Deer flies often attack large mammals, including humans, but some species were also recorded to feed on ravens, crows, ducks and robins. Many tabanids are selective in attacking specific body regions of their hosts, regardless of color.

Control

Tabanid control is difficult to achieve. A given area typically has multiple species with different seasonal occurrences and biological characteristics. Typical host contact is only about 4 min per fly during blood feeding, which may occur only once every 3-4 days.

References

• Burger, J.F.: Catalogue of Tabanidae (Diptera) of North America north of Mexico. Contrib. Entomol. Int., 1, 1995, 1-100
• Fairchild, G.B.: Climate and the phylogeny and distribution of Tabanidae. Bull. Entomol. Soc. Am., 15, 1969, 7-11
• Mackerras, I.M.: The classification and distribution of Tabanidae (Diptera). I. General review. Aust. J. Zool. 2, 1954, 431-454
• Mullens, B.A.: Horse flies and deer flies (Tabanidae). In: Mullen, G., Durden, L. (eds.): Medical and Veterinary Entomology. 2002, Academic Press, London, pp. 263-277