Tabanids are large biting flies generally called horse flies, although other
vernacular names include greenheads (some species of Tabanus),
clegs and stouts (Haematopota) and deer flies
(Chrysops). All belong to the family Tabanidae, which
comprises about 4300 species and subspecies in 133 genera. Medically the most
important are species of Tabanus, Chrysops and
Haematopota. Tabanids have been incriminated in the spread
of anthrax and tularaemia and might be involved in the transmission of Lyme
disease (usually transmitted by hard ticks), but this remains unclear. But their
main medical importance is that species of Chrysops, mainly
C. silaceus and C. dimidiatus, are vectors
in West and Central Africa of the filarial worm Loa loa.
The Tabanidae have a worldwide distribution. Species of Tabanus
and Chrysops are found in temperate and tropical areas, but
Haematopota is absent from South America and Australasia
and is uncommon in North America.
External morphology
A generalized description is presented of the Tabanidae, with special reference
to the genera Chrysops, Tabanus and
Haematopota.
Tabanids are medium to very large flies (6–30mm long). Many, especially
Tabanus species, are robust and heavily built, and this
genus contains the largest biting flies, some with a wingspan of 65 mm. The
colouration of tabanids varies from very dark brown or black to lighter reddish
brown, yellow or greenish; frequently the abdomen and thorax have stripes or
patches of contrasting colours (Fig. 7.1). The head is large and, viewed from
above, is more or less semicircular (Fig. 7.2); it
is often described as semilunar. The head has a conspicuous pair of compound
eyes which in life may be marked with contrasting
iridescent colours, such as greens and
reds or even purplish hues, arranged in bands, zigzags or spots. Adults are
sexed by examining their eyes. In the
female there is a distinct space on top of the head separating the eyes: this is
known as a dichoptic condition (Fig. 7.2a). In
females of some species this space between the eyes may be narrow, whereas in
others, especially Chrysops, it is quite large. In males the
eyes are so large that they occupy almost all of the head and either touch each
other on top of the head or are very narrowly separated, this being known as a
holoptic condition.