Tabanids are large biting flies generally called horse flies, although othervernacular names include greenheads (some species of Tabanus),clegs and stouts (Haematopota) and deer flies(Chrysops). All belong to the family Tabanidae, whichcomprises about 4300 species and subspecies in 133 genera. Medically the mostimportant are species of Tabanus, Chrysops andHaematopota. Tabanids have been incriminated in the spreadof anthrax and tularaemia and might be involved in the transmission of Lymedisease (usually transmitted by hard ticks), but this remains unclear. But theirmain medical importance is that species of Chrysops, mainlyC. silaceus and C. dimidiatus, are vectorsin West and Central Africa of the filarial worm Loa loa.

The Tabanidae have a worldwide distribution. Species of Tabanusand Chrysops are found in temperate and tropical areas, butHaematopota is absent from South America and Australasiaand is uncommon in North America.

External morphology

A generalized description is presented of the Tabanidae, with special referenceto the genera Chrysops, Tabanus andHaematopota.

Tabanids are medium to very large flies (6–30mm long). Many, especiallyTabanus species, are robust and heavily built, and thisgenus contains the largest biting flies, some with a wingspan of 65 mm. Thecolouration of tabanids varies from very dark brown or black to lighter reddishbrown, yellow or greenish; frequently the abdomen and thorax have stripes orpatches of contrasting colours (Fig. 7.1). The head is large and, viewed fromabove, is more or less semicircular (Fig. 7.2); itis often described as semilunar. The head has a conspicuous pair of compoundeyes which in life may be marked with contrastingiridescent colours, such as greens andreds or even purplish hues, arranged in bands, zigzags or spots. Adults aresexed by examining their eyes. In thefemale there is a distinct space on top of the head separating the eyes: this isknown as a dichoptic condition (Fig. 7.2a). Infemales of some species this space between the eyes may be narrow, whereas inothers, especially Chrysops, it is quite large. In males theeyes are so large that they occupy almost all of the head and either touch eachother on top of the head or are very narrowly separated, this being known as aholoptic condition.