Vectors: Tabanidae

E M NEVILL, R J PHELPS AND B R STUCKENBERG

Introduction

The reason for the inclusion of a chapter on flies of the family Tabanidae in this book is their great potential as mechanical transmitters of infectious disease agents. Tabanidae are commonly known as horse flies, deer flies, clegs, and, in South Africa, as blindevlieë (Afrik.) (blind flies). The large size and variety of genera and species of Tabanidae in southern Africa should have made them an obvious choice for study by biologists, but studies on them have been mostly restricted to their taxonomy while those on their biology, role in disease transmission and control are limited.

Biosystematics

The taxonomy of the Tabanidae of the Afrotropical Region embraces three subfamilies (Pangoniinae, Chrysopsinae and Tabaninae) containing 31 genera and 727 species.^{9, 10, 37–39} Four hundred and ten of these species occur in southern Africa, and of them about 80 per cent belong to only five genera, namely Haematopota, Tabanus, Philoliche, Rhigioglossa (= Mesomyia), and Chrysops.

Flies belonging to the Pangoniinae are usually of little known economic importance.³⁰ Various species of Philoliche of the subfamily Pangoniinae are, however, persistent pursuers of blood meals from cattle and horses in localized situations.⁴⁶ Of the Tabanidae recorded from southern Africa, three genera of the Chrysopsinae, namely Rhigioglossa (39 species), Chrysops (23 species) and Tabanocella (10 species); and five genera of the subfamily Tabaninae, namely Haematopota (137 species), Tabanus (68 species), Hybomitra (nine species), Atylotus (six species) and Ancala (four species) appear in a list of tabanid-transmitted disease agents.³²

The taxonomy of tabanids of Angola,^{14, 16} Mozambique, ¹⁵ Zimbabwe²⁴ and South Africa⁴⁸ has been reviewed.

In general, the biology of the Tabanidae has been poorly studied throughout the world, but this is even more the case in southern Africa. Chainey and Oldroyd¹⁰ summarized the larval biology of these flies very succinctly as follows:

Tabanid larvae live in wet places but need to reach the atmosphere periodically as they breathe air through a posterior siphon. The usual habitat is the wet mud at the margins of ponds, lakes and streams, but those of some species live in sand on the seashore. Some Tabanus larvae inhabit pockets of damp soil in generally dry areas, and Haematopota larvae occur mainly in areas of wet soil. Tree-hole breeding occurs in a few Tabanidae; larvae of Thaumastocera for example living in the accumulated debris in rot-holes of palms and forest trees . . . larvae of Haematopota and Tabanus feed on other insects, crustacea, worms or their own kind, but many tabanid larvae, such as those of Chrysops, feed on vegetable debris.

Adult tabanid flies are generally large, usually with well-developed eyes. Their mouthparts are adapted for both sucking and lapping, with both sexes feeding on nectar. The females of most species also feed on blood, which is needed for the development of the ovaries. The mandibles and maxilla are serrated and are used to cut into the skin and lacerate the dermal tissues, and thus induce the formation of a pool of blood which is then lapped up. A small amount of blood is trapped within the component parts of the mouthparts when they are withdrawn. Mechanical transmission of disease organisms is thus facilitated by this action. The coarse structure of the mouthparts and the mode of feeding results in painful bites causing cattle to try to dislodge or kill the flies by striking them with their tongues.^{5, 30} Further details on the life cycles and bionomics of Tabanidae can be obtained from a chapter in the book by Kettle.³⁰

The density of tabanid populations is closely associated with breeding sites. Populations around dams, vleis and natural wetlands may be high and composed of only a few species or perhaps even a single dominant one, which in South Africa is often a species of Haematopota.

In southern Africa, the breeding sites of many tabanids in Malawi^33–35 and the larval feeding habits of Tabanus biguttatus Wied. and Amanella emergens Oldroyd in South Africa⁸ have been recorded, but published information on the biology of adults of species of the Afrotropical Region is rather sparse and deals mainly with flight seasons, abundance and nocturnal activity.^{10, 48} It is considered that certain physical adaptations aimed at distracting the attentions of visually orientated biting flies have evolved in some African wildlife species.⁴⁵ It has been demonstrated in the north-eastern KwaZulu-Natal Province of South Africa that the striping of zebra makes them virtually undetectable to tsetse flies (Glossina spp.) and tabanids.²⁵Field observations also indicate that antelope species with a countershaded haircoat pattern, characterized particularly by a pale belly that eliminates the rounded appearance of the abdomen which is enhanced by shadowing in sunlight, are evidently unattractive to tabanids.⁴⁵ In contrast, mammals with uniform colouring, such as elephant, rhinoceros and hippopotamus, which present a strongly rounded appearance in sunlight, are highly attractive to large tabanid species such as Tabanus taeniola P. de B. and Philoliche aethiopica Thunberg. Experimental evidence has shown that a dark, single colour, horizontal cylinder is a most attractive shape for tabanids, and indeed the Harris Fly Trap represents essentially such a form.²⁶.

Tabanidae...