HORSE FLIES

Tabanids are relatively large stout flies belonging to the sub-order Brachycera. They have acquired various colloquial names including horseflies mainly for Tabanus spp, clegs mainly for Haematopota spp, and deerflies mainly for Chrysops. Body length ranges from 5-25mm and the compound eyes are well developed. As with most haematophagous diptera it is only the females that take blood in addition to nectar with the males being solely nectar feeders. The principal mechanism for finding their hosts is sight and so the large eyes serve this function well. There is also evidence that CO₂ acts as an odour source especially in some Chrysops spp. The eyes are coloured and are also used to sex the flies. Males are holoptic and the females dichoptic. Eye coloration varies between species with unicolourous or horizontally banded patterns seen in Tabanus, spots in Chrysops, and zigzag bands in Haematopota. The antennae are stiff and project forward. Wing veination is pronounced but not used for species diagnosis. At present there are over 3,000 known species of Tabanids. The three main genera of economic importance are the Chrysops, Tabanus, and Haematopota. The three genera differ in their distributions although there is considerable overlap. Tabanus has a cosmopolitan distribution with Chrysops being predominantly holoartic and oriental. Haematopota is found in the palaeartic, the orient and afrotropical regions. Tabanus and Haematopota both show similar morphological features in having reduced ocelli and a proboscis which is shorter than the head. Chrysops has a longer proboscis although not longer than the head and the ocelli are functional.

The life cycle of the genera discussed above all have life cycles associated with water to some extent. The eggs are laid in large masses ranging from 200-1000 eggs and oviposition varies according to genus. Eggs are not laid directly in water but rather on overhanging vegetation or protruding rocks and debris. The eggs darken from a creamy white to a dark grey to black. Hatching usually occurs around four days after being laid, although this timescale depends on the ambient temperature. The development site for the larva can be divided into three distinct habitats. The division is based mainly on the water content of the substrate in which the larvae develop. Chrysops larvae are found in substrate with the highest water content of the three genus and are thus termed hydrobionts. The larvae of Tabanus are found in somewhat drier substrates and have a wider distribution than the other two genera. These larvae are called hemi-hydrobionts, whilst the larvae of Haematopota are edaphic, being found in the soil. The larvae use a special hatching spine to exit the egg case. The larvae usually pass through six to nine instars before pupation. The larval stage often takes several months and allows over wintering in temperate climates. The temperature which dictates the development of T.taeniola for example ranges from a constant high of 32⁰C to 35⁰C at which temperature no prolonged prepupal stage occurs, to 22⁰C at which development takes considerable longer. The length of development varies from ten to eleven weeks at the higher temperatures, to 42 weeks at the lower temperatures.

First instar larvae moult soon after emergence and the second stage larvae is positively phototactic moving across the surface of the substrate. This second instar is a non-feeding stage and in a further three to six days the third instar stage is reached. The third instar shows behaviour the converse of the second instar, being negatively phototactic and burrowing down in to the substrate. The larvae of Chrysops feeds on organic matter within the substrate, whilst the larvae of both Tabanus and Haematopota are carnivorous equipped with piecing mandibles. The larvae feed on other insect larvae, crustaceans, snails and earthworms. They have also been observed to be cannibalistic . The consequence of this is that these larvae are found at low population densities per unit area of substrate. This contrasts with the larval densities of Chrysops per unit of substrate which can become relatively dense. When the larvae pupate they migrate up to the top one or two inches (2.5-5.0cm) of soil or towards aquatic environments in the case of Chrysops and to some extent Tabanus.

The pupa have a series of spines projecting from the abdominal segments with a spiny aster formed from bristles on the terminal segment. These spines allow purchase on the substrate when the pupae moves, although movement is limited and only undertaken in order to avoid adverse conditions. The pupal stage lasts from one to three weeks. When the fly emerges from its pupal case females will seek a blood meal, whilst the males feed on nectar. The next priority for a newly emerged fly is to mate. This usually takes place during the early hours of the morning. Males come together in swarms above the forest canopy and females are attracted to these swarms. Females enter these dipteran "lekking" sites and copulation is initiated in the air, the act is completed on the ground and takes approximately five minutes. The recognition of females by males is thought to be primarily by sight although it is not known at present if a aggregation pheromone causes the early morning swarms of males. Certain species do have different mating habits such as C. fuliginosus in this species the males rest on vegetation and will dart out as females fly past. The initial visual cue is easy to elicit by throwing a small stone past a resting male, this will cause him to dart quickly after the stone. According to species a blood meal may or may not be needed before egg production can occur, although autogenous species are relatively rare.

Tabanids are major mechnical transmitters of Trypanosoma evansi, causing "surra" in horses, camels, and dogs. It also effects cattle and other mammals but this is to a lesser extent. Other trypanosomes transmitted include T. vivax viennei, in cattle and sheep, T. simiae in pigs, T. theileri of cattle and African trypanosomes. Major species being:

In addition to these trypanosomes the bacterium Francisella tularensis a zoonotic disease is transmitted, anthrax California encephalitis, Western equine encephalitis, rinderpest, anaplasmosis and equine infectious anaemia virus. The arterial worm Elaeophora schneideri of sheep is also transmitted

Opposite:Chrysops the vector of human loiasis

All these pathogens are carried within a blood pool from one host to another. The term mechanical transmitters is used as the pathogens are not actually ingested by the fly and then injected into the next host. It is the contact of blood on the mouth parts of the fly that may then contact blood from a new bite on a different host which allows pathogens entry into the new host. This process is probably best understood by knowledge of a tabanids mouth parts and feeding technique.
Diagram showing structure of tabanid mouthparts

I = Maxillae

II= Hypopharynx

III= Mandibles

IIII= Labrum

IV= Labella

The diagram opposite shows the layout of the various mouthparts. The maxillae and mandibles bear teeth. These are used to cut through the hide or skin in a scissor type action on an inward sweep. The resulting cut is deep and painful resulting in a pool of blood. The labrum is then used to ingest the pooled blood. Due to the cutting nature of the bite the fly is frequently disturbed whilst feeding. As the blood wells up and forms a pool there will be a considerable amount of blood on the various feeding structures. On being disturbed a tabanid will fly only a short distance and then return. When feeding on livestock there is a high probability of the fly returning to an adjacent animal. As the fly cuts this animals skin so the blood in its mouthparts will mix to some extent with that of the new host thus transferring any pathogens present.

Horseflies are frequently important pests to both livestock and humans. Even if there is no disease transmission the bites themselves cause considerable discomfort and disrupt the normal grazing or outside activities of people. Very rarely it is possible to change a small local environment to eliminate the breeding grounds of these flies, such as small ponds. This method however is not effective in most cases and rarely are ponds removed in an attempt at controlling a horsefly problem. This is due mainly to the close locality of another site which cannot be removed and the flight and dispersal of the adult flies. The main attempts at present are the use of traps, pour on and spot on insecticides, and for humans the use of repellents.

Traps are usually large, conical and have a collection chamber at the apex of the cone into which the flies rise, become trapped and then die. The trap uses the behaviour of the fly to catch them. A large black round ball is suspended so that half of the ball is visible below the netting forming the cone. The flies are attracted to the ball and land to investigate. Once on the ball the flies crawl over it usually ending up on its upper surface. From here, having decided that there is nothing worth biting they fly directly upwards. This brings them into the conical netting. The collection chamber at the top of the trap is transparent and so the flies are attracted up into it. The flies enter through a funnel and once inside the chamber they die from dehydration in a day or so. These traps can be effective in localized areas and involve killing horseflies without the need for insecticides in traps which can easily be home made.

The use of insecticides as spot ons and pour ons is used mainly to treat against other livestock flies such as blowflies and warble flies. There is evidence that insecticides such as the pyrethroids are effective when first applied but the effects are short lived in relation to horsefly control. The use of self applicating methods such as face bags or back scrubbers has been used but although extremely effective against other flies, such as hornflies and faceflies, they are not very effective for horseflies. As far as humans are concerned the methods employed can be either repellents or traps or a combination of both. Traps are most effective when placed around the periphery of an area such as a garden to create a horsefly free zone. The positioning and amount of traps however need to be determined by trial and error as the population density and species will differ in different localities. The use of repellents involves either topical application onto the body with chemical such as diethyltoluamide-DEET-, citronella oil or eucalyptus oil. DEET can cause certain plastics to degrade, spectacle frames, watch faces etc, and no repellents should contact the eyes or lips. The use of these repellents is fine for the occasional trip out but become both expensive and laborious to apply when wanting to go out into the garden. In these situations it is probably better to use a impregnated patch or article of clothing. A light cotton jacket or a stick on patch can be effective at repelling horseflies. A garment can be soaked in DEET diluted in water to give effective protection. A cotton jacket (DEET affects some artificial fibres) weighing 120 grams can be impregnated by pouring 30ml of DEET into 250ml of water and immersing the garment into the solution. When not in use the jacket can be sealed in a plastic bag to retain the life of the DEET. A garment so soaked can be effective for several weeks.

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