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Identification & Distribution:

Identification

Hybomitra bimaculata is a medium-large horsefly, with a body length of 13-16.5 mm. The female has a number of different colour forms. Probably the commonest has the abdomen dark grey with clearly marked whitish triangles along the dorsal midline (= form bisignata) (see first picture below). In a second form there are lateral reddish spots on the abdomen which reach to the third segment (= form collini) (see second picture below). In another form of Hybomitra bimaculata the lateral reddish spots on the abdomen, stop short on the second segment (= form bimaculata). There are many intermediates between these named forms.

 

The multiple abdominal colour forms of the female Hybomitra bimaculata can make identification tricky. Useful distinguishing features are

  1. the upper parts of the tibiae are orange and
  2. there are unusually long hairs on the anterior and posterior surfaces of the mid tibiae (see first picture below).

 

Like most Hybomitra species, it has hairy eyes (see second picture above) with three stripes on each eye (see picture below).

The male (not pictured here) has reddish yellow side margins on one to three abdominal tergites. They also have a long posterior hair fringe on the hind tibiae which distinguishes male Hybomitra bimaculata from male Hybomitra lurida. The upper facets of the eyes are hardly enlarged, the vertex has a distinct tuft of long black hairs and the notopleural callus is black.

Distribution & Seasonal Occurrence

Hybomitra bimaculata is active from May to August in woodland edge habitats, especially heath woodland, and sheltered fens and marshes. In Britain it is mainly found in southern England and parts of the West Midlands. It is generally absent from upland areas. It is distributed widely in Europe, Russia, Mongolia, China and Japan.

 

Biology & Ecology:

Resting behaviour & Swarming

Male hairy-legged horseflies can sometimes be observed hovering, like 'strange-looking blackish hoverflies' (see Falk on Flickr ). Surprisingly this is the only reference we have found to hovering/swarming behaviour by this species.

Blood feeding

Grayson (1997)  reports that female Hybomitra bimaculata generally attack humans in ones and twos, encircling the legs and lower torso several times before alighting. Parmenter (1950)  reports it being attracted to cattle where it feeds on their flanks and bellies.

Nectar feeding & puddling

Males have been observed drinking at wet mud (Jones, 1922 ). They have also been recorded 'dipping' where they dive at the water surface and then shoot away again after touching it very briefly. (Goffe, 1931 )

Breeding sites

The predatory larvae develop in wet soil, wood detritus and even waterbird nests. Surcouf (1921)  found many larvae in a layer of oak leaves in the bottom of a shallow pool.

Trapping & odour attractants

Our observations on a small swarm of female Hybomitra bimaculata that had been attracted to a moving car gave some information on the responses of this species to different colours. The tabanids had a choice of colours for settling - white (the bodywork), black (the tyres and rubber seals) and red (the brake lights). The picture below shows a female Hybomitra bimaculata that settled on the white body of the car.

However, black and red seemed to be their preferred colours, with fewer settling on white than would expected if they showed no preference.

 

Grayson noted that Hybomitra bimaculata is strongly attracted to black objects. Bracken et al. (1962)  found that all tabanid species in his area were attracted to black and red spheres, but most were not attracted to white spheres. Browne and Bennett (1980)  reported that Hybomitra spp. (and Chrysops spp) were attracted to blue or red, but were consistently not attracted to black, yellow or white.

Some work has been done to test the effectiveness of different odour attractants for Hybomitra bimaculata. In Croatia Krčmar et al. (2006)  showed that canopy traps baited with aged cow urine collected many more Hybomitra bimaculata females than did unbaited traps, and more than traps baited with aged horse, sheep, or pig urine. Differences were claimed to be significant, but only pooled data were analyzed which is invalid because of pseudoreplication. There was some evidence that 4-methylphenol (one of the active chemicals in cow urine) increased the catch of Hybomitra bimaculata, although differences were not significant (Krčmar, 2007 ).

We have only once found this species commonly - at Park Corner Heath in East Sussex in June, 2009 where a number of females had been attracted to a moving vehicle. In 2014 we caught just one (shown above) in our cow urine/acetone - baited NG2F trap survey in southern England - in mixed deciduous/coniferous woodland at a site where Tabanus bromius and Hybomitra distinguenda were abundant (Brightwell & Dransfield, 2014 ). The scarcity of Hybomitra bimaculata in the traps may simply have resulted from it being late in the season for this species.

 

Disease transmission

Trypanosoma theileri has been found in Hybomitra bimaculata (Fominykh et al., 1980 ).

References

  • Affolter, F. et al. (1981). La biocenose des habitats larvaires de Hybomitra bimaculata (Macquart) (Dipt. Tabanidae). Revue Suisse de Zoologie 88, 965 - 675.

  • Bracken, G.K. et al. (1962). The orientation of horse flies and deer flies (Tabanidae: Diptera): II. The role of some visual factors in the attractiveness of decoy silhouettes. Canadian Journal of Zoology 40 (5), 685-695. Abstract 

  • Brightwell, R. & Dransfield, R.D. (2014). Survey of Tabanidae (horseflies) in southern England 2014. A preliminary survey of tabanids using odour-baited NG2F traps. 14 pp. Full text 

  • Browne, S.M. & Bennett, G.F. (1980). Color and shape as mediators of host-seeking responses of Simuliids and Tabanids (Diptera) in the Tantramar marshes, New Brunswick, Canada. Journal of Medical Entomology 17, 58-62. Abstract 

  • Fominykh, V.G. et al. (1980). The finding of Trypanosoma theileri Laveran, 1902 in Hybomitra bimaculata. Voprosy biologii 1980, 21-22.

  • Goffe, E.R. (1931). British Tabanidae (Diptera) with an account of the principal variation, with descriptions of a number of new forms , and of some additions to the British list. Trans. ent. Soc. S. Engl. No. 6, 43-114.

  • Grayson, A. (1997). Personal notes on attacks by female tabanids. Larger Brachycera Recording Scheme Newsletter 15Full text 

  • Jones, H.P. (1922). Some notes on the habits of male Tabanidae. Entomologist 55, 40-42.

  • Krčmar, S. et al. (2006). Response of Tabanidae (Diptera) to different natural attractants. Journal of Vector Ecology 31(2), 262-265. Full text 

  • Krčmar, S. (2007). Response of Tabanidae (Diptera) to canopy traps baited with 4-methylphenol, 3-isopropylphenol, and napthalene. Journal of Vector Ecology 31(2), 188-192. Full text 

  • Parmenter, L. (1950). The Diptera of Bookham Common. London Naturalist 1949, 98-133.

  • Surcouf, J. (1921). Diptera: fam. Tabanidae Genera Insectorum 175,182 pp.