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Identification & Distribution:

Identification

Tabanus sudeticus is a very large rather dark species (body length about 25 mm) with small equilateral pale median abdominal triangles which do not reach the foregoing tergites, and (usually) little or no lateral reddish colour on the abdomen. These characteristics should distinguish Tabanus sudeticus from the very similar Tabanus bovinus, which has the abdomen distinctly reddish-orange at the sides and median triangles usually longer and reaching the foregoing tergite. In addition the tergites of Tabanus sudeticus have black or dark brown bands, whilst the tergites of Tabanus bovinus have brown or pale reddish-brown bands.

 

The first picture above was taken in the Scottish Highlands, and shows a typical, strongly marked Tabanus sudeticus sitting on my (large black) beating tray, to which it was attracted. The second picture shows a more reddish specimen captured in the New Forest, Hants in 1964. We thought at first this could be Tabanus bovinus but the pale median abdominal triangles do not reach the foregoing tergites which suggests it is Tabanus sudeticus.

Other characteristics enable us to confirm identification of the set specimen above. The 3rd antennal segment of Tabanus sudeticus is reddish-brown on the basal part (including the dorsal tooth) and blackish brown apically, with the antennal flagellar segments black (see first picture below). Tabanus bovinus has the antennae mainly black with only the extreme base of segment 3 reddish-brown. On the underside, sternite 3 of Tabanus sudeticus has a full width dark band(see second picture below). Tabanus bovinus has only a median dark patch on sternite 3.

 

In life the eyes of females of Tabanus sudeticus are blackish-brown with a coppery sheen (compared to Tabanus bovinus wheose eyes are emerald green) (Brauer in Austen, 1906 ). The parafacials have abundant black hairs and there are no eye bands.

Males of Tabanus sudeticus (not shown here) have the abdomen extensively yellow-orange. The facets in the upper two thirds of the compound eye of Tabanus sudeticus are, with the exception of those on hind margin, at least four times the size of the rest. In Tabanus bovinus the eye facets are of fairly even size (if you are uncertain about this character go to Diptera.info  for an excellent photo). In life the eyes of males of Tabanus sudeticus are blackish with a coppery sheen, whilst those of Tabanus bovinus are entirely green. (Brauer in Austen, 1906 ).

Distribution & Seasonal Occurrence

The dark giant horsefly flies in July and August and commonly feeds on the blood of cattle and ponies. In Europe it appears that while Tabanus bovinus occurs in May & June, Tabanus sudeticus flies from the end of June and through July and August. Krčmar (2005)  reports that it reaches its maximum abundance in third week of July. In Britain it mainly lives in boggy areas in the north and west, although it is also quite common in the New Forest. Tabanus sudeticus is distributed widely in northern Europe into Russia.

 

Biology & Ecology:

Resting behaviour & swarming

As with so many Tabanidae, there is surprisingly little information available on the biology of Tabanus sudeticus. Most information appears to have been gathered by Brauer back in the 1880s; it is summarised by Austen (1906).  Brauer reports that males hover and swarm above the highest mountain tops in the twilight before sunrise.

Blood feeding

Tabanus sudeticus is anautogenous - it must first take a blood meal before it can lay eggs (Krčmar & Maríc, 2007 ). The dark giant horsefly undoubtedly prefers feeding on horses, cattle and deer, but it will bite man if available, as many have found to their cost (see below). It makes a deep hum when flying around a host, but this stops abruptly just before it settles.

Nectar feeding & puddling

Aside from feeding on live hosts, Tabanus sudeticus has been recorded feeding on mammal carcasses, presumably upon the decaying juices. Gu et al. (2014)  observed females feeding on a red deer carcass for about one week, and on a two-week old carcass of a European bison.

Breeding sites

Tabanus sudeticus breeds in boggy areas, although it seems that few larvae have ever been found. Andy Grayson suggests  that the larval habitat of Tabanus sudeticus is bogs and boggy flushes, whereas the larval habitat of Tabanus bovinus will prove to be the margins of ponds and lakes.

Trapping & odour attractants

Some work has been done to test the effectiveness of different odour attractants for Tabanus sudeticus. In Croatia Krčmar et al. (2006)  showed that canopy traps baited with cow urine collected many more Tabanus sudeticus females than did unbaited traps. However traps baited with aged horse, sheep, or pig urine were ineffective.

 

Nuisance value & Disease transmission

Although many tabanid bites are painful, very few are likely to have serious consequences. However, Quercia et al (2009)  report that a bite by Tabanus bovinus/sudeticus can cause a systemic reaction - in severe cases including anaphylactic shock and death . I have personal experience of this since, when camping in the New Forest with my parents, a dark giant horsefly bit my father on the hand. A short time later his hand swelled up like a balloon and he was briefly hospitalized.

There are several similar accounts on the web, for example Simon Davey  reported a severe reaction to a bite, although he thought that the chances of getting bitten by such a 'clumsy noisy beast' were very small. A quite different assessment of the 'threat level' is given by one of the contributors to the ranger's blog  "Be very careful around them, they land ever so gently and you don't feel them until it's too late". This blog includes several reports of extreme swelling following a bite. It seems that horses also tend to swell up when bitten, and can show a very vigorous reaction when the flies land on them.

References

  • Austen, E.A. (1906). Illustrations of British blood-sucking flies.  Full text 

  • Gu, X. et al. (2014). Carcass ecology - more than just beetles. Entomologische berichten 74 (1-2), 421-424. Full text 

  • Krčmar, S. (2005). Seasonal abundance of horseflies (Diptera: Tabanidae) from two locations in eastern Croatia. . Journal of Vector Ecology 30(2), 316-321. Full text 

  • 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. & Maríc (2007). The role of blood meal in the life of haematophagous horse flies (Diptera: Tabanidae). Periodicum biologorum 112(2), 207-210. Full text 

  • Quercia, O. et al. (2009). A case of anaphylaxis: Horse-fly or Hymenoptera sting? European Annals of Allergy and Clinical Immunology Journal 41(5), 152-154. Full text