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Check out this massive fly

Matthew Chatfield

This weekend I discovered that the biggest fly in the UK was much bigger than I’d previously thought. Yes, we heard you wanted to see a picture of a simply gigantic horsefly. So here it is.

Tabanus sudeticus (c) Cat James


This is Tabanus sudeticus, sometimes called the dark giant horsefly. It seems, oddly enough, that this impressive insect has not really got a commonly-accepted English name. It’s referred to in one place as the “dark behemothic horsefly”: a charmingly descriptive name, albeit a little cumbersome. Yes, it’s sitting on my finger and no it didn’t bite me. They can be up to 25mm long (that’s one inch) and 50mm across the wings – a massive fly and the largest dipteran in Europe (I think it was bigger! Having measured the Ranger’s finger, the fly could’ve been at least 30mm – The Cat). Horseflies are big, fast-flying creatures, and they will bite any big mammal, including humans. The bite is very painful, and as horseflies cut the skin when they bite (rather than pierce it), horsefly bites can take a long time to heal, and can cause infection. Unlike flies such as mosquitoes which surreptitiously puncture the skin with needle-like organs, horse flies have mandibles like tiny serrated scimitars, which they use to rip and slice flesh apart. So I’m quite glad my new acquaintance didn’t take a nibble out of me. Given the size of it, it might have left me rather drained and anaemic.


I was walking in a wet meadow in West Hampshire, where the thick, lush vegetation was buzzing with life. Swallows swooped overhead, and we were admiring the bee-flies that seemed to be almost swarming around us. Suddenly, my companion Cat shuddered to a halt having nearly trodden on what she at first thought to be a moth. It turned out to be the biggest fly we had ever seen. It was soon captive in a jamjar and being admired safely through glass. In the video above a 5p coin is in the jar for scale.

Tabanus sudeticus (c) Cat James


Adult horse flies feed on nectar and sometimes pollen, and the female flies drink a blood meal before laying eggs. Males don’t drink blood – so which is this character? The key is in the eyes. Like many big-eyed dipterans, the males have holoptic eyes which meet in the middle, whereas the females have a bar separating the two big eyes. Clearly, in the picture you can see that this one is a female. Mmm. Good job she wasn’t hungry. After a bit of cooing and gawping; we decided to let the captive free, as she seemed to be getting a little irritated by her imprisonment. It shook itself, cleaned itself for a moment, and flew off with a noise that sounded more like the low drone of a stag beetle or moth than the buzz of a fly. We’ll report the sighting to the Hants fly recorder in due course as the species is not very widely distributed. One would imagine that any sightings would be quite often noted, as the things are so spectacular. Anyway, we didn’t have a saddle that would fit it, otherwise we might have tried to ride it home.

Matthew Chatfield

Uncooperative crusty. Unofficial Isle of Wight cultural ambassador. Conservation, countryside and the environment, with extra stuff about spiders.

65 thoughts on “Check out this massive fly

  • Known Human

    If you think our British horseflies are big, you don’t want to see the ‘flying teeth’ horseflies of French Guiana. I don’t know their ‘official’ non-scientific name in English or French, let alone their binomial taxonomic name. When I briefly work in Kourou, the ‘flying teeth’ as my fellow British colleagues called them were everywhere and bit everyone of us leaving triangular patches of missing skin a couple of mm long. I imagined that “flying teeth” was translated from some informal French name for them. Their bodies typically measured 2″ / 50mm long or longer. I have no photos and my internet search for them has failed.

    • Matthew Chatfield

      You are correct, I don’t want to see those. Or not too close, anyway.

  • John Hurst

    One on our pond today – North East corner of Anglesey. Probably having a rest before nipping off to Ireland. What an impressive beast!


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