Have you noticed that there seems to be more horseflies present in the UK during summer 2017 compared to previous years? There are certainly more where I live in Shropshire, I’ve seen them at locations where they have never previously been present and also in much higher numbers at known haunts. A quick search of the internet (e.g. here, here, here and here) suggested that this is probably not just a localised phenomenon, with horror stories of an ‘invasion’ abounding (along with a trend of sharing images of bites on social media). Whether this apparent increase is real, or a reflection of increased awareness (largely due to increased social media use) is unclear but, if the increase is real it is likely a result of favourable weather conditions resulting in the survival of more horsefly larvae.

Horseflies are true flies in the family Tabanidae of the order Diptera. There are around 28 British species of Horsefly, with the Common Horsefly (or Notch-horned Cleg Fly) Haematopota pluvialis being generally regarded as the most unpleasant. Unlike most flies this species flies silently, allowing it to approach unnoticed. The Dark Giant Horsefly Tabanus Sudeticus is the largest horsefly in the UK, a truly formidable insect!

Dark Giant Horsefly – one of the UKs largest

Selected Horsefly Facts

  • They are among the world’s largest flies (up to 3cm in length) and occur around the world with the exception of extreme northern and southern latitudes.
  • They possess scissor like mandibles that tear and cut, with the mouth parts of the female resembling jagged saw blades.
  • Only females bite and they use protein obtained from blood to facilitate egg development.
  • Their bites are painful because the females cut a hole in the skin in order to soak up the blood that emerges.
  • Male horseflies feed on pollen and nectar, and do not possess biting mouth parts.
  • Horsefly eggs are laid on plants in or near water, and once hatched they spend 1-2 years growing before transforming into adult flies which only live for a few days.
  • Blood loss is a common problem in some animals when large flies are abundant. Some animals have been known to lose up to 300 millilitres of blood in a single day to Tabanid flies, a loss which can weaken or even kill them.
  • As well as attacking mammals, horseflies are known to feed on reptiles and amphibians, but few seem to attack birds.
  • Horseflies are also known as ‘clegs’, ‘glegs’ or ‘clags’, which comes from Old Norse and may have originated from the Vikings.

You’ll probably be glad to know that In the UK horsefly numbers peak during June and July, so the chances of being bitten are diminishing (at least for this year!). But, if you are bitten, advice is available here.

This slideshow requires JavaScript.