Monday, 7 July 2014

Horseflies - my insect nemesis

I've never been a fan of horror films but I think I can understand the attraction; I get what I guess is a similar tingle of fear whenever I see a horsefly close up. I'm one of those who react badly to horsefly bites, typically it swells up like an egg and I suffer several days of intolerable itching (as I write I've got an unsightly red 'wound' on my arm after a bite six days ago). I have to admit though that they do have a kind of macabre beauty, I mean, just look at the eyes on this one.

A deer fly Chrysops viduatus 21/7/2005 Topsham
As in many families of flies, the sex can be told from the eyes - meeting on top of the head = male, separated (as here) = female. It pays to know this since only females bite (males prefer nectar) and the offensive hardware, the proboscis, is clearly visible under those green and purple eyes.

A word on Common/English names: given their predatory habits, biting flies tend to get noticed and a few of these have acquired common names. Since folk taxonomy rarely discriminates beyond the genus specific names have been invented in an attempt to make them 'more accessible'. In my opinion if you're interested enough to specifically identify this 'Square-spot Deerfly', you aren't going to be put off by it's scientific name - Chrysops viduatus Tabanidae.

This is one of about 30 spp. of horsefly (Tabanidae) found in Britain and this family contains the worst biters in my opinion - apart from mozzies of course, but that's an entirely different subject. They're often big, they're ugly (or are they beautiful, I can't decide) and their bites can itch like hell - I'd rather be stung by a wasp than bitten by a cleg (see below).  Deerflies may prefer deer but I've been bitten a few times - they usually go for the head and neck. Unlike these next two which tend to go for the arms or the back of the legs.

A cleg Haematopota pluvialis 9/7/2006 Topsham
Eyes not meeting on top so a biting female - be afraid.

A cleg Haematopota crassicornis 22/7/2009 Dunsdon
Eyes meeting on top so a non-biting male - relax, it's harmless.

Haematopota spp. (literally 'blood drinker') also go by the name of 'cleg' (always enjoyed the name - seems so apt for these quite unpleasant little beasts - and I make no political comment here). These are tough critters and can survive a fairly hard swat with the hand, after a few seconds they just pick themselves up and fly off - or more likely come back for another go at the back of your knee.

Clegs and deerflies are fairly modest sized flies - about the same as a housefly - but there are some alarmingly larger species out there. The larger 'horseflies' proper are nowadays divided between two genera, Tabanus and Hybomitra; There are about 17 spp. but many are rare and only half of them bite - the females, that is. Tabanus bromius (below) is actually one of the smaller examples but still about wasp-sized.

Tabanus bromius 27/6/2009 Salcombe Regis
 Eyes not meeting - a biting female - run away.

Next is a fairly large one, T.autumnalis. You'll notice I say 'fairly large', there are 'giant' horseflies out there - T.sudeticus should occur around these parts (though I've never noticed it) and can be an inch long, or about the size of a Small Elephant Hawk-moth!

Tabanus autumnalis 13/7/2005 Topsham
Eyes not meeting - a biting female.

The very similar genus Hybomitra differs most obviously in their hairy eyes - though you have to get uncomfortably close to see this. Here's H.distinguendua, a fairly large species.

Hybomitra distinguenda 22/7/2005 Topsham
Eyes separated - I think you know what that means.

I've never knowingly been bitten by one of these larger flies and I think I'd have noticed, in fact I'd probably need hospital treatment. I suspect they prefer to bite cattle or horses, humans just aren't worth the effort.

All species photographed and identified by me (in some cases verified by the good folks at using the keys in the excellent Stubbs and Drake (2001).


Stubbs, A.; Drake, M. (2001) British Soldierflies and their allies. BENHS


  1. A fascinating post Tim. Superb photos too. The eyes look simply incredible. Look forward to more insect posts!

    1. Thanks Matt, plenty more where these came from.

  2. Stunning shots,are these also known as Clegflies.

    1. Hi John, yes, clegs, cleggs, clegflies, klags - all the same thing. To some people all horseflies are clegs but I've always just used the name for Haematopota spp.