I can\u2019t imagine there are many of you out there who have yet to meet a cleg in the wild, but just in case such people exist, a cleg is a horsefly. Along with deerflies, horseflies are part of the Tabanidae family. Globally, there are almost 4500 tabanid species, and very generally speaking they are robust, blood-feeding flies with painful bites. <\/p>\n\n\n
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There are 30 or so species of horsefly in the UK, the largest and most intimidating of which is undoubtedly the Dark Giant Horsefly (Tabanus sudeticus<\/em>). At around an inch long, it\u2019s quite the thing to encounter, plus you know an insect is big when you measure it in inches rather than centimetres or millimetres! The horseflies we\u2019re most familiar with though, are less intimidating in appearance and much smaller.<\/p>\n\n\n\n The word \u2018cleg\u2019 apparently comes from the old Norse word \u2018kleggi\u2019, meaning horsefly. Norwegian and Danish haven\u2019t strayed far from that since then, today using klegg and kl\u00e6g respectively. In Scotland many people use \u2018cleg\u2019 in reference to any of the biting horsefly species they encounter. But in my experience, and for the sake of making things simple here, the clegs most of us encounter are very likely to be one of the species in the Haematopota <\/em>genus – most likely Haematopota pluvialis<\/em>, the Notch-horned Cleg Fly. It\u2019s Haematopota <\/em>I\u2019m focusing on for this article, as they\u2019re the ones people commonly think of when they talk about clegs, and are the ones who have been bugging me of late.<\/p>\n\n\n\n Unlike a mozzie or a midge, both of which are on the delicate side, clegs are robust. A mere swat won\u2019t necessarily kill a cleg, it\u2019ll just knock it for six. That\u2019s assuming you can actually make contact with it of course. Clegs might not have the lightning-fast reaction times of a house fly (they\u2019re about three times faster), but they\u2019re still on the quick side. <\/p>\n\n\n\n However, assuming you don\u2019t swat them away, you\u2019ll notice that the Haematopota<\/em> species are small (about 1cm long), slender and sturdy. They have rather beautifully mottled brown-grey wings, which when resting are held together over their back, like the apex of a roof.<\/p>\n\n\n\n Look closer still (oh go on!) and you\u2019ll notice the eyes. Oh the eyes! It\u2019s the female you\u2019re likely to encounter, and her eyes easily rank among the most peculiar and dazzling of the insect world – a lovely sheen of metallic reds, greens and yellows. The male eyes aren\u2019t nearly so dazzling, but the female\u2019s eyes have horizontal, irregularly zig-zagging brown\/red stripes across them, as though charged with sparking electricity. Those aren\u2019t pigments making her eyes that colour, rather it\u2019s the thousands of colourless optical units of different densities called ommatidia. It is the passage of light through these, and the subsequent refraction of that light as it does so, that produces the colours.<\/p>\n\n\n\n Both male and female clegs have large, wide eyes. The male\u2019s eyes touch one another at the centre of the fly\u2019s head, but the female\u2019s are very obviously separated slightly. Let\u2019s be honest though, the finer details of cleg anatomy will be lost on you out in the field as you frantically swat the thing away or flee at speed. All you\u2019ll be familiar with is a fairly weighty-feeling insect, a blur of grey, as you instinctively swat it away.<\/p>\n\n\n\n Both sexes feed on nectar from flowers, which is energy-rich, but the female alone needs a blood meal to obtain the extra protein and iron for laying her eggs. As with midges, it\u2019s therefore only the female that bites.<\/p>\n\n\n\n <Enter, stage left, the unsuspecting hillwalker><\/p>\n\n\n\n Yep, that\u2019s you.<\/p>\n\n\n There are many different insect beasties that pester us, but they do so with their own unique styles. Midges are weak flyers and don\u2019t arrive quickly. That midge cloud around your head kind of fizzes into existence \u2013 slowly at first and then more quickly as word spreads around the midge community that food has been served.<\/p>\n\n\n\n Mozzies are slow too, delicately circling around you before hesitantly deciding on a landing spot. Clegs however, are like heat-seeking missiles, coming at you at speed. There\u2019s no faffing with deceleration before contact, no Top Gun finesse, no aerial reconnaissance to find the best landing spot. They just come at you directly, almost colliding in fact. I\u2019d therefore rate them as the most confident of the wee beasties, given their laser-like targeting. But how do they find you? What gives them that confidence?<\/p>\n\n\n\n Detailed studies of pluvialis <\/em>are all behind academic paywalls, so I couldn\u2019t easily find a study that explained to what extent various biological phenomena play in a cleg\u2019s predation. I\u2019ve referred to their flight pattern as \u2018heat-seeking\u2019, but as far as I can tell, even though they can detect body warmth when they\u2019re very close enough to their target, or have landed on them, heat doesn\u2019t play a significant role in how clegs first identify and target their prey. Nor does carbon dioxide, which contrasts with other blood-feeding insects, such as our beloved midge. A cleg\u2019s initial targeting seems to have more to do with light and motion, and the fact that its eyes are extremely sensitive to polarised light.<\/p>\n\n\n Normal (unpolarised) light becomes polarised when it reflects off certain types of shiny surface and subsequently gets filtered into a uniform pattern. I won\u2019t pretend to even half understand what light waves look like before and after they\u2019ve been polarised, but essentially they\u2019re a confused mess beforehand, coming from all directions depending on what the source was and what else they\u2019ve bounced off. It\u2019s a jumble. But when those waves hit a shiny surface, such as sleek or oily animal fur, it\u2019s like passing that light through a grid or a sieve. The light is filtered at the moment of reflection – some if it is scattered in all directions, but the light that is successfully filtered is no longer a jumble. It is reflected away with a uniform alignment or pattern. It has been polarised.<\/p>\n\n\n\n Whatever that uniform pattern looks like, we can\u2019t see it. But clegs can, and it apparently makes the source object stand out brightly against its background. Having this behavioural response to the polarisation of light is called Polarotaxis.<\/p>\n\n\n\n This means clegs can instantly identify host animals with sleek, oily or reflective hides and fur, even when there\u2019s lots of visual \u2018noise\u2019 in the environment. A single, large area of polarised light indicates a potential host. And if it\u2019s moving, this makes it stand out even more.<\/p>\n\n\n Objects obviously shine more brightly and reflect more strongly when they\u2019re in bright sunlight, but on cloudy days when the light is diffuse, with no strong light from any direction, a potential target won\u2019t stand out as well to a cleg among surrounding dark vegetation. It\u2019s little wonder, therefore, that clegs tend to be a problem on those lovely sunny days when we long to be outside. Certainly in my experience, it\u2019s when it\u2019s hot and still and sunny that I am most plagued by clegs. Those hot days in June and July when it\u2019s not too humid. <\/p>\n\n\n\n Interestingly, white animal coats reflect light with a much lesser degree of polarisation compared to coloured \/ dark ones – something to do with the way light reacts when striking the melanin pigments that give hair its colour. A Hungarian study showed how horsefly attacks on dark livestock were four times more likely than on white livestock.<\/p>\n\n\n\n Bright sunlight not only assists clegs in seeing polarised light more clearly, but it also means that the surface of their potential target will be warmer than it would be in shade. Horseflies, like most insects, are cold blooded and therefore need ambient heat in order to function. They can be sluggish in cooler temperatures. Researchers, again in Hungary (not a great advert for the Hungarian tourist board!) found that horseflies targeted warmer hosts more readily than colder ones.<\/p>\n\n\n\n They found that the \u2018escape success\u2019 of a horsefly (i.e. not being killed after biting) was driven by temperature, and that horseflies therefore appear to hold off an attack until conditions are optimal for survival. The warmer the host, the quicker the cleg\u2019s reactions are likely to be, and the more chance it has of surviving its attack.<\/p>\n\n\n\n All of the above means that clegs therefore show a preference for sunlit objects over shaded ones, and for dark objects over light or white ones. So if you\u2019re a moving, dark-coloured, sunlit large object, watch out!<\/p>\n\n\n\n Clegs are famed for their stealth, but they aren\u2019t completely silent in flight. Their wings do make noise as they cut through the air, however they are nowhere near as buzzy as most buzzing things, so in the insect world at least, yep, clegs can be considered stealthy. <\/p>\n\n\n Certainly they\u2019re quiet and fast enough that in an outdoor setting you\u2019re unlikely to hear them coming. But their stealth is also down to the fact that they\u2019re ambush predators. They don\u2019t fly around perpetually searching for a target, rather they sit and wait. And once they\u2019ve identified you as a host, they make a direct line for you, at speed.<\/p>\n\n\n\n Upon landing, clegs tap the surface they\u2019ve landed on with their palps. These detect heat, revealing whether blood vessels are close to the surface and whether it\u2019s worth making a bite. This always feels a protracted process to me when I watch a cleg setting the dinner table, but it\u2019s probably just a few seconds. Then, when they\u2019re happy with the spot, they pivot like a see-saw, their front end heading downwards and their rear end heading up. <\/p>\n\n\n\n Unlike the surgical, needle-like proboscis of a mozzie, clegs have an altogether rougher assemblage of mouthparts, the reality of which is revealed by a sharp, sudden nip that has you instinctively reaching to swat whatever is responsible away.<\/p>\n\n\n\n A cleg\u2019s mouthparts are a collection of six structures, comprising two mandibles, two maxillae, the hypopharynx and the labellum. The mandibles move in a sideways fashion, left to right, like a pair of scissors. These make the initial cut into the skin, and while the inside edges are very sharp, presumably to aid the cutting, the outer edges are serrated, which I\u2019m guessing helps to anchor the cleg to the skin and prise the wound open. Close-up images of their mandibles clearly show fine serrations along their outer edges, like the teeth on a saw.<\/p>\n\n\n\n A typical horsefly has, on its mandibles, about 20 \u2018teeth\u2019 per 50 microns. A micron is 1000th<\/sup> of a millimetre. To give a sense of just how finely serrated that is, an average human hair is about 70 microns thick.<\/p>\n\n\n\n The cleg also has a second pair of jaws, called maxillae, which are finely toothed. These don\u2019t move side to side like scissors, instead they move up and down like saws in a sawmill, cutting capillaries and rupturing blood vessels. Nice eh?<\/p>\n\n\n\n The hypopharynx delivers an anticoagulant to prevent the blood from clotting, which helps it pool on the surface, and then finally the labellum acts as a mop, with which the cleg drinks the blood. It\u2019s a slash and mop method, which is why the bite can be both painful and bloody.<\/p>\n\n\n\n I\u2019ve always found it almost malevolent, the way that clegs are so capable of approaching and landing without being detected, only to depart with such a visceral and painful goodbye. It\u2019s like rubbing your nose in it. <\/p>\n\n\n\n \u2018HAH! GOT YOU!\u201d, I imagine them saying as they whizz off into the sunshine.<\/p>\n\n\n\n What makes a cleg especially formidable as a predator, and indeed so very successful in biting humans, is that it doesn\u2019t have to land directly on bare skin to be able to bite you. The mouthpart assemblage is at least 1mm long, so while it won\u2019t get through thicker, tighter-woven clothing, it is quite capable of cutting through thin, skintight, or loosely-woven fabrics such as leggings, base layers or even t-shirts. This means you won\u2019t necessarily feel them when they land, and they\u2019ll go unnoticed right up to the moment they bite you.<\/p>\n\n\n This is why I always put insect repellent even on some parts of my clothing, especially if it\u2019s a thin layer\u2026which it is invariably is on the kinds of days that clegs favour. Those sunny, hot days. My shoulders and around the neck always seem especially prone.<\/p>\n\n\n\n Cleg bites are generally larger and more severe than midge bites. Mine certainly are, but I also find that the severity depends on how quickly I managed to swat the cleg away.<\/p>\n\n\n\n At the very least, I always get a large red welt that is insanely itchy. If I scratch it, it develops a circular red rash around it, or it might develop a small nodule or blister on the surface. More unlucky people than me might get an acute inflammatory response, a painful lesion, or even a localised burning sensation. But some people fare much worse than that.<\/p>\n\n\n\n Given that cleg bites are so keenly felt by the host, clegs don\u2019t tend to get their fill from just one meal before being chased off. They need to feed numerous times, potentially on many different hosts. This behaviour exposes them to many different environments and pathogens, and increases their potential to pass on bacterial infections.<\/p>\n\n\n\n A greater risk however, comes from secondary infection from bacteria in the environment. It is the slashing action and the nature of the wound that makes cleg bites so susceptible to infection, not least from the surface of your skin. Streptococcus for example, which can result in cellulitis if it gets into a wound, lives quite happily on your skin. For this reason scratching a bite, especially with dirty hands, should be avoided\u2026\u2026but I\u2019m sure we\u2019re all speaking from personal experience when we say that\u2019s much easier said than done!<\/p>\n\n\n\n We\u2019ve learned that clegs are hardwired to identify dark-coloured, moving, sunlit objects that have sheen-like or reflective surfaces. On hot sunny days therefore, it seems there\u2019s little chance of outwitting a cleg. Baselayers are shiny, as are sweaty arms and t-shirts. <\/p>\n\n\n I\u2019ve not tried it myself, but I suppose you could try to minimise how much you stand out by wearing light colours rather than dark, or choosing fuzzy, fleecy material that scatters (depolarises) light in all directions? I\u2019m tempted to give the white clothing a go, but I\u2019d draw the line at wearing thick, woolly, fleecy, light-scattering garments in hot sunny weather, as I\u2019m not fan of hot-weather walking at the best of times!<\/p>\n\n\n\n Bizarrely, another option could be to wear zebra-stripes, as horseflies seem averse to landing on them. In field tests, horseflies do approach zebras from a distance as though the animal was any other large, potential host target, because from a long way away they can\u2019t see the individual stripes. But when they get within a couple of metres they can suddenly see the stripes, and this sudden appearance is thought to confuse the insects\u2019 depth perception to such an extent that they veer off at the last second, unable to complete the landing.<\/p>\n\n\n\n I\u2019m not sure I fancy roaming the hills in zebra print, but if anyone has tried it, please do let me know. Personally, I always try to wear long-sleeved tops on days like that. Mainly to stop my arms getting burned, but also to deter clegs. Ultimately though, we do love those hot sunny days, and are therefore almost certain to be outside wearing shorts and t-shirts or shiny baselayers, with exposed, shiny, sweaty skin, polarising light wherever we go and doing our very best \u2018large animal\u2019 impressions.<\/p>\n\n\n\n We\u2019ve only got ourselves to blame, really!<\/p>\n","protected":false},"excerpt":{"rendered":" Yes, I know I wrote about ticks last time. And yes, I know that makes it two articles on the trot about nasty wee beasties, but inspiration for subjects comes from whatever has crossed my path in the previous month\u2026..and I\u2019m afraid to say very little has crossed my path in July other than clegs! Indeed, as I\u2019m writing this, I can feel the crazy itching of a cleg bite on my thigh and am desperately trying to ignore it, which is difficult when you\u2019re writing about the very thing that caused said itching. What is a cleg? I can\u2019t…<\/span> <\/p>\n <\/a><\/p>\n","protected":false},"author":6,"featured_media":29576,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[37,8,4],"tags":[],"class_list":["post-29559","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-features","category-magazine","category-conservation"],"_links":{"self":[{"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/posts\/29559","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/comments?post=29559"}],"version-history":[{"count":8,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/posts\/29559\/revisions"}],"predecessor-version":[{"id":29579,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/posts\/29559\/revisions\/29579"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/media\/29576"}],"wp:attachment":[{"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/media?parent=29559"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/categories?post=29559"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.walkhighlands.co.uk\/news\/wp-json\/wp\/v2\/tags?post=29559"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/www.walkhighlands.co.uk\/news\/wp-content\/uploads\/2026\/08\/P1030303-800x567.jpg\")
How they find you<\/strong><\/h3>\n\n\n\n
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Stealth biters<\/h3>\n\n\n\n
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The bite itself<\/strong><\/h3>\n\n\n\n
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Ouch!<\/h3>\n\n\n\n
Is there hope?<\/strong><\/h3>\n\n\n\n
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