Posted in science

Why are horsefly bites so painful?

Like most students, I have the mixed blessing of moving back home for the duration of the summer holidays. On the one hand, I get to catch up with my family, there’s no rent and I can indulge in large amounts of free food. However, as with everything, there are downsides to returning to the rural idyll that is the countryside. Ranking only slightly below the lack of public transport in my local area, the ridiculous abundance of horseflies is very high on my list of utter nuisances. I don’t know about you, but when I am bitten by one of those godforsaken creatures I swell up like a balloon and itch like you wouldn’t believe. So, obviously, I wanted to find out why.

According to the rather dubious source that is Wikipedia, horseflies have appeared in literature since someone in Ancient Greece complained about them ‘driving people to madness through their persistent pursuit’. Personally, I entirely relate to this historical experience. They are known by a large number of common names, such as gad-flies, dun-flies and breeze-flies to name but a few. Rather terrifyingly, it has been recorded that they can fly at speeds of up to 15 mph (although I cannot verify the veracity of this information).

The official dictionary definition of a horsefly (or gad-fly or whatever you want to call it) is as follows: ‘a stoutly built fly, the female of which is a bloodsucker and inflicts painful bites on horses, humans, and other large mammals’. Painful? Damn right. Most of the time, both genders of horsefly consume plant nectar in order to survive. However, when it comes to reproducing, the females of the species need the protein found in blood to produce their eggs. As such, they then turn into evil creatures that attack unsuspecting bystanders.

Unlike mosquitoes (which pierce the skin with a needle-like projection instead of inflicting a genuine bite) horseflies use sharp mandibles to tear open the skin. This provides easier access to the blood underneath, and it also has a very beneficial evolutionary side effect – because the bite is more painful, the poor mammal is usually more concerned with tending to the newly inflicted wound than it is with killing the perpetrator. Once it has bitten, the horsefly will return in order to drink the blood with its sponge-like mouthparts. Yuck. When it is feeding, salvia containing an anticoagulant is injected into the wound in order to maximise blood flow. It is this saliva that causes the intense allergic reaction in some people; the main symptoms are painful weals, itching, dizziness, weakness and wheezing. The reactions are caused by the increased production of histamine by the body, which then floods the area.

Unfortunately, there is no fool-proof way to get rid of/avoid being bitten by the ever-present horror that are horseflies. Never fear though, because there are ways to minimise the likelihood of being turned into a walking buffet. Horseflies tend to be most active during the day, especially around noon and in warm, sunlit areas. They tend to avoid shady areas, and are completely inactive at night. My recommendation is to avoid the areas/times they frequent. They seem to be attracted to potential victims in a number of ways, such as movement, warmth, the pattern/colour of the clothing/hide and the amount of carbon dioxide exhaled. There is some evidence to suggest that horseflies may be deterred by stripes, but this has not been extensively researched. I think stripes might be in this season? Fashionable and functional?

So to conclude, the reason why horsefly bites are so damn itchy and painful is this: they tear open your skin to form a wound, which is then filled with a mixture of saliva and anticoagulant which triggers an intense histamine reaction from the body. Fabulous.

Tabitha Watson


2 thoughts on “Why are horsefly bites so painful?

  1. And this is why our beasties are disguised as zebras all summer. In horses we thought that the darker the colour the more they are bitten. I postulate that sweat might attract at them..or that might be heat? Are female mammals bitten more than males? In other words are hormones attractive to them? I have been told by local farmers that the more you are bitten, the less you react. Is this a histamine – related response? All very interesting. Can’t you just stop the damned things??? I now know a number of horsey types who have invested in horse-fly catchers …they seem to work!


    1. As far as I understand it, both the scent of the sweat and the heat trail that mammals leave behind them are factors that can attract horseflies to a potential victim. I haven’t done enough research into any potential gender bias to be able to answer about that conclusively, but it does seem very plausible that some hormones may smell more palatable than others.

      When a horsefly bites you it is indeed a histamine driven response, although it really depends on the individual as to whether the severity of the reaction decreases with greater exposure. Think of it in the terms of any other allergic reaction – if someone with a peanut allergy was repeatedly fed peanuts the severity of the reaction probably wouldn’t decrease very noticeably (then again, micro-dosing is a thing so I guess it depends on the size of the dose).

      I am by no means a professional in this field though, so if you do spot a mistake or discrepancy please do let me know.



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