Horse fly

2009 July 25

So, I came home on July 15, and Sam proudly showed me this fly sitting on a magazine. She had chased it around the house so much that it was too exhausted to move, and so it just sat there while I took pictures:

It’s one of the horse flies, which are some of the more obnoxious biting flies [1]. The things that make them obnoxious are that they are big flies (over a centimeter long), have a tendency to circle my head for a long time before trying to bite, and then when they finally *do* land to bite (usually right on the back of my head, at that point where the hair swirls around in a circle[2]), the bite hurts like the dickens and bleeds freely, because their mouthparts look like this:


There’s none of the slim, elegant, needle-like mouthparts that mosquitos have. No, they have this huge dagger of a mouth that slashes brutally through the skin, no anaesthetic or anything. They are going for the quick meal – go in, slash, suck up the blood, and then flee. On the plus side, the wound bleeds freely and doesn’t end up with a lot of fly saliva in it, so there usually isn’t the same level of itching and swelling that you get with black flies and mosquitos. It’s just the high level of immediate trauma and pain that makes them unpleasant.

Of course, they do have their attractive side, note the iridescent striping on the eyes, which are apparently interference patterns due to small changes in the size of the ocelli that make up those huge compound eyes:


Those bands of color are almost hypnotic, and seem out of place on an insect that is otherwise kind of on the drab side.


There are a lot of kinds of horse fly, I think this one is one of the members of the genus Tabanus. There are several species on BugGuide that look kind of like this one, but none that look quite right, so we’ll just leave it at that.

Like most other biting flies, the females are the only ones that bite[3], because they need nitrogen (as protein) and salt for their eggs. And the reason they need extra nitrogen and salt, is because their larvae are aquatic, and the freshwater aquatic environment is notoriously poor in these two nutrients. Unlike most other biting flies, though, horse fly larvae are carnivorous. I haven’t found anything about what other animals, exactly, they most like to eat, but I kind of expect that they will eat the larvae of other biting flies (like black flies and mosquitos) if they are given half a chance. This makes me a bit ambivalent about them; on the one hand they are noxious in their own right, but on the other hand, if one horse fly does away with 100 black flies as it grows up, they might be worth it.

Anyway, they are mainly called “horse flies” because they do like the horses and other large mammals. They used to drive our cattle crazy on the farm, for example. There are related biting flies that are commonly called “deer flies”, but that doesn’t mean that they prefer deer while the horse flies prefer horses. They both bite all large mammals pretty indiscriminatly. I think it’s just that people tend to associate a big, burly fly with horses, and the smaller, more delicate flies with deer.

[1] As it turns out, this is only the second species of biting fly that I’ve posted so far. The first one was the infamous black fly, and it was posted almost exactly two years ago. Then, just about a year ago, Edward Vielmetti left a comment on the black fly page pointing out, quite reasonably, that I was seriously short of biting fly entries. I promised to get right on it, but what with one thing and another, it took another year to do anything about it. So much for promptness . . .

[2] I haven’t tried it yet, but you can get these big pieces of two-sided tape to stick on the back of your head (or, more realistically, onto your hat on the back of your head – sticking it straight to the head would probably make a real mess in your hair). The horseflies will supposedly tend to land on this tape (thinking it is that spot on your head that they love so much), and get stuck there, leading to their grisly demise. At which point, we all cheer.

[3] Some years ago, somebody made some comment to the effect that a particular person was such an insect expert that he could tell the *sex* of an insect[4], and I replied, “Oh, that’s easy! If it stings you or bites you, it’s probably female. If it makes loud noises, flashes light, or otherwise draws attention to itself, it’s probably male.” This isn’t always true (both male and female fleas bite, for example), and there are a lot of species where it is no help because neither sex bites, stings, or makes itself obvious, but it is accurate an uncomfortably large fraction of the time.

[4] Although actually, it is frequently easier to tell the sex than it is to tell the correct genus. Like the way that the vast majority of male spiders tend to have swellings at the end of their pedipalps (making it easy to tell their sex), even while most of the time only an expert with a microscope can make identification even down to the family level.

10 Responses
  1. July 27, 2009

    I hates ’em, I do! I really, really hates ’em! Durned horse flies. I don’t think their predatory nature makes up for the stabbings.

  2. Sandra H permalink
    July 29, 2009

    When I worked as a timber cruiser these guys used to like to land on the mirror of my fold-out compass. I held the compass cupped in my hand and found that if I was quick, I could slam the compass shut and smoosh them. It didn’t help anything but it made me a little bit happier.

  3. August 3, 2009

    I love the visual effect of photographing an insect on news print.

  4. August 14, 2009

    I wasn’t thinking about it at the time, but the text behind the fly does make an amusing background (I think it was an article about a financier going to jail, if I’d been thinking about it maybe I could have arranged for some appropriate words to be visible, like “criminal”, “theft”, or “bloodsucker”)

    Pity the light wasn’t better, though. She was in direct sunlight, and I didn’t have the settings correct for the conditions and she ended up kind of washed out.

  5. bebe permalink
    September 4, 2009

    Nasty, nasty things! Just yesterday one landed on my horse’s rump while I was riding. My friend was yelling, “He’s going to buck! He’s going to buck!” Then, “Kill it! Kill it!” Not easy to kill a horse fly on your horse’s rump while he’s trying to buck it off. Luckily, they are not very quick, at least not compared to a house fly. I was able to smack the evil thing with my bare hand and kill it. My poor horse (who was the intended meal) heaved a huge sigh of relief.

    Horses know the second one of these flying devils land on them and they freak out because they know they’re going to get a vicious bite. This time, however, I killed it before it bit.

  6. September 12, 2009

    do horse flys move or sometimes?

  7. September 14, 2009

    Julia: I’m not quite sure what you mean. They are certainly strong fliers, and they walk around on surfaces the same way that other flies do.

  8. September 22, 2009

    they are ugly

  9. samantha permalink
    September 22, 2009

    they are an interesting animal to see up close

  10. Steve permalink
    September 25, 2009

    I sprayed a horsefly about as big as my thumb, with wasp killer (it was harassing us). It flew into our chain link fence post with an audible “dink” noise, bounced off then flew on.

Comments are closed.