Horsehair worm

2011 June 8

Last week, Alex Wild brought up the topic of Longform Photoblogging – his was a rather long mayfly. Well, in that spirit, here is my contribution to the longform format:

That’s probably as long of a picture of a living thing as I am likely to get (the tricky bit isn’t to find something that is just long. I have to get it to lay out straight, too. This one was in a shallow dish of water and just happened to straighten out while I took the picture).

It’s a horsehair worm, phylum [1] Nematomorpha. It was only about a half-millimeter wide but pretty close to 8 centimeters long. Sandy found it the last weekend in May on a paver-block sidewalk that she’d just built, after some fairly heavy rain. She said it was holding up one end well clear of the wet ground, and actually looked quite a bit like a grass shoot. These are also known as “gordian worms” (after the “Gordian Knot”), because they tend to tie themselves up into complex, 3-dimensional knots when they are mating or get a bit dried out, like this:

This particular one has interesting coloration, with parts of it being darker than others. I would normally think that this was due to having food in parts of its gut, except that these worms aren’t supposed to eat as adults. Which suggests that maybe the darker portions are eggs.

So, if the adults don’t eat, how do they get so long? Well, the immature forms parasitize certain fairly-large arthropods, particularly grasshoppers, crickets, roaches, some beetles, and crustaceans. The adults mate in water or very moist areas, and the eggs hatch into very tiny larvae that grab onto a host and get into its digestive tract. They then grow in there until they are many times longer than their host. When mature, they come out, get into water, and find a mate so they can lay more eggs. I gather the host generally does not survive.

Some species of horsehair worm don’t depend on their hosts accidentally being somewhere moist when it is time for them to emerge. Some of them actually take control of their host’s brain, so that when they come near water the host jumps in and drowns itself. The worm then emerges from the unfortunate corpse in the exact sort of wet environment that they need.

There are differing opinions about these worms in our household. After hearing about their lifestyle, Sandy declared them “disgusting”. Sam, on the other hand, spent a long time watching it coil and uncoil in a shallow dish, and said it was “cute”.

[1] Yes, I said “phylum”. Horsehair worms are not particularly closely related to any other animals, not even to nematodes (which they kind of resemble). Not only are they not in the same phylum as arthropods, they aren’t even in the same phylum as segmented worms like earthworms. Their only connection to arthropods, is that they parasitize them.

13 Responses
  1. JRR permalink
    June 8, 2011

    Thanks for this entry. I found some of these in a lake in Canada when I was a kid, and kept them in a jar that week. They were fascinating to watch, but I never knew exactly what they were. My dad told me they were horsehair worms but I never looked them up at the time and I’d mostly forgotten about it.

  2. June 8, 2011

    I’m with Sandy. You really do need to create a “truly icky” category in the sidebar.

  3. June 8, 2011

    Some of them actually take control of their host’s brain, so that when they come near water the host jumps in and drowns itself.

    That is freakishly cool.

  4. June 8, 2011

    JRR: You’re welcome. It just goes to show that, with the Internet, even answers to questions you didn’t know you had are likely to fall into your lap.

    Anne: Yes, I could do that. Ideally, I suppose people could *vote* on it, but I can probably work out which things people will find disturbing in any case.

    Andy: The whole “parasites mind-controlling their hosts” thing has really only been discovered in the last couple of decades, and I get the impression that it is turning out to be more common than anybody thought at first. Wikipedia even has separate lists for “Mind-altering parasites” and “Suicide-inducing parasites”. It may even happen to humans: the parasitic protozoan Toxoplasma gondii makes mice and rats unafraid of cats, and therefore more likely to be eaten (which lets the parasite infect the cat). But then, when humans catch the parasite from the cat feces, it is possible that the parasite then causes a bunch of changes in human behavior, too.

  5. June 8, 2011


    Awesome stuff. It’s cool how such behavior and chemical changes could evolve to that level of complexity. It reminds me of this:

    Cordyceps: parasitic fungi attacks insects

    …which is about a parasitic fungus called “Cordyceps”. It infects insects, and takes over their minds so they start climbing to higher elevations, after which the fungus can sprout and release its seeds more effectively. Nasty stuff, but again, pretty cool.


  6. Carole permalink
    June 8, 2011

    This is so cool I must think of someone to forward it to.

  7. June 9, 2011

    I’m not sold on the brain control aspects. The parasite would have to manage a creature more complex than itself. That is, the parasite does not have eyes, yet the host does. The parasite would have to have brain capacity to deal with non-native sensor inputs.

    Hmm. Sounds more than just tricky, sounds improbable.

  8. June 10, 2011

    You have a good point that “control” usually means something a bit stronger than what these parasites do to their hosts. What the parasites do is more of overlaying a strong impulse to do something over the host’s normal behavior. These worms don’t take control of the host’s body, use their senses to locate water, head to the water, and then force the host to jump in. What happens instead is that, when the host gets near water, it’s normal instinct to move away from the water gets turned into an urge to jump in. It’s as if there is normally a “don’t drown yourself” routine in the host’s brain, and the worm flips a switch to turn it into a “drown yourself” routine. The worm does not actually consciously do anything – which is a good thing, seeing as how it evidently doesn’t have a brain.

  9. Liz Davis permalink
    July 22, 2011

    I’m thinking this is what I found in the moist dirt at my barn. They were only in one spot and our horses are wormed regularly. I took some pictures of the bunch and a few videos of them and you can see them move. They were gone the next day so I knew they couldn’t be worms from the horses.

  10. Della3 permalink
    July 31, 2011

    Now you’ve got me wondering about the possibility of some human psychoses being caused by parasites. Wouldn’t it be wonderful if, say, schizophrenia could be cured just by killing off a parasite that is altering the person’s brain?

  11. Anna permalink
    August 31, 2011

    This is so great! I found one of these in a big green insect…not sure what it was. Anyway! It was in my kids’ room and I got so scared that I looked it up. I’m glad that it can’t hurt my kids and now I’m truly interested in finding out more.

    The part about “controlling” the insect was interesting. It made me think about how our bodies tell us when we are thirsty and hungry and when we get cravings. Cravings are like mind-controlling parasites if you will, telling us that we need a certain vitamin or nutrient of some sort. Problem with this is sometimes it makes us want weird things like laundry detergent and dirt. Comparing it to the insect being controlled, the insect feels like it needs water because of the parasite, so it goes to the water and can’t get enough to quench it’s thirst and ends up drowning.

    very cool! err not for the bug, but greatly entertaining for us!

  12. September 1, 2011

    Glad you found it useful, Anna. Did the big green insect the worm came out of have very long jumping legs (which would make it a katydid), or was it something else?

  13. Anna permalink
    September 13, 2011

    You know, I think that is what it was. I should have known because that’s what we call our daughter. πŸ˜› I did a lot of research on it when she was a baby so I could explain it to her when she was older…I seem to have forgotten a lot of that info now. πŸ˜› Ah, parenthood. πŸ˜›

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