The Parasitic Mind-Controlling Worms of Yosemite!

November 20, 2015 Posted by: BA - Park Ranger (Yosemite Valley)

I am often approached in the Yosemite Valley Visitor Center with oddities found while exploring Yosemite National Park. About a week ago, I was brought some particularly interesting photos of a long and skinny worm found in a puddle near North Pines Campground. I was excited to see photos of a worm that I had never heard or seen anything about. I resolved to find some answers.

I discovered that the worm was a horsehair worm. They are slender worms, 1/25 to 1/8 inch wide, but oftentimes reach a foot in length. Some have been recorded up to six feet long. A common but incorrect misconception is that these worms are horsehairs brought to life after falling into water. Though they certainly can look like animated hair, the reality of their life cycle is perhaps even more curious than the myth.

The larvae of these worms develop as parasites within the gut of an invertebrate host, usually a beetle, cricket, or mantid. After several months of parasitizing the host, the worm somehow compels the usually aquaphobic invertebrate to cannonball into a water source, where the worm can then emerge as a free-living aquatic adult. It is usually while or after escaping from their hosts that these worms are discovered. It is difficult to differentiate between infected and non-infected invertebrates.

Hold up. Just to be clear, we’re talking about a parasitic worm that effectively thieves nutrients over the course of several months until it almost fills the entire body cavity of its victim host. The worm then takes control of the host’s mind and effectively turns it into a zombie, drawn to water and its own demise instead of to brains.

Do not worry: horsehair worms are not harmful to humans, domestic animals, or plants.

I am struck by the apparent lack of bounds that adaptation and evolution display as organisms have developed extraordinary methods to grow and reproduce. Again I am humbled by the incredible power of nature to excel given sufficient time. Yosemite National Park and the natural world never fail to excite and surprise me.

Skinny worm squirming around in a person's hand

Yosemite Valley, Ranger Notebook

11 Comments Comments icon

  1. Michele
    July 04, 2018 at 08:53

    Found one in my pool as I was using the net to clean debris. Thought it was a piece of thread and suddenly it unraveled itself and was moving. I was freaked out, did not go back into the pool and put shock in the pool that night!

  2. Judy
    January 26, 2018 at 03:15

    I think I had one of these worms on the front porch of my trailer tonight when I got home. It was climbing up my ramp and was almost to the top. But it wasn't just one, there was one very long one and a couple very small ones. The picture on your website sure looks like what was on my porch. I have a picture and a video of it but don't see a way to upload it here for you to see it. I was not sure what it was as I have never seen anything like it before. I was afraid it was something that would want to get into my animals so I swept it up and put it in a sealed (zip lock sealed) garbage bag. I do live out in the country and have lots of crickets, beetles, etc that hang out on my porch. It was raining and there are a lot of puddles on the ground and on my ramp. Very unusual worm, if that is what I had, glad to hear it wouldn't do any harm to my animals or myself.

  3. Yu
    November 24, 2015 at 03:55

    @Judy This little worm embodies so much wonder (and awe) in its evolutionary path. The larvae cyst in their intermediate hosts to increase the chances of survival and transmission and prolong the lifespan (from a mere one week for bare larva to more than 6 month in a cyst). Once reaching to the definite/final hosts, the worm makes changes to the final hosts' body - most noticeably altering the hosts' neural system (manipulated behavior change) and reproductive system (evolutionary selection). It absorbs nutrition and water from the host directly from its skin, promotes increased level of taurine in the insects' brain, which lead to their deluded feeling of thirst hence the aggressive water-seeking behavior. Interestingly it sterilizes the male hosts but only partially the female ones, i.e. letting the female crickets develop eggs only after they successfully find water and releases the adult worm. Hanelt, Thomas and Schmidt-Rhaesa had a very comprehensive review article in 2005 detailing the most recent (up to then i guess..) research progress on this little cunning being. [ref] Advances in Parasitology Volume 59, 2005, Pages 243–305

  4. November 23, 2015 at 05:51

    When I was young we would find the horse hair worms and tie them into a knot and watch them unwind! Incredible!

  5. November 23, 2015 at 01:13

    What do they do or add to the Eco system?

  6. November 23, 2015 at 01:05

    Absolutely fascinating. Awesome post. Love the zombie reference, as well as the appreciation and wonder. Nicely done!

  7. November 23, 2015 at 01:05

    Absolutely fascinating. Awesome post. Love the zombie reference, as well as the appreciation and wonder. Nicely done!

  8. November 23, 2015 at 01:04

    Absolutely fascinating. Awesome post. Love the zombie reference, as well as the appreciation and wonder. Nicely done!

  9. November 23, 2015 at 12:47

    I HAVE seen them but, don't know that I knew they were parasitic. I have seen them in the Arroyo of O'Neill Regional Park in puddles in the past. Maybe again this year!

  10. Kim
    November 20, 2015 at 07:12

    This is very interesting! I've never seen them before!

  11. Kim
    November 20, 2015 at 07:11

    This is very interesting! I've never seen them before!

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Last updated: November 20, 2015

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