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

YosemiteValley, RangerNotebook

Last updated: November 20, 2015

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