UPDATED SEPT 2, 2014
While collecting aquatic insects last week in this pleasant stream in Malibu, CA (left) I came across this little wriggler, commonly called the Horsehair worm but more fancily called Nematomorpha (right). I placed the worm next to a pen so you can get a better perspective, it comes in at over 13″ long. That’s one heck of a parasitic worm. Luckily for me, it is no parasite of humans, it prefers the much smaller prey. Here’s a video of me moving it around a bit:
These worms have an amazing story. They are endoparasites of arthropods, which means that they grow up feeding off the insides of things like crabs, crickets, cockroaches, and beetles. One particularly cool species is a grasshopper parasite. Once the worm has grown to its massive adult size, it infects the brain of the grasshopper, takes over some of its natural instincts, and ‘instructs’ the grasshopper to jump into water, an otherwise very unnatural behavior.
Once in the water, the grasshopper drowns, the worm escapes, and it goes on to live happily as a free-living adult in the stream/pond/puddle that the grasshopper jumped in to. There it meets other horsehair worms, mates, lays a goopy mass of eggs, and the story goes on.
Here’s where people can get scared though: sometimes, these worms end up in your toilet. This can cause an immediate alarm in people, thinking they might have some parasitic worm living in their intestine. Lucky for us, being infect with a horsehair worm is incredibly rare and - if you see one in your toilet it probably contributed to one less cockroach running around your house by parasitizing it. However, these worms being inside a human is not unheard of though are more likely caused by eating (intentionally or not) an infected insect.
That being said, if you do find a worm in your toilet I do recommend that not-so-lovely task of collecting it and bringing it to your doctor to make sure your insides are parasite-free.
As seen above, male horsehair worms have this forked end, whereas females have a straight round end.
These worms got their name from people in the olden days believing that they formed from horse hairs coming to life when they fell into a stream. As cool as that would be, I think their actual natural history story is even better.