That's a disgustingly bizarre horsehair worm.
The Horsehair Worm. Pretty Gross Parasite, Super Cool Story.
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. Luckily, there is an easy way to distinguish our harmless horsehair worms from the potentially nasty human parasite. Horsehair worms have a distinctly clefted/forked rear end, as seen in the picture below, while human parasites have a hooked/blunt end.
So, if that worm in your toilet has a forked end you’re likely in luck, it probably contributed to one less cockroach running around your house by parasitizing it. However, if there is no forked end, I’d recommend the not-so-fun task of putting it in a jar and visiting your doctor.
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.