Q. I am totally freaked out by this thing that I found in my toilet! It looks like a very long worm, or worms, I can’t tell since it’s sort of a mass, and it moves. I managed to put it in a jar because I’m worried that it’s a tapeworm or something that came out of one of us! Have you ever heard about anything like this?

A. We don’t hear about this worm very often, but when we do, people are pretty panicked. Don’t be. If your worm matches this description, it is probably a horsehair worm, also called a Gordian worm, and it’s harmless. These blackish worms are very thin (like a horse’s hair!) but can be more than a foot in length. They’re usually found knotted up in a loose ball which is how they got the name, Gordian worm. Supposedly, the ball resembles one created by King Gordius in a Greek myth.

Horsehair worms are not insects or tapeworms; they are semiaquatic roundworms that are related to nematodes. This worm didn’t come from a human or pet and it doesn’t parasitize humans or pets. It parasitizes insects and no doubt came out of the body of a cricket or other insect that ended up in your house.

Outside, horsehair worms are often found in water sources such as ponds, rain puddles, swimming pools, and drinking troughs. They got their name because of an old belief that they developed from horsehairs that feel into horse watering troughs and came alive! Occasionally, they’re found indoors in sinks, toilets, tubs, or pet water bowls.

Here’s how that horsehair worm ended up in your toilet. A female horsehair worm deposits a string of eggs in water. The parasitic larvae that hatch need to complete their development inside the body of a relatively large insect. The larvae are ingested by a cricket, cockroach, beetle, or other insect that can then find its way into your home. When that insect gets near water or even a wet area, the mature parasitic worm suddenly emerges from its body, hoping to continue its aquatic life. The worm is whitish when inside the insect’s body but turns dark after emerging. It’s unusual, but your host insect must have been in or on the toilet when the worm emerged from its body. If you think about it, you may remember seeing the dead or dying host insect in the toilet or nearby.

Horsehair worms are not a common occurrence and it’s rare to find more than one horsehair worm in a home. They’re more common in homes in the fall when crickets and other insects tend to move indoors. Horsehair worms do not parasitize humans or pets, so if one was accidentally swallowed, it would pass through the body without harm.