♪ (narrator) The day starts normally enough.

You give your pet some food and water, but later, in your pet's water dish, you find this-- a hairworm.

It didn't get here on its own.

It came out of a little cricket.

Don't believe me?

Okay.

These hairworms are gnarly parasites.

They actually control a cricket's mind to get to their home, the water.

The hairworm's journey starts innocently enough in a river as an egg, one of many in this long string.

The eggs grow into squiggly larvae, which get eaten by a mayfly larva that also lives in the river, and inside the mayfly is exactly where the hairworm needs to be.

The hairworm uses this pointy part to burrow into the mayfly's flesh.

Then it curls up and waits because, really, it's not after a mayfly.

It's after a cricket... [chirping] ...so it sits tight while the mayfly larva turns into an adult and heads to dry land where it just might get eaten by a cricket that has no idea what it's in for.

Inside the cricket, the hairworm goes at it, eating all the cricket's stored-up fat for about a month.

The cricket loses its chirp, but the hairworm doesn't kill the cricket because the worm needs a lift back to the water.

Crickets usually avoid bodies of water.

They're not great swimmers, so the worm takes over, boosting chemicals in the cricket's brain, which make the cricket walk around mindlessly until it happens to reach water.

Scientists in France watched this infected cricket make a beeline for the pool.

The hairworm makes a break for it.

Still going.

Ugh, that's just--ugh.

But don't worry.

They don't target humans.

Ready for more?

This one at the University of New Mexico has a whole lot of hairworms inside it.

They don't waste any time, curling around each other to mate even before they're fully outside the cricket.

But it's more than a gruesome spectacle of nature.

Learning about these hairworms could help scientists understand parasites like toxoplasma that make us very sick.

As for the crickets, don't feel bad.

If they don't drown, most of them survive their ordeal.

At least, that's what scientists have seen in the lab.

They go back to being crickets and hopefully stay on dry land.

Hi, it's Lauren.

Who's hungry for more after that?

We'd love to make more videos of amazing critters up close all year-round, but we need your help.

We're a member-supported PBS show from KQED in San Francisco.

That means, to grow, we need you-- our YouTube fans-- to support us on Patreon.

Are you in?

Link is in the description.

Thanks.