(Describer) Titles: KQED. PBS.

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(Describer) Against a white background, brown organisms like half-cooked strands of spaghetti move around. Title: Deep Look.

(Describer) Someone walks down steps outside and sets down two bowls.

(narrator) The day starts normally enough. You give your pet some food and water, but later,

(Describer) One of the strands moves in a bowl.

in your pet's water dish, you find this-- a hairworm.

(Describer) Its end is rounded. It slides along the inside of the bowl just above the water surface.

It didn't get here on its own. It came out of a little cricket. Don't believe me? Okay.

(Describer) Hairworms crawl out of one.

These hairworms are gnarly parasites. They actually control a cricket's mind to get to their home, the water.

(Describer) A few of them pull out of the cricket's back end.

The hairworm's journey starts innocently enough in a river as an egg,

(Describer) A white string falls.

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.

(Describer) The pointy part sticks out under a microscope.

Then it curls up and waits

(Describer) The mayfly sits on water.

because, really, it's not after a mayfly. It's after a cricket...

[chirping]

(Describer) One vibrates its wings in grass.

...so it sits tight while the mayfly larva turns into an adult and heads to dry land

(Describer) Some mayflies take off.

where it just might get eaten by a cricket that has no idea what it's in for.

(Describer) A cricket eats.

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.

(Describer) A stream flows over rocks.

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.

(Describer) The cricket goes to a swimming pool and drops in. It's labelled as a wood cricket: Nemobius sylvestris.

The hairworm makes a break for it.

(Describer) As the cricket flails, the hairworm emerges from the back end.

Still going.

(Describer) More of it comes out.

Ugh, that's just--ugh.

(Describer) Much longer than the cricket, it keeps swimming back and forth, pulling itself out.

But don't worry.

(Describer) It breaks free.

They don't target humans.

(Describer) It swims on its own.

Ready for more?

(Describer) In a dish of water...

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.

(Describer) One wraps around two others as they keep pulling out.

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.

(Describer) One stretches out longer.

As for the crickets, don't feel bad. If they don't drown, most of them survive their ordeal.

(Describer) One walks off concrete.

At least, that's what scientists have seen in the lab. They go back to being crickets and hopefully stay on dry land.