Why Hairworms Are One Of Nature's Creepiest Creatures

A grasshopper walks calmly to the edge of a pond, hesitates for a second, and jumps in. It can't swim. It drowns within minutes. As its body sinks, a thin brown worm — longer than the grasshopper itself — uncoils out of the back end and swims away.

The worm is a horsehair worm, Spinochordodes tellinii, and it has been living inside the grasshopper's body cavity, eating it slowly from the inside, for most of the insect's adult life. To complete its life cycle it needs water. The grasshopper doesn't. So the worm makes the grasshopper want water.

Researchers in France isolated the proteins the worm releases just before its exit. The chemicals mimic insect neurotransmitters and act directly on the grasshopper's central nervous system, specifically the parts that respond to light reflecting off water. The infected insect doesn't feel possessed. It just suddenly, urgently, finds water beautiful.

Once in the pond, the worm reproduces, the eggs hatch into microscopic larvae, and the larvae are eaten by mosquito larvae. The mosquitoes grow up, fly out of the water, and are eaten by grasshoppers. The cycle restarts.

What makes the hairworm uncomfortable to look at is its size. A worm coiled inside a grasshopper can be three times the insect's body length, packed tight like a watch spring. Pull one out of a single host and it can stretch 30 centimetres long.

They're harmless to humans, but they live almost everywhere with fresh water — including, occasionally, in dog water bowls left outside. The worm doesn't bite, doesn't burrow, doesn't infect anyone bigger than a cricket. It just waits.

And somewhere right now, an insect is taking a walk it doesn't understand.