How worms trick grasshoppers into leaping to their doom

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 alter the insect's central nervous system, confusing its response to light and gravity, which compels the host to seek out the polarized light reflecting off water surfaces. The infected insect doesn't feel possessed. It just suddenly, urgently, finds water attractive.

Once in the pond, the worm reproduces, the eggs hatch into microscopic larvae, and the larvae are ingested by small aquatic insects or form cysts on vegetation near the water, which are then accidentally eaten by the grasshopper as it forages. 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.