Suicidal Crickets

A hairworm can grow longer than the cricket that carries it, coiled up tight inside the insect's body. When the worm is ready to breed, it needs water — so it makes proteins that imitate the cricket's own brain chemicals and floods its nervous system with one urgent command: find water and jump.

The cricket leaps into a stream and drowns. The worm wriggles out within about a minute and goes looking for a mate. Oddly, only the cricket's water-seeking behavior changes; in every other way it acts completely normal.