Cordyceps is a genus of parasitic fungi that invade insects such as ants, spiders and caterpillars. The fungus infects the brain of their host, modifying its behaviour. The insect is suddenly driven to climb plants so that the maximum amount of fungal spores can be distributed. The insect will eventually die as the fungus erupts out of its body, causing some pretty dramatic displays.
(Source: pixie-sabre, via smurfberries)