The Dicrocoelium dendriticum

This tremadote parasite spends the adult stage of its life cycle in the liver of cows and sheep. The eggs exit with the feces of the mammalian host and are eaten by land snails, which serve as hosts for an asexual stage of the parasite life cycle. Two generations are spent within the snail before the parasite forms yet another stage, the cercaria, which exits the snail enveloped in a mucus mass that is ingested by ants. About fifty cercariae enter the ant along with its meal. Once inside, the parasite bore through the stomach wall and one of them migrates to the brain of the ant (the subesophagal ganglion), where it forms a thin-walled cyst known as the brain worm. The other cercariae form thick-walled cysts. The brain worm changes the behavior of the ant, causing it to spend large amounts of time on the tips of grass blades. Here the ant is more likely to be eaten by livestock, in whose bodies parasites may continue their life cycle.

The brain worm loses its ability to infect the mammalian host. It sacrifices its life and thereby helps to complete the life cycle of other parasites in its group.