During its life cycle Dicrocoelium dendriticum lives in three hosts—a snail, ant and a cow.
One of the most active hitchhikers has to be a lancet fluke, or flatworm, called Dicrocoelium dendriticum. During its life cycle, the parasite lives in three hosts. First, a snail eats cow dung rife with the worm’s eggs. The eggs hatch inside the snail, and in defense, the snail produces a slime that entraps the larvae. Eventually, the snail hocks a slimy, larvae-filled loogie. Then, an ant comes along and slurps up the slime. The parasites set up two outposts, one around the nerves that control the ant’s mandibles and another in its head. Here is where it gets tricky. The parasite needs to spend its adulthood in the liver of a cow, so it has to get a cow—an herbivore—to eat the ant it has infected. With a little mind control, the parasite gets the ant to crawl up to the top of a blade of grass each night and bite down to stay in place. This way, a cow is more likely to chomp on it while grazing. In the cow’s liver, adult worms reproduce, and the cow later defecates the eggs.