The snail parasite Leucochloridium is a worm that eats the internal organs of snails, but it must complete its life cycle by breeding in the intestines of birds. Snails that are infected crawl out on leaves during the day, and the parasite pulses in their eye stalks in a way that resembles a caterpillar moving. Birds attack and try to eat the caterpillar. This bizarre adaptation likely evolved through selection to reduce the number of worms in an individual snail, increase the probability that infected snails will be removed from the host population, reduce the virulence of the parasite which can sometimes castrate the host, increase the transmission rate among host snails, and reduce the density of the host population below the threshold value necessary for the parasite to spread.