Why would a caterpillar protect wasp cocoons from invaders? The wasps (of the genus Glyptapanteles) are controlling the caterpillar. "Just before the wasp [larva] emerges from the caterpillar, it produces a toxin that causes the caterpillar to be a bodyguard" for the wasp larvae, explains David Hughes, a research fellow at the University of Exeter. "It doesn't allow the caterpillar to do all the other normal caterpillar stuff, like walk away" to escape, he adds.

Researchers are still not sure how the caterpillar is engaged in this final duty. "It's a behavior that's induced by a parasite that is not even in the host anymore," says Edward Levri, an associate professor of biology at The Pennsylvania State University. Other caterpillars have been forced to create cocoons for parasitic wasp larvae that emerge from them, he adds. "The caterpillar is almost dead, and then its last goal in life is to create a cocoonlike house" for the vulnerable wasp larvae. As he points out, "if [the larvae] aren't covered up by the web, another parasitoid will inject eggs into them," initiating another meta-level of parasitism.