A great, full-body-squick-inducing article in National Geographic provides an overview of the current research on parasites that use a combination of techniques to control their hosts' behavior, making them sacrifice themselves for the sake of the parasites and their offspring.
Out of the whole (very fascinating) bunch, the coolest is undoubtedly the Dinocampus coccinellae wasp, which lays its eggs under ladybugs' shells, then, after the larvae have hatched out, the wasp's sting somehow paralyzes the ladybug to stand guard over its progeny, killing any predators that approach the gestating pupa. Once the wasp has crawled free of its cocoon, the adult wasps tunnel through their guards' bodies and fly away, leaving the dying zombie ladybug behind.
But it turns out that the wasp itself is acting on behalf of another parasite:
And what of D. coccinellae and its hapless ladybug host? While at the University of Montreal, Fanny Maure and her colleagues made a startling discovery: In turning its victim into a willing bodyguard, the wasp itself may only be acting as the extended phenotype of yet another organism. The researchers found that when a wasp injects an egg into a ladybug victim, she also injects a cocktail of chemicals and other substances—including a virus that replicates in the wasp's ovaries. Some evidence suggests it is this virus that immobilizes the ladybug, protecting the wasp's cocoon from intruders.
The virus and the wasp have the same evolutionary interests; turning a ladybug into a bodyguard produces more wasps, and more wasps beget more viruses. And so their genes work together to make the ladybug their puppet. The D. coccinellae wasp may not be the puppet master it once seemed. Instead it hides another puppet master within.
Mindsuckers [Carl Zimmer/National Geographic]
(Photo: Anand Varma)