A Brazilian wasp has evolved a very peculiar mind-control power in order to reproduce: It induces love in a species of caterpillar. The wasp lays its eggs in a baby caterpillar, which grows normally as the eggs grow inside it. Eventually, larvae burst out of the caterpillar's body, and that's when things get weird. The caterpillar covers the larvae with silk, and will protect them quite violently until they are full-grown wasps (you can see that in this picture). In fact, the caterpillar refuses to eat or leave until the wasps hatch.
A group of researchers observing this Brazilian insect drama in the wild say it's the first time they've been able to prove scientifically that parasites essentially mind-control their hosts to ensure the parasites' survival.
According to a release from PLoS One:
Inside the caterpillar host, a cruel drama takes place: the eggs of the parasitoid hatch and the larvae feed on the body fluids of the host. The caterpillar continues feeding, moving and growing like its unparasitized brothers and sisters. When the parasitoid larvae are full-grown, they emerge together through the host's skin, and start pupating nearby. Unlike many other combinations of host and parasitoid, the host remains alive but displays spectacular changes in its behaviour: it stops feeding and remains close to the parasitoid pupae. Moreover, it defends the parasitoid pupae against approaching predators with violent head-swings.
The caterpillar dies soon after the adult parasitoids emerge from their pupae, so there can be no benefit whatsoever for the caterpillars . . . The research team found that, in the field, parasitoid pupae which were guarded by caterpillars suffered half as much predation as those which had no bodyguard. Hence, the behavioural changes of the host result in increased survival of the parasitoids.
In other words, this caterpillar is made to love those wasps so much that it will protect them at all costs, including its own life. Now imagine if these researchers decided to figure out whether this wasp behavior mod could be ported to the human brain. A squirt of wasp juice could make you a super soldier, willing to give your life to protect whatever your "parasite" might be.
Parasitoid Increases Survival of Its Pupae By Inducing Host to Fight Predators [PLoS One via Science Daily] (Thanks, Brian!)