We’re all at the mercy of parasites. You, me, your dog or cat or even pet rabbit—if you’re one of those people. But rather mercifully, parasites like the tapeworms that invade our guts don’t typically kill their hosts, on account of that meaning the end of their food and shelter. One group of parasites known as parasitoids, though, shows no such mercy. They invade their hosts, use them, and unceremoniously dispatch them when they’ve had their fill.

Few parasitoids are more bizarre or disturbing than the wasps of the genus Glyptapanteles, whose females inject their eggs into living caterpillars. There, the larvae mature, feeding on the caterpillar’s fluids before gnawing through its skin en masse and emerging into the light of day. Despite the trauma, not only does the caterpillar survive—initially at least—but the larvae mind-control it, turning their host into a bodyguard that protects them as they spin their cocoons and finish maturing. The caterpillar eventually starves to death, but only after the tiny wasps emerge from their cocoons and fly away.

Because he has awesome ideas and not because he’s some kind of sadist, ecologist Arne Janssen of the University of Amsterdam brought this remarkable lifecycle into the lab a few years back to study it. What he and his colleagues confirmed for the first time is that not only do Glyptapanteles larvae actively manipulate the behavior of their hosts, but by transforming caterpillars into bodyguards, they greatly boost their chances of survival compared to their unprotected comrades.

A caterpillar stands guard over the Glyptapanteles wasps that erupted from its body. That's a bit like you—OK there's really nothing in the world to compare this to. Ton Rulkens/Flickr

It goes a little something like this. A female Glyptapanteles wasp pounces on a caterpillar, drilling into its flesh with what is known as an ovipositor (literally, “egg placer”), and pumps up to 80 eggs into its body cavity, according to Janssen. When the eggs hatch into larvae, they begin feeding on the caterpillar’s bodily juices, taking care to avoid attacking vital organs—somewhat of a rarity for parasitoids.

“Most parasitoids eat the host completely empty,” said Janssen. “The Glyptapanteles don't do that. We don't know exactly why, but one of the reasons may be that if you kill the host it cannot defend you afterwards.”

Inside the caterpillar, the larvae will go through several stages, or molts, to shed their exoskeletons as they expand. During all of this, the caterpillar, which grows more and more bloated as the larvae mature, isn’t yet showing any signs of being manipulated. Incredibly, you can’t even tell it’s behaving any differently, even as it swells to the point where it looks like it’s going to burst, like a can of soda in a freezer ... that's filled with parasitic larvae instead of soda, I guess.

Inevitably, though, the larvae must make their exit. All 80 at once. Over the course of an hour. They release chemicals that paralyze the caterpillar, then each individual begins gnawing its way out. It’s a horrific happening, as you can see in the amazing National Geographic video below, yet keep in mind that the caterpillar survives this incredible trauma.

How? Well, it’s thought that the larvae time their final molt to coincide with the exit, so as they squeeze through the caterpillar’s skin, the exoskeleton they leave behind blocks the exit hole. Thus they perform their own slapdash surgery on their gravely wounded host.

If You'll Be My Bodyguard, I Can Be Your Long-Lost Pal

As the larvae congregate in a mass and begin spinning their cocoons, the caterpillar snaps out of it and helps them, using its own silk to construct a protective covering. And you can imagine it has somewhat conflicted feelings about all of this, much like Kevin Costner's emotional struggles in The Bodyguard.

Once everyone is done spinning, the caterpillar switches into defense mode, lashing out at not only predatory insects, but other wasps known as hyperparasitoids. The Glyptapanteles pupae (the final stage before they complete their development), you see, don’t have it so easy. In a nice little bit of poetic justice, these hyperparasitoids will inject their own eggs into Glyptapanteles.

But not if the caterpillar has anything to do with it. The bodyguard doesn’t wander, and it doesn’t eat. It dutifully stands sentry over the pupae, rearing up on its hindmost legs and violently lashing out with swift swings of its head at anything that approaches. “And we've also seen that occasionally they take predators into their mouthparts and just throw them away,” said Janssen. Not exactly what you’d call normal behavior for a placid vegetarian. (Though there really are some incredibly vicious carnivorous caterpillars out there.)

What Janssen found is that when he removed the caterpillar and left the pupae to fend for themselves, twice as many fell prey to either predators or hyperparasitoids. It would seem, then, that Glyptapanteles has evolved this behavior to boost its chances of survival. Interestingly, though, the caterpillar itself attracts predators that can also opportunistically attack the pupae.

"This suggests that there may also be costs involved with the behavioral changes in the caterpillar: Behavioral changes might attract some predators against which the caterpillar cannot defend the parasitoid pupae," Janssen and his colleagues wrote in a paper. "Nevertheless, the overall effect of caterpillar presence on survival of parasitoid pupae was positive.”

But one big question remains: How on Earth are the pupae able to mind-control the caterpillar after they’ve left its body? Given the long span between the injection of the eggs and the behavioral changes, Janssen reckons it couldn’t be the initial sting. And it probably isn’t the larvae exiting the caterpillar either, because “the caterpillars do not respond strongly to disturbance during egression, but only one to two hours after the event,” he wrote.

Janssen and his colleagues may have found their answer when they dissected caterpillars that had given painful birth to larvae three to four days before. Remarkably, they found one or two larvae still hanging out inside. It could well be that they were staying behind to mind-control the host with some kind of cocktail of chemicals in order to protect their siblings, which “would represent a cost of host manipulation: some offspring are sacrificed for higher survival of their kin,” Janssen wrote.

So it seems that this horror story could actually be a rather touching … well, horror story still. But as the tiny wasps hatch and flutter away, and as the exhausted caterpillar wavers and collapses dead of starvation, we must remember the larvae that we lost, the brave souls that stayed behind and gave their lives so their siblings might live.

And with that, I’ve written the next Jerry Bruckheimer movie. We’ll need to work in some explosions, though.

Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.

Reference:

Grosman AH, Janssen A, de Brito EF, Cordeiro EG, Colares F, et al. (2008) Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators. PLoS ONE 3(6): e2276. doi:10.1371/journal.pone.0002276