Wild Things

Nature’s Most Delightfully Depraved Parasites

Zombie enthusiasts in Berlin in 2012.

Photo by Sean Gallup/Getty Images

Somewhere in Japan, a wasp swoops onto a spider and jabs it with a stinger. The wasp’s venom paralyzes the spider for 10 to 15 minutes, during which the wasp secretes a tiny egg.

By the time the spider wakes up, it is doomed.

Soon, a larva will hatch from the egg and immediately begin siphoning juices out of the hapless spider. The larva will grow quickly, never once releasing its death grip.

To put this in perspective, imagine carrying a blood-sucking maggot on your back that grows to the size of a carnival teddy bear.

And oh, by the way, that teddy bear has a command for you. It’s just one task, but you have to do it for the rest of your life.  

Orb weaver spiders parasitized by this species of wasp, Reclinervellus nielseni, are basically turned into zombies. To add to the indignity, they are forced to embark upon an Extreme Makeover: Web Edition.

First, the spider removes the sticky spiral section of its web that it uses to catch food. Then it disassembles the struts that radiate out from the web’s center until only a few support beams remain. The zombie spidey then reinforces what’s left—its spinneret churning out layer after layer of silk like a hormonal teen etching his beloved’s name in a desk during study hall.

When the remodeling is finished, the larva sucks the last sip of suds from its host, crunches up its carcass like an empty can of beer, and tosses the spider unceremoniously into the weeds. The video below has no sound, or we’d no doubt hear the ungrateful larva belch before weaving its cocoon and hunkering down for metamorphosis.

I think it’s safe to say that we’re living in the age of peak zombie. Elijah Wood, Arnold Schwarzenegger, and even the Terminator’s son Patrick Schwarzenegger have zombie flicks coming out this year or next. Pride & Prejudice & Zombies is getting a movie, and World War Z is getting a sequel. This month, AMC is premiering a spinoff of The Walking Dead, which is itself still running.

In most of these pop culture depictions, zombies are chaotic, mindless killing machines. They don’t think, they don’t feel, they merely chase, claw, and chomp until the credits come to save us.

But nature is also full of zombies—slightly more nuanced zombies—and we’re learning more bizarre things about them all the time.

We’ve known about the zombie spiders for more than a decade, but new research just published in the Journal of Experimental Biology is the first to show that the web the larva forces the spider to build has superior tensile strength.

As to why the larva should require such a bunkerish web, the study’s lead author, Keizo Takasuka, an ecologist at Kobe University in Japan, has a few ideas.

The webs the zombified spiders create are actually very similar to those that healthy, nonpossessed spiders make just before they molt. (Molting occurs when spiders and other arthropods grow too big for their exoskeletons and have to do the fat-guy-wiggling-out-of-a-little-coat dance.)

However, a molting spider requires just 48 hours to shrug out of its skin, whereas the wasp larva needs more than 10 days to make its transformation to an adult. During that time, the wasp and its web are vulnerable to falling debris, wind, rain, and many other kinds of damage. A super-strong web gives the pupating wasp the best chance of moving on to its next stage of life. The spiders even adorn their webs with a few cottony patches of ultraviolet-reflective web so that birds and big insects can see the structure and won’t run into it.

This sort of mindhack plays out across many different species of spiders and wasps.

Tommy Leung of the University of New England—who runs an excellent blog called Parasite of the Day—says real-life zombies are more complicated than the ones we see on the screen. “It’s not so much that the hosts become zombies in the classical sense of being mindless, shuffling husks, but that they are acting under the influence,” says Leung.

Put another way, it’s as if hosts come preprogrammed with certain behaviors, and the parasites have learned that it’s easier to hijack those programs rather than write a bunch of new lines of code.

For example, each autumn, Japanese yellow hornet queens—the ones that decapitate honeybees—go out in search of new nests. When they find the perfect one, say in a nice decaying log, the hornets carve out a little hole and start ordering cute prints from Etsy.

However, Leung has written about a parasitic nematode that can make these queens think it’s autumn when it’s actually spring. The queens then fly from potential nest to potential nest, stopping just long enough to let off a load of nematodes. When true autumn rolls around and nonparasitized hornets show up to start making nests, the nematodes are already sitting at the dinner table, clutching forks and knives, ready to infest any eggs she lays.

Does the parasite make its host do anything crazy? Nope, the nematode just rearranges the hornet’s day planner to better fit its own schedule. (Oh, and uses the hornet like an Uber car while sterilizing the queen in the process.)

Elsewhere in the animal kingdom, there are caterpillars who like to climb up and down trees every day. Parasitic viruses simply disable the “down” button, the better to position the caterpillars for when they turn into exploding sacs of infectious viral particles.

In another example of zombification, ants leave their colony when sick, which is thought to protect their nest-mates from infections. However, there’s a fungus that uses this script for its advantage, commandeering the ill ant’s self-sacrifice code and guiding the insect high up onto a branch. The ant’s carcass then sprouts a stalk of death spores and rains them down on all the other ants from above.

But my favorite example is that of the crab-castrating barnacle.

Now I’m aware that the barnacle, as an animal, is not high on the list of most human’s interests. (Darwin, though—he loved a good barnacle.) Most barnacles are sedentary, dully colored, and well, kind of pathetic.  

But Sacculina carcini and its ilk are not your average barnacles. As larvae, the females float around searching for a suitable host, which they then inject themselves into and eviscerate with long, probing roots. If the host crab is unlucky enough to be male, his genitals are replaced with the barnacle’s own reproductive organ.

After awhile, a free-floating male barnacle comes along and fertilizes the Master Blaster hybrid of female barnacle and castrated crab, and the parasite begins to produce thousands of larvae. It keeps these in big white sacs where the crab would normally harbor its own young.

So where does the zombie part come in? Regardless of which sex the crab was before the barnacle took up residence, it will now behave like a mother—to the parasite’s brood.

The crab wafts fresh, oxygenated water over the parasite’s developing larvae, keeps them clean, and protects them from predators. The crab stops eating and, possessing no genitals of its own, abandons any attempts at passing on its own genes.

The study of how this sort of possession works on the molecular level is, as Leung says, very much in its infancy. The wasps may be keying in on a hormone that tells the spiders its time to molt, but myriad other proteins and molecules may do the dirty work in other species. In fact, some parasites manipulate their host’s behavior simply by showing up.

Leung says he spent several years studying a fluke called Curtuteria australis, a parasite that likes to burrow into the foot of New Zealand clams. Now, one fluke in your foot is no big deal if you’re a clam, but these flukes love company.

“As the clam gets infected with more and more of the parasites, the foot becomes something less like a muscular organ and more like a bag of parasite cysts,” says Leung.

Eventually, the fluke-ridden foot becomes useless, and the clam can no longer dig itself into the sand. This makes it more likely that the clam will be found and devoured by a hungry bird, which, it just so happens, is exactly what the fluke requires to complete its life cycle.

Now, you could look at all of this and start to feel bad for the billions of hosts who have their minds warped each day, all those exploding caterpillars and hijacked hornets. But I think that’s the wrong takeaway.

Instead, I feel like we should be raising a glass to the parasites—nature’s original social engineers. After all, what evolutionary trick could be more impressive than the ability to pirate another species’ evolutionary tricks?