Walking Dead: How Wasp Overlords Control Spider Zombies
A wasp larva kills its spider host once the host has completed its web-building tasks.
Credit: Keizo Takasuka

Like a mindless zombie controlled by a menacing overlord, the spider scampers back and forth, reinforcing its silky web. Not long from now, the subservient arachnid will be dead, its web transformed into a shelter for the spawn of the creature that once controlled it, according to a new study.

No, this isn't science fiction; it's the somewhat terrifying (but very real) tale of the orb-weaving spider Cyclosa argenteoalba and the parasitic wasp Reclinervellus nielseni, two species that carry out a strange relationship in Hyogo prefecture, Japan.

Together, the wasp and the spider provide a perfect example of host manipulation — an ecological process in which one species (the parasite) and its young (the parasitoids) manipulate the behaviors of another species (the host) to their advantage. [Zombie Animals: 5 Real-Life Cases of Body-Snatching]

Just how a parasite turns its host into a zombielike slave varies from species to species, and sometimes, researchers aren't sure what the mechanism is that makes a host do its parasite's bidding. That's the case for the orb-weaving spider and parasitic wasp of Japan. Researchers in that country want to find out how R. nielseni controls C. argenteoalba. Does it use a neurotoxin, or perhaps some kind of hormone?

But to solve that mystery, scientists first need to answer another question: What, exactly, does the wasp make the spider do?

Walking dead

The manipulative relationship between the wasp and the spider begins when a female wasp attacks the orb weaver in its web. She deposits her egg onto the back of the spider's abdomen but doesn't kill it. Firmly attached to the spider, the egg develops into a larva, which eventually does kill its host, but not before the spider serves it as a slave throughout the early stages of development, said Keizo Takasuka, a postdoctoral fellow at Kobe University's Graduate School of Agricultural Science in Japan and lead author of a new study exploring the relationship between R. nielseni and the orb weaver. [Watch the Zombie Slave Spider Do the Wasp's Bidding (Video)

A cocoon web, finally completed by the host spider and occupied by the parasitoid wasp.
A cocoon web, finally completed by the host spider and occupied by the parasitoid wasp.
Credit: Keizo Takasuka

Over the past several years, Takasuka has headed to the Shinto shrines of Hyogo prefecture to collect spiders enslaved by the parasitic larvae of R. nielseni.

"I looked for already-parasitized spiders in shrines … because the spiders prefer to construct webs particularly in artificial structures and stone materials," Takasuka told Live Science in an email. He's not sure why the spiders prefer the shrines, but he said these arachnids can also be found in other habitats.

In the lab, Takasuka and his colleagues observed the behaviors of the parasitized spiders — mainly the precise way in which the arachnids built their webs — and then compared this behavior with that of orb-weaving spiders that weren't controlled by parasitoids.

The zombie slave spiders tended to build a particular kind of web, one that was quite different from the webs created by parasitoid-free spiders, the researchers found. First, the parasite-ridden spiders took apart their old webs (some even abandoned them altogether), and then they started building new ones that resembled the web an orb weaver would build if it were about to molt, or shed its exoskeleton (something spiders do in order to grow).

Rest in peace

Known as a "resting" web, the pre-molting web is distinct from the spiral-shaped web the spider usually weaves to catch prey. When molting, the spider is soft-bodied, vulnerable and unable to eat. So it stays huddled in the center of the resting web, which has no "capture" areas to snag prey but is instead outfitted with fibrous thread decorations (FTDs), which are strands of silk meant to make the web stand out. [Goliath Birdeater: Images of a Colossal Spider]

You might think that spiders would want to keep their webs inconspicuous, but a molting spider's web is under constant threat from flying birds and other, larger animals. If the web is visible to these animals, they will be less likely to crash into it, and the spider will be more likely to survive the molting process. With that in mind, the spider adorns its home with extra strands of ultraviolet (UV) light-reflecting silk, which passersby are not likely to miss.

The resting web, a safe haven during times of transformation, is the perfect place for a wasp larva to transition into the pupal phase (the stage of transformation in which the insect envelopes itself in a cocoon). An orb weaver's resting web can keep its occupant safe for about two days, which is how long it typically takes the spider to molt. But a web that lasts only two days isn't going to cut it for R. nielseni, which needs to remain ensconced in the spider's web for at least 10 days once it has wrapped itself up in a cocoon.

"[The] cocoon web has to endure falling debris, the elements and animal strikes for a long time — at least four to five times longer than [a] resting web," Takasuka said.

That's why R. nielseni doesn't just direct its host to build a resting web; it instructs the spider to build a superstrong resting web, one chock-full of reinforced threads that hold the web — and the wasp-filled cocoon at its center — in place for long stretches of time, the researchers found.

Using a tensile machine, Takasuka and his colleagues tested the breaking forces (how much force a material can handle before breaking) of the radius and frame silks used to construct a so-called "cocoon" web and found that they were at least 2.7 times greater than the breaking forces of the silks that made up both the orb and the resting webs of C. argenteoalba.

Horrifying hormones

When a zombie spider is finished doing its parasitoid's bidding, it returns to the center of the web, but its ordeal is far from over. With its UV light-reflecting, reinforced shelter in place, the wasp larva no longer needs the spider, so it slaughters it. After chucking the spider's corpse off the web, the larva spins itself a comfy cocoon and hunkers down for nearly two weeks to complete its metamorphosis.

The parasitic wasp's ability to manipulate its host in such a specific and subtle way is not unique. In Costa Rica, another parasitic wasp, Hymenoepimecis argyraphaga, ups the horror by depositing its eggs inside of its host arachnid (Plesiometa argyra), which builds a cocoon-worthy web before being consumed from the inside out by larvae.

And, in Brazil (as well as other countries), there are fungi that infect many species of ants, turning these insects into a host of zombies. The ants climb to the highest point they can find and then die as fungal stalks shoot through their skulls, dispersing the fungus' spores into the wind.

In the case of the fungi-entranced ants, scientists know that the fungi actually release a cocktail of chemicals into the ants' brains, inducing them to do the fungi's bidding. But entomologists are still actively studying the ways that wasps and other insect parasites might control their hosts.

Takasuka suspects that, in the case of R. nielseni and C. argenteoalba, the mechanism controlling the spider's web-strengthening preferences is somehow related to the hormone that is naturally released in the spider just before molting. This hormone is what motivates the spider to start building a resting nest. In the near future, Takasuka hopes to study the chemicals present in the larvae to determine how those chemicals might be related to the resting-web hormone and others.

The researchers' study was published today (Aug. 5) in The Journal of Experimental Biology.

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