Tuesday, 16 July, 2019

The tiny horror story of how wasps create “zombie” spiders to do their bidding


Zombies are real – at least, in the insect world. The most famous example is a fungus that controls the minds and bodies of ants to help itself reproduce, but it’s not the only thing to use the morbid tactic. Certain species of parasitic wasps have been found to “zombify” spiders, and now a new review paper has examined exactly how the insects pull off this feat.

It reads like a horror story: First, the wasp lays its eggs on a spider’s back. Soon after the larvae hatch, they apparently take control over the spider, forcing it to build an unusual type of web that acts like a cocoon for the larva. With the work done, the wasp young eat the hapless spider and then settle in to pupate, eventually emerging as adult wasps.

A parasitic wasp prepares to attack a spider, in order to "zombify" it

This same creepy behavior plays out with a number of different wasp and spider species, but the mechanism at work wasn’t clear. How does the wasp larva gain access to the spider’s brain? The new study set out to find an answer, by pulling together a review of previous reports and studies on these wasps, as well as new observations.

The team found evidence that the wasps are hijacking the spiders’ molting behavior. After all, the webs that the zombie spiders build for the wasps closely resemble those that healthy spiders build to protect themselves during the vulnerable process of molting. The researchers also noted that spiders that had just built cocoon webs had high levels of a hormone called ecdysone in their bodies, which is known to play a big role in the spider’s molting cycle.

The idea goes that the wasp larvae inject extra ecdysone into the creature, essentially tricking it into thinking it’s time to molt. In response the spider builds that special, protective type of web – ultimately becoming dinner before it gets a chance to use it itself.

After the web is built, the wasp larva eats the hapless spider

“Now that we have a proposed mechanism, we can ask a new set of questions,” says William Eberhard, co-author of the study. “Because the lines in spider webs represent precise records of their behavior, we could study ‘zombification’ in unprecedented detail by looking at the lines in cocoon and molting webs. We discovered that both web types vary, and more importantly, that the variations only overlap partially. The larvae probably tweak the spider’s molting web construction behavior to gain added protection. The mechanisms by which these additional modifications are obtained may result from differences in the timing or amounts of ecdysone, or modifications in the ecdysone molecules themselves, but they remain to be documented.”

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