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When parasites take over their hosts, they can change their hosts’ behavior. The effects can range from simple aggressiveness to losing their ability to make decisions.

But sometimes it gets very weird…. And scientists just observed a new parasitic behavior between a spider and a wasp, where the spider re-learned an ancestral behavior!. In a study published in 2018, researchers described parasitism by Zatypota wasps in Ecuador that were causing some never-before-seen behaviors in a species of social spider, Anelosimus eximius.

This particular spider spends its life in a large, messy, communally built web that contains thousands of other spiders. In fact, this spider only builds a web as a group, it does not build solitary webs. And they almost never leave their home webs.

Researchers have actually identified the gene that’s associated with leaving their colony, and it turns out it’s been suppressed; just turned off. After all, the colony’s protection is very important for these little spiders, because there are many predators in the rainforests they call home. The only time they might venture out of the communal web is as an adult female, setting out to lay eggs and expand the colony.

And even then, she might take some of her original colony with her as protection. Except, that is, when these spiders are parasitized by a Zatypota wasp. These wasps are known for targeting spiders that make large, messy webs.

That’s because these types of webs come in handy for their larva’s development. Zatypota wasps lay eggs on a spider, so that when they hatch, the larva can feed on the hemolymph, a spider’s equivalent of blood. And the larva force the spider to modify its behavior and the type of web it builds.

When the spiders are parasitized, they wander away from the colony. Yes - the spider goes away, on its own, and builds a cocoon. Even though evolution suppressed these behaviors long ago.

After they wander away, they spin this protective cocoon. Then the larva eats its spider host and then tucks itself away in the cocoon, emerging as a full-grown wasp 9 to 11 days later. The researchers hypothesize that the wasp larva inject a hormone, known as an ecdysteroid, into the spider’s hemolymph as the larva feeds.

Spiders produce this hormone before they molt, and it has been shown to change their web building behavior. But cocoon building isn’t part of molting; that only happens when the wasps control them. Wasps produce this same hormone, to use for their own molt cycles.

So the Zatypota wasp larva may be capitalizing on this hormone to awaken a long-buried behavior in social spiders. Not only are these spiders a food source for the larva, but they’re essentially being zombified to do the larva’s bidding. And by targeting social spiders, the wasps have a constant stream of hosts, the same researchers found that a large social spider community has many parasitized individuals.

By manipulating a hormone already present in both creatures, these wasps have evolved a way to ensure their own survival, by changing the forgotten behavior of their unlucky spider hosts. Thanks for watching this episode of SciShow! We’ve made thousands of educational videos over the years, and we’ve been able to do that because of our patrons on Patreon.

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