Wasp Forces Zombie Spiders to Enhance Cocoons for Young

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The larva of the parasitic ichneumon wasp clings to its host spider shortly before eating it. Keizo Takasuka

Many insects make cocoons when they are young to protect themselves from the big, bad world, full of predators (and cable news channels). Ichneumon wasps have a found a way to make this process easier and safer. Their larvae grow up clinging onto the bodies of spiders, and program the arachnids to make extra-sturdy webs, ideal for holding up the cocoons the larvae will eventually spin. After growing larger than the spider, which dutifully lays the groundwork for the wasp's next stage of life, the larva devours the arachnid and weaves a cocoon.

New research sheds light on how the wasp accomplishes this unusual feat. In a study published August 5 in The Journal of Experimental Biology, Japanese scientist Keizo Takasuka and colleagues have shown that the spider’s cocoon-bearing web, within which the wasp larva develops, is not dissimilar in structure to its own “resting web,” which the arachnid crafts for itself to shelter in before it molts, or sheds its skin to become larger.

In other words, the wasp has not figured out how to induce a completely new spider behavior, but rather to change it slightly to benefit itself. But there are important differences between these resting webs and the cocoon-containing webs the zombie spiders build: The latter are three to 40 times stronger and more reflective, perhaps making them less likely to be run into and accidentally destroyed by flying insects or other animals, Takasuka says. Both increase the likelihood that the lil’ wasp larva will have a safe place to develop after snacking on spider.

The exact mechanism by which the wasp elicits this behavior isn’t known, but experts suspect that the larva secretes a hormone that tells the spider it's time to molt, or mimics the hormone the spiders produce before molting, says Gavin Broad, a senior curator at the Natural History Museum in London.

“This discovery—of enhanced behavior as opposed to merely switched behavior—is completely new, impressively demonstrated, and rather unexpected I think,” says Mark Shaw, an expert on ichneumon wasps at the National Museum of Scotland, who like Broad was also not involved in this study.  

Here is a video of the ichneumon wasp (Reclinervellus nielseni) larva eating the spider, after the spider has done its bidding and created an extra-strong web for its cocoon:


Here is the larva, slightly smaller, piggybacking on the spider as it builds its highly reflective cocoon web: 

And here is a video of the zombie spider altering its usual web to accommodate the cocoon, making it more streamlined and stronger: