In the forests of Costa Rica, there lurks a sinister variety of wasp, bent on hijacking the minds of hapless spiders for its own ends. Left unmolested, a variety of orb spider known as Plesiometa argyra spends every day of its life carefully reconstructing its perfectly round web, and feasting on the insects unfortunate enough to become snagged upon it. But should one of these spiders fall victim to this as-yet-unnamed species of wasp, the spider is stripped of its free will, and made to spend the last evening of its existence building a protective shelter for the larvae that infect it.

It is a true example of mind control in nature, and though scientists are well aware of the method of infection, they are uncertain exactly how the mind control is accomplished. When a wasp successfully attacks a host spider, the spider is temporarily paralyzed as the wasp lays eggs on the tip of the spider's abdomen. Once the wasp departs, the spider regains its ability to move, and it continues its daily web construction for the next two weeks as though nothing has changed. Meanwhile, the wasp's growing larvae cling to the spider's belly and feed on its juices through a number of small punctures.

On the night before the parasites kill their host, events take a bizarre turn. Through some unknown mechanism, the larvae compel their host spider to build a web that is very different from that it has always constructed before. Instead of a flat, round web, the spider builds a stout, reinforced platform which is much smaller. Once the new web is complete, the larvae kill their host, and cocoon themselves on the structure. It is ideal for the task, being resistant to wind and rain, and safe from the ants that inhabit the forest floor.

This behavior was first observed by Dr. William G. Eberhard at the university of Costa Rica. His observations have led him to believe that the mind control is most likely accomplished through a fast-acting chemical secreted by the larvae, but what that chemical is-- and how it works-- is a mystery. What he has found is that the spider's usual five-step web building process is reduced to two when held captive by these larvae, resulting in the alternate design; and he has also discovered that if he removes the larvae on the last day, just before the spider is killed, the spider will often recover after a few days of spinning the abnormal web.

It is true that many parasites are able to shape their host's behavior subtly, but never before has science observed a parasite that can manipulate its host in such a detailed, specific way. As evidenced by this finding, biology certainly has many fascinating secrets yet to be discovered.

Written by Alan Bellows, posted on 07 September 2005. Alan is the founder/designer/head writer/managing editor of Damn Interesting.
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