writes: "The New York Times has an article about a bizarre Costa Rican spider parasite. This tiny wasp larva forces its host, an orb spider, to do its bidding before killing it. Instead of building a normal round web, the spider spends its last night stringing together a frame. The larva then kills the spider and uses the frame to support its cocoon. The scientist who discovered the behavior still doesn't know how the parasite does it."
Since this is an older article, there's probably some more recent information available about this critter.