Following is a great illustration from the book, “King Me” by Steve Farrar…
“If you’re an argyra spider, you want to avoid the hymenoepimecis wasp at all costs. This wasp is a parasite. It turns the spider into something it was never intended to be. And it all begins when the wasp stings the spider.
When the stinger enters the spider, the spider is temporarily paralyzed. The wasp then lays its egg inside the abdomen of the spider. The spider soon recovers and does the normal work of a spider, which, of course is spinning webs. The spider does this for the next ten to fourteen days while the wasp eggs grow inside it.
At that point, the egg secretes a chemical that forces the spider to do what the spider has never done before. Instead of spinning a web, the spider begins to build a cocoon for the unborn wasp inside its abdomen.
By nature, spiders don’t build cocoons; they build webs. But, when the wasp penetrates the insides of the spider, the spider begins to exhibit traits that are not normal to spiders. When the spider is so infiltrated by the wasp, the spider loses its natural purpose and begins to express behaviors that are contrary to its nature.
When the cocoon has been constructed for the unborn wasp, it kills the spider and continues to grow in the cocoon until it is born.”
~from the Journal of Arachnology 29 (2002), pages 354-66