In a forest in Italy, the parasitic wasp Sinarachna pallipes hunts for a type of spider called an orb weaver. When the wasp finds one, it delivers a sting that temporarily paralyzes the spider. The wasp uses this opportunity to lay an egg on the spider’s abdomen. Then the wasp flies off. The spider regains its ability to move and builds a web as if nothing happened—that is, until the wasp egg hatches.
For two weeks, the wasp larva clings to the spider’s abdomen, feeding on its blood. Events then take another strange turn: The larva releases chemicals that force the spider to do something unusual. Instead of making a typical flat web with circles spiraling toward its center, the spider weaves a three-dimensional web to protect the developing parasite.
When the structure is complete, the larva eats the spider. Then the parasite retreats into its spider-built den, which shields it from rain, wind, and predators on the forest floor. Tucked inside the shelter, the larva forms a cocoon where it will safely grow into an adult wasp.