Meet the Wasp that Turns Spiders into Zombie Construction Workers
Wasp lays egg in spider. Baby wasp hatches, lives on spider “blood”. Once wasp becomes juvenile, it injects spider with chemical causing the spider to modify ITS BEHAVIOUR, and strengthen the web to suit the wasp. Wasp kills spider and eats it. Wasp builds cocoon on web and adult wasp emerges. Wasps are arseholes.
Pic and title, with a better description, from here.