Acrotaphus wiltii

On September 13 I photographed an Ichneumon wasp, Acrotaphus wiltii, that is widespread in the eastern US. It has only one known host, a species of orb spider, Neoscona arabesca, that must be equally widespread.
According to info that I found online, the wasp stings the spider to immobilize it briefly and lays an egg that hatches into an external parasite. The larva grows rapidly, kills the spider, pupates, and eventually emerges as an adult.