A wasp that eats its host alive from the inside shook Darwin's faith

Ichneumon wasps lay their eggs inside living caterpillars; the larvae then eat their host from within, keeping it alive as long as possible for freshness. Darwin wrote to a friend that he couldn't 'persuade myself that a beneficent and omnipotent God would have designedly created' such a creature, citing it as evidence against a benevolently designed world.