Stereotaxis in the Wild: How a Parasitoid Wasp Finds its Host's Brain

The parasitoid Jewel Wasp Ampulex compressa uses cockroaches (Periplaneta Americana) as hosts for its larvae. To render a cockroach a live and yet immobile food supply for the larva, the wasp uses its stinger to penetrate the cockroach head and inject neurotoxic venom directly inside the brain. Previous studies suggest that the wasp uses sensory feedback to locate the cockroach brain inside its head capsule, however, the underlying mechanism is yet unknown. In the current study we used several methodologies, including Electron Microscopy, behavioral analysis and electrophysiology, to identify the mechanism by which the wasp’s stinger discriminates the brain from non-neuronal tissue inside the cockroach head.
Electron Microscopy reveals numerous cuticular structures along the shaft of the wasp’s stinger, and especially along the dorsal valve which penetrates the cockroach brain during the sting. The morphology and ultrastructure of some of these structures resemble those of known mechanoreceptive campaniform-like sensilla. When the cockroach brain is surgically removed prior to a wasp’s sting, the stinging duration is significantly prolonged. However, when the cockroach brain is surgically removed and replaced by an agar pellet which roughly resembles the consistency of a cockroach brain (2.5% w/v), the stinging duration is normal and venom is injected inside the agar pellet. In contrast, the stinging duration is significantly prolonged and no venom is injected when the cockroach brain is surgically removed and replaced with an agar pellet with lower consistency (0.6% w/v). The later suggests that the wasp indeed uses mechanoreceptive feedback to target the cockroach brain. In further support of this, extracellular recordings from stinger afferents reveals significantly stronger and unidirectional responses to hard (2.5%) agar pellets compared with soft (0.6%) agar pellets. We therefore conclude that the wasp uses mechanosensory feedback from specialized sensilla on its stinger to accurately localize and discriminate the brain target inside the cockroach head.