Claws that snap: the raptorial mechanism of dryinid wasps

Abstract

Prey capture is a key selection pressure, often favoring extreme performance morphologies. Many mechanisms of prey capture, however, remain poorly understood. In this study, we document the remarkable raptorial behavior and evolutionary biomechanics of the "pincer wasps" (Hymenoptera: Dryinidae). Dryinid claws, previously thought to function as simple vises (Nachtigall-&-Nachtigall-1974), are shown to possess "snap traps", or claws that close rapidly upon prey contact. Detailed morphological analysis, using synchrotron-radiation microtomography (SR-{micro}-CT), reveals that all long-clawed dryinids can overcenter their specialized anterior pretarsal claws, presumably allowing elastic energy storage in the pretarsal apodemes. Uniquely, explosive release of this energy, leading to claw closure, appears to occur not through active withdrawal of a latch, but as the direct consequence of contact with the prey: impact hyperextends a specialized trigger-joint complex of the tarsus, pulling the claw back into undercenter alignment, and so freeing the stretched apodeme to fully recoil. Such a "contact trigger" is eminently useful, as these trap-claw wasps prey on insects with explosive jump capacity; by linking trap shutting with prey contact, success chances are increased. Paleontological and allometric analyses suggest that the raptorial behavior likely preceded the evolution of overcentering, which appears to have evolved via release from developmental constraint on anteroposterior symmetry, leading to a distinct scaling rule. These findings expand the documented diversity of spring-loaded biological mechanisms and establish Dryinidae as a promising system for studying how developmental constraints, mechanical demands, and functional innovation shape ecomorphological diversity. HIGHLIGHTSO_LIDryinidae catch prey with spring-loaded trap claws, relying on a unique direct-action trigger. C_LIO_LIRaptorial behavior evolved first, followed by the functional derivation of two separate components. C_LIO_LIThe evolution of novel raptorial mechanism was enabled by release of ancestral developmental constraint. C_LI