Multifunctional mandibles of ants: Variation in gripping behavior facilitated by specific microstructures and kinematics

The elongated mandibles of certain ant species are dexterous grippers that can output a wide range of forces as needed for various tasks. Our combined experimental and theoretical research reveals the multifunctionality of the mandibles of Harpegnathos venator that is facilitated by specific microstructures and characteristic kinematics. First, we found that H. venator can pull off a spider's (Heteropoda venatoria) leg by closing its long mandibles. We observed that the ant usually clamps the spider's leg using the distal or middle part of its mandibles. In contrast, the ant can grip its egg with the proximal parts of its mandibles without causing damage. Our results showed that the spider's legs are always fractured at the coxa-trochanteral joint. Second, we found that the force required to fracture the spider's leg can be up to 500 times the ant's body weight. On the other hand, the maximum force can be controlled to less than 2×10^{- 6}N while gripping an egg. By combining microstructure imaging, kinematic tracking and mathematical modeling, we uncovered that the sharp teeth and dense bristles on the internal side of the mandibles determine the high adhesion force, while the concave teeth and biaxial rotation of the mandibles facilitate gentle gripping. We validated our findings by constructing an artificial mandible pair. This work expands the knowledge of the physiological multifunctionality in ant mandibles, and provides novel ways to reveal the multifunctionality in insect appendages by applying the tools of mechanical analysis and related experimental devices.