Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect

Joint Action Many small insects are impressive jumpers, but large leaps and small bodies pose biomechanical challenges. Burrows and Sutton (p. 1254) show that the nymphal planthopper Issus has interlocking gears on their hindleg trochanters that act together to cock the legs synchronously before triggering forward jumps. At the final molt, the gears are swapped for a high-performance friction-based mechanism because the risk of breaking a gear is high, the options for repair during molting are gone, and, moreover, the animal is bigger and stronger. Functional gears are involved in the ballistic jumping movements of the flightless planthopper insect, Issus. Gears are found rarely in animals and have never been reported to intermesh and rotate functionally like mechanical gears. We now demonstrate functional gears in the ballistic jumping movements of the flightless planthopper insect Issus. The nymphs, but not adults, have a row of cuticular gear (cog) teeth around the curved medial surfaces of their two hindleg trochantera. The gear teeth on one trochanter engaged with and sequentially moved past those on the other trochanter during the preparatory cocking and the propulsive phases of jumping. Close registration between the gears ensured that both hindlegs moved at the same angular velocities to propel the body without yaw rotation. At the final molt to adulthood, this synchronization mechanism is jettisoned.