BENT LEGS BEAT BREAKAGES DURING TAKE-OFF

Abstract

As an insect hops from leaf to leaf looking for food or jumps away to escape a peckish predator, the last thing it needs is to either break its leg or stumble because the supporting leafy surface collapses. However, in order to defy gravity, insects need to generate high accelerations as they push off and this in turn requires strong forces that act on both the insect's legs and the leaf. So, how do insects generate enough acceleration to become airborne but not produce too much force that would break either their legs or their launch pads? Cesare Stefanini from The BioRobotics Institute, Italy, and his colleagues decided to investigate (p. 1270).The potentially damaging forces mainly occur during the initial take-off phase, from when the insect readies itself to jump to just after it's airborne, and with take-offs occurring in as little as 1 ms, they are not easy to capture. Undaunted, Stefanini turned to the green leafhopper, Cicadella viridis, which takes off in a relatively leisurely 5.6 ms. By filming their sky-bound launches at a rapid 8000 frames s−1, the team were able to calculate that it took off at an average velocity of 0.9 m s−1 with a near-constant acceleration rate of 152 m s−2.This near-constant acceleration rate also means that there is constant force being exerted at the foot–ground interface. However, muscles, which power the jump, are naturally elastic and cannot provide a constant force. In fact, peak forces that are generated by the muscles are high enough to damage the thrusting insect's leg or the leaf. Using the data from their videos, the team calculated that breakages are prevented because of the way the leafhopper's legs moves its legs. As energy is released from the contracted muscles in the thorax, the femur portion of the leg rotates and transmits the motion to the tibia. In doing so it converts the variable muscular force to a constant force acting on the tibia, which pushes off the ground without damaging the leg or the launch pad. So, good posture is key for jumping.