Control of high-speed jumps: removing rotation from the jumps of locusts (Schistocerca gregaria).

Abstract

To jump, locusts (Schistocerca gregaria) use a latch-mediated spring-actuated (LaMSA) mechanism in their metathoracic legs, which gradually stores and then rapidly releases elastic energy, propelling them into the air. This system consistently results in a head-up, tail-down pitch of the body immediately after take-off, caused by a thus far unknown underlying mechanism. In this study, we aimed to test the hypothesis that head-up, tail-down angular rotation is a product of the locust's centre of mass being positioned off-centre to the force vector produced by the metathoracic legs; therefore, shifting the centre of mass forwards should counterbalance any torque acting around it. This was achieved experimentally by attaching a small mass, ranging from 3.4 to 13.6% of the locust's body mass, to the head of a locust before filming it jump to a target substrate. The resultant angular velocity from these jumps decreased as the added mass increased, with a mass equivalent to 10.2% of the body mass resulting in the complete elimination of angular velocity and therefore an entirely linear jump. When added mass totalled 13.6%, negative angular velocities were recorded (head-down, tail-up). This was matched with a corresponding decrease in the rotational energy percentage of each jump's total energy budget, and a slight decrease in linear velocity for the largest added mass. These findings strongly support the theory that locusts do not independently control their body pitch at take-off, and that changes to the body mass distribution can counter it.