Locusts use a composite of resilin and hard cuticle as an energy store for jumping and kicking

Abstract

SummaryLocusts jump and kick by using a catapult mechanism in which energy is first stored and then rapidly released to extend the large hind legs. The power is produced by a slow contraction of large muscles in the hind femora that bend paired semi-lunar processes in the distal part of each femur and store half the energy needed for a kick. We now show that these energy storage devices are composites of hard cuticle and the rubber-like protein resilin. The inside surface of a semi-lunar process consists of a layer of resilin, particularly thick along an inwardly pointing ridge and tightly bonded to the external, tanned, black cuticle. From the outside, resilin is visible only as a distal and ventral triangular area that tapers proximally. To reproduce the bending of the semi-lunar processes that occurs during kicking and jumping, the extensor tibiae muscle was stimulated electrically in a pattern that mimicked the normal sequence of its fast motor spikes recorded in natural kicking. Externally visible resilin was compressed and wrinkled as a semi-lunar process was bent by the muscular contraction without moving the hind leg. It then sprung back to restore the semi-lunar process rapidly to its natural shape. Each nymphal stages jumped and kicked and had a similar distribution of resilin in their semi-lunar processes as adults; the resilin was shed with the cuticle at each moult. It is suggested that composite storage devices that combine the elastic properties of resilin with the stiffness of hard cuticle, allow energy to be stored by bending hard cuticle by only a small amount and without fracturing. In this way all the stored energy is returned and the natural shape of the femur is restored rapidly so that a jump or kick can be repeated.