Natural arrangement of micro-strips reduces shear strain in the locust cuticle during power amplification

As one of the key components in power amplification of locust, semi-lunar process (SLP) cuticle has five different portions where its portion II stores a large amount of strain energy when locusts kick. The SLP portion II is composed of regular chitin micro-strips that non-uniformly arranged in its transverse plane. However, it is still unknown whether the natural arrangement of micro-strips affects the power amplification of the SLP cuticle. To address this, SLP portion II samples of adult locusts were mechanically tested along three orthogonal directions, corresponding to the length, width, and thickness axes of the micro-strips, respectively. Four different arrangements of micro-strips in the portion II were compared using finite element method. It was found that the micro-strip set of the portion II is orthotropic elastic and its elastic modulus is maximal in the length axis of the micro-strip. The inclusion of the micro-strips offers more strain energy than the isotropic portion II (without micro-strips). Compared to the horizontally- and vertically-arrangements of the micro-strips, their natural arrangement does not affect the strain energy but reduces the in-plane shear strain in the portion II. This research helps us deeply understand the power amplification mechanism of locusts and may guide optimization of catapult device design in bioinspired robots.