Adhesion–stiffness competition in caterpillar crawling enhanced by non-linear elasticity and buckling

Caterpillars move very efficiently traversing arboreal, geometrically uneven and extremely varied environments through a complex mechanism that allows highly adaptable, stable and bone-free locomotion. The gait efficiency is vital for these animals at their larval stage, since it significantly increases their survival chances in the very short period in which they have to eat, grow quickly and shed their skin multiple times for finally performing the metamorphosis, one of the most profound and fascinating transformations in nature. As demonstrated recently, the signature of the caterpillar crawling gait can be identified in the way it operates, coordinating periodic muscle contractions and substrate grip ruled by a unique competition between body elasticity and adhesion of suction-cup-like abdominal prolegs, which test shapes, anchor surfaces and recognise roughness of material substrates to adapt and orient locomotion modes and speeds. To upgrade previous mechanical models and more faithfully capture further experimentally observed caterpillar gait features, which in fact appear more pronounced in some species, we propose to incorporate large deformations, nonlinear elasticity and buckling instability of the body segments, showing how the synergy of all them makes evident additional important strategies actuated by caterpillars for enhancing their crawling gait, maximising elastic energy release, skipping intermediate states and selecting the most energetically favourable locomotion sequence. It is felt that, by deciphering how soft invertebrates exploit non-linear elasticity, adhesion and buckling to optimise and stabilise their movement, this work might contribute to better understand animal biomechanics and to design next-generation bio-inspired climbing and multi-legged soft robots.