Allowing caterpillar locomotion on any substrates: towards hybrid soft robots design

Soft robotics draws inspiration from nature to develop efficient locomotion strategies. Among natural organisms, caterpillars serve as excellent biomimetic models by exploiting body elasticity and substrate adhesion. Building on previous studies, this work employs a crawling model that represents the body as spring-linked masses, interacting with the environment via threshold-based adhesion and Winkler-like springs. To experimentally further validate this theoretical framework, we analysed the locomotion of Pieris brassicae larvae across substrates with diverse morphologies: wood, steel, slate and glass. Our results highlight a strong correlation between substrate roughness and crawling performance. While caterpillars seamlessly adapt their movement to predominant surface patterns, they struggle on smooth surfaces like glass-where microscale asperities are insufficient for proleg grip-confirming that adequate adhesion is crucial to trigger and sustain movement. Driven by the model's insights on these physical constraints, we explored how crawling mechanics could be artificially enhanced. Through proof-of-concept experiments, we equipped larvae with artificial armour to increase body stiffness and additional masses to improve effective friction and adhesion, allowing them to cross non-walkable glass surfaces. Ultimately, these findings validate the model's predictive accuracy, envisioning possible novel design criteria for bioinspired soft crawlers capable of adapting movements in complex habitats.