The exoskeleton of scorpions' pincers: Structure and micro-mechanical properties

Since scorpions exist almost all over the world, some expected body differences exist among the species: undoubtedly, the most evident is the shape and size of their pincers or chelae. The scorpion chela is a multifunctional body component (e.g. attack/defense, mating and protection from the environment) that leads to the development of different stresses in the cuticle. How such stresses in the cuticle are accommodated by different chelae shape and size is largely unknown. Here we provide new comparative data on the hierarchical structure and mechanical properties of the chela cuticle in two scorpion species: Scorpio Maurus Palmatus (SP) that has a large chela and Buthus Occitanus Israelis (BO), with a slender chela. We found that the SP exocuticle is composed of four different sublayers whereas the BO exocuticle displays only two sublayers. These structures are different from the exocuticle morphologies in crustaceans, where the Bouligand morphology is present throughout the entire layer. Moreover, the scorpion chela cuticle presents an exclusive structural layer made of unidirectional fibers arranged vertically towards the normal direction of the cuticle. Nanoindentation measurements were performed under dry conditions on transversal and longitudinal planes to evaluate the stiffness and hardness of the different chela cuticle layers in both scorpions. The chela cuticle structure is a key factor towards the decision of the scorpion whether to choose to sting or use the chela for other mechanical functions. STATEMENT OF SIGNIFICANCE: Many arthropods such as lobsters, crabs, stomatopods, isopods, and spiders have been the subject of research in recent years, and their hierarchical structure and mechanical properties extensively investigated. Yet, except for a limited number of pre-1980 publications, comparatively little work has been devoted to the terrestrial scorpion. The scorpion chela is a multifunctional part of the body (e.g. attack/defense, mating and protection from the environment) that involves the development of various stresses in the cuticle. How these stresses in the chela cuticle are managed by different chelae shape and size is still unknown. The lack of a single study that integrates morphological characterization of the entire hierarchical structure of the scorpion chela cuticle, and local mechanical properties, significantly affects the scientific knowledge regarding important structural approaches that can be used by nature to maximize functionality.