Comparative Analysis of Mechanical Properties of Arolium and Euplantulae in Medauroidea extradentata (Phasmatodea), Using In Vivo Atomic Force Microscopy, Supports Functional Specialization of both Types of Attachment Pads

Abstract

Stick insects possess two functionally different smooth attachment pads, the pretarsal (arolium) and tarsal (euplantulae) producing adhesion and friction during locomotion across diverse substrates. While their functional roles are well understood, the mechanical properties driving these differences require further investigation. Using atomic force microscopy (AFM), this work measures mechanical properties of both pads on the same tarsus under near-physiological conditions at different indentation depths and velocities. The results reveal that the arolium exhibits a lower effective elastic modulus (EEM) and higher work of adhesion (WOA), suggesting its efficacy in generating adhesion forces. Conversely, the euplantulae demonstrates a higher EEM and lower WOA, supporting their role as friction pads. Furthermore, it is demonstrated that altering the velocity and indentation depth has a significant impact on the mechanical properties of both pads revealing their viscoelastic characteristics. This study provides the first comparative analysis of the mechanical properties of both types of attachment pads on the same tarsus in living stick insects, validating the complementary roles of the arolium and euplantulae. These results contribute to better understanding of structure–function relationships in insect attachment devices and might be of interest for biomimetic engineering of adaptive adhesives and soft robotics.