Comparative analysis of the ultrastructure and adhesive secretion pathways of different smooth attachment pads of the stick insect Medauroidea extradentata (Phasmatodea)

Abstract

The mechanism by which insects achieve attachment and locomotion across diverse substrates has long intrigued scientists, prompting extensive research on the functional morphology of attachment pads. In stick insects, attachment and locomotion are facilitated by two distinct types of smooth cuticular attachment pads: the primary adhesion force-generating arolium and the friction force-generating euplantulae. They are both supported by an adhesive secretion delivered into the interspace between the attachment pads and the substrate. In this study, we analysed and compared internal morphology, material composition and ultrastructure, as well as the transportation pathways in both adhesive organs in the stick insect Medauroidea extradentata using scanning electron microscopy, micro-computed tomography, light microscopy, and confocal laser scanning microscopy. Our observations revealed structural differences between both attachment pads, reflecting their distinct functionality. Furthermore, our results delineate a potential pathway for adhesive secretions, originating from exocrine epidermal cells and traversing various layers before reaching the surface. Within the attachment pad, the fluid may influence the viscoelastic properties of the pad and control the attachment/detachment process. Understanding the material composition of attachment pads and the distribution process of the adhesive secretion can potentially aid in the development of more effective artificial attachment systems.

The effect of age on the attachment ability of stick insects (Phasmatodea)

“…Blue indicates less sclerotized cuticle, green indicates rather sclerotized cuticle, and red colors indicate strongly sclerotized cuticle [ 50 , 55 ]. The general appearance of the autofluorescence and its distribution was uniform for all tarsi examined and corresponds to the signals known for stick insect tarsi [ 56 ]. Differences in color between the pads and the cuticle of the tarsus were clearly visible.…”

“…Less activity would also mean less circulatory support by muscular activity, intensifying the circulatory problems. A relatively large fraction of the tarsus is filled with hemolymph [ 30 ], including the volume of the attachment pads [ 56 , 81 ].…”