Cuticular Hydrocarbon Composition of Adhesive Secretions from Functionally Different Attachment Pads of the Stick Insect Medauroidea extradentata (Phasmatodea)

During the evolution of insects, two principal types of attachment systems, hairy and smooth, evolved that enable them to attach to and move across diverse surfaces. Both rely on attachment supported by a thin film of fluid that enhances functionality. In some insects, multiple pad types occur on the same leg, each fulfilling complementary roles during attachment. Although their structural and mechanical differences are well understood, the secretion’s chemical composition and its functional implications remain largely unexplored. Stick and leaf insects (Phasmatodea) provide an ideal model to investigate these aspects, as they possess two morphologically and functionally distinct smooth attachment pads on the same tarsus: the pretarsal arolium, responsible for adhesion, and the tarsal euplantulae, specialised for friction. Using polydimethylsiloxane-coated solid-phase microextraction fibres and gas chromatography–mass spectrometry, we analysed the hydrocarbon profiles of both pad types and the body surface of Medauroidea extradentata. These data were compared with published profiles from other insect taxa possessing either hairy or smooth attachment systems to place our findings in a broader phylogenetic and functional context. Our analyses revealed distinct hydrocarbon compositions consistent with the specific mechanical roles of each structure. The arolium secretion was enriched with long-chain, methyl-branched hydrocarbons, likely enhancing fluidity and capillary forces, whereas the euplantulae secretion contained shorter, less methyl-branched hydrocarbons, suggesting higher viscosity, improving frictional contact. The body surface contained only n-alkanes, likely forming a wax-like protective layer. Comparisons across taxa revealed significant differences between smooth and hairy systems, reflecting both functional and phylogenetic influences on fluid chemistry.