Attachment performance of stick insects (Phasmatodea) on convex substrates

Abstract

Phasmatodea (stick and leaf insects) are herbivorous insects well camouflaged on the plant substrates due to cryptic masquerade. Also their close association with plants makes them adapted to different substrate geometries and surface topographies of the plants they imitate. During past years, stick insects gained increasing attention in attachment- and locomotion-focused research. However, most studies experimentally investigating stick insect attachment have been performed either on single attachment pads or on flat surfaces. In contrast, curved surfaces, especially twigs or stems of plants, are dominant substrates for phytophagous insects, but not much is known about the influence of curvature on their attachment. In this study, by combining the analysis of the tarsal usage with mechanical traction and pull-off force measurements, we investigate the attachment performance on curved substrates with different diameters in two species of stick insects with different tarsal length. We provide the first quantitative data for forces generated by stick insects on convex curved substrates and show that the curvature significantly influences the attachment abilities in both species. Within the studied range of substrate curvatures, traction force decreases and the pull-off force increases with increasing curvature. Shorter tarsi demonstrate reduced forces, however, the tarsus length only has an influence for diameters thinner than the tarsal length. The attachment force generally depends on the number of tarsi/tarsomeres in contact, tarsus/leg orientation and body posture on the surface. Pull-off force is also influenced by the tibiotarsal angle, with higher pull-off force for lower angles, while traction force is mainly influenced by load, i.e. adduction force.