Ultrastructural architecture and mechanical properties of attachment pads in Tettigonia viridissima (Orthoptera Tettigoniidae)

Natural releasable attachment systems of insect legs, where attachment-detachment performances are often very fast, seem to be optimized to get a maximum of real contact to the substratum, Tarsi of Tettigonia viridissima bear flexible attachment pads with unusual ultrastructural architecture of the cuticle. The indentation of the attachment pads was measured under different loads using a force-tester. Since the mechanical properties are influenced by material structure, the freeze-substitution experiments were undertaken to investigate the influence of loads on material structure. Both profile changes of the surface and the orientation of cuticle microfibrils were visualized by means of scanning electron microscopy followed by fracturing of the frozen material. The results show that the flexible pad material deforms replicating the substrate profile down to the micrometer roughness. The pad material showed both elastic and viscous behavior under loads. Elastic deformation of the pad occurred under normal force applied for 4-6 s (elastic modulus 27.2 +/- 11.6 kPa). Two viscous relaxation processes were found, of time constants tau (1) = 1.88 +/- 0.616 s and tau (2) = 41.2 +/- 9.95 s. Low stiffness of material studied here aids in surface replication and increase of area of real contact between the pad and the underlying substrate.