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SUMMARY Pitcher plants of the genus Nepenthes efficiently trap and retain insect prey in highly specialized leaves. Besides a slippery peristome which inhibits adhesion of insects they employ epicuticular wax crystals on the inner walls of the conductive zone of the pitchers to hamper insect attachment by adhesive devices. It has been proposed that the detachment of individual crystals and the resulting contamination of adhesive organs is responsible for capturing insects. However, our results provide evidence in favour of a different mechanism, mainly based on the stability and the roughness of the waxy surface. First, we were unable to detect a large quantity of crystal fragments on the pads of insects detached from mature pitcher surfaces of Nepenthes alata. Second, investigation of the pitcher surface by focused ion beam treatment showed that the wax crystals form a compact 3D structure. Third, atomic force microscopy of the platelet-shaped crystals revealed that the crystals are mechanically stable, rendering crystal detachment by insect pads unlikely. Fourth, the surface profile parameters of the wax layer showed striking similarities to those of polishing paper with low grain size. By measuring friction forces of insects on this artificial surface we demonstrate that microscopic roughness alone is sufficient to minimize insect attachment. A theoretical model shows that surface roughness within a certain length scale will prevent adhesion by being too rough for adhesive pads but not rough enough for claws.

SUMMARY Knowledge of both surface structure and physical properties such as stiffness and elasticity are essential to understanding any adhesive system. In this study of an adhesion surface in the tree frog, Litoria caerulea White, a variety of techniques including atomic force microscopy were used to investigate the microstructure and properties of an epithelium that adheres through wet adhesion. Litoria toe pads consist of a hexagonal array of flat-topped epithelial cells, separated by mucus-filled channels. Under an atomic force microscope, this `flat' surface is highly structured at the nanoscale, consisting of a tightly packed array of columnar nanopillars (described as hemidesmosomes by previous authors), 326±84 nm in diameter, each of which possesses a central dimple 8±4 nm in depth. In fixed tissue (transmission electron microscopy), the nanopillars are approximately as tall as they are broad. At the gross anatomical level, larger toe pads may be subdivided into medial and lateral parts by two large grooves. Although the whole toe pad is soft and easily deformable, the epithelium itself has an effective elastic modulus equivalent to silicon rubber (mean Eeff=14.4±20.9 MPa; median Eeff=5.7 MPa), as measured by the atomic force microscope in nanoindentation mode. The functions of these structures are discussed in terms of maximising adhesive and frictional forces by conforming closely to surface irregularities at different length scales and maintaining an extremely thin fluid layer between pad and substrate. The biomimetic implications of these findings are reviewed.

Adhesive organs like arolia of insects allow these animals to climb on different substrates by creating high adhesion forces. According to the Dahlquist criterion, adhesive organs must be very soft exhibiting an effective Young's modulus of below 100 kPa to adhere well to substrates. Such a low effective Young's modulus allows the adhesive organs to make almost direct contact to the substrate and results in van der Waals forces beside capillary forces. In previous studies the effective Young's moduli of adhesive organs were determined using indentation tests yielding their structure to be very soft indeed. However, adhesive organs show a layered structure, thus the measured values comprise the effective Young's moduli of several layers of the adhesive organs. In this study, a new approach is illustrated to measure the Young's modulus of the outermost layer of the arolium, i.e. of the epicuticle, of the stick insect Carausius morosus. As a result of the epicuticle being supported by upright fibres tensile tests allow the determination of the Young's modulus of the epicuticle hardly affected by subjacent layers. In our tensile tests arolia of stick insects adhering on a latex membrane were stretched by stretching the membrane while the elongation of the contact area between an arolium and the membrane was recorded. For analysis mathematical models of the mechanical system were developed. When fed with the observed elongations, these models yield estimates for the Young's modulus of the epicuticle of about a hundred megapascal. Thus in arolia a very thin layer (~225 nm) of a rather stiff material, which is less susceptible to abrasion, makes contact to the substrates while the inner fibrous structure of arolia is responsible for their softness.