Slippery surfaces of pitcher plants: Nepenthes wax crystals minimize insect attachment via microscopic surface roughness

Abstract

SUMMARYPitcher 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. Supplementary material available online at

Plant surfaces with cuticular folds are slippery for beetles

“…reduced on average by 88 per cent and in comparison to the reference substrate glass even by 96 per cent. Our findings support the roughness hypothesis ( proposed in [7,8,14,22,23]), assuming that a critical roughness reducing the real contact area between the insect's attachment devices and the substrate is a crucial parameter to minimize its attachment ability. According to the dimensions of the single setae of the Colorado potato beetle, the tip of a seta may come to lie on several folds.…”

“…These asperity diameters are similar to the 0.5 mm thickness of medium cuticular folds showing the strongest reduction in adhesion in our study. In Scholz et al [23] friction measurements were performed with juvenile stick insects on polishing paper of different roughness, finding a strong reduction of friction force on polishing paper of an approximate grain size of 3 mm, with a mean spacing of surface irregularities of 1.55 mm. Within the same study a similar spacing of 1.3 mm was recorded for 3D epicuticular waxes within the slippery zone in Nepenthes alata pitchers.…”

Role of locust Locusta migratoria manilensis claws and pads in attaching to substrates

“…The other type of insect pads, occurring in ants and grasshoppers, are covered by rather soft deformable structures with an extremely flexible cuticle [10][11][12]. Regardless of the discriminations in such microstructures, both hairy pads and smooth pads duplicate the unevenness of the surface profile at different scales to guarantee a maximum real contact area, and thus achieve sufficient attachment force [9,12,13]. Measurements of the friction force of insects on polishing papers and other artificial surfaces have demonstrated that the microscopic roughness alone can minimize insect attachment, and a correlative model showed that surface roughness with a certain scale can prevent adhesion by restricting pads but not claws [13].…”

“…Regardless of the discriminations in such microstructures, both hairy pads and smooth pads duplicate the unevenness of the surface profile at different scales to guarantee a maximum real contact area, and thus achieve sufficient attachment force [9,12,13]. Measurements of the friction force of insects on polishing papers and other artificial surfaces have demonstrated that the microscopic roughness alone can minimize insect attachment, and a correlative model showed that surface roughness with a certain scale can prevent adhesion by restricting pads but not claws [13]. Therefore, insects depend on claws to generate mechanical interlock and pads to cause flexible attachment for walking or standing freely on substrates.…”

It's Not a Bug, It's a Feature: Functional Materials in Insects

“…For instance, pitcher plants, from the carnivorous Nepenthes genus, have developed epicuticular wax crystals to serve a variety of purposes from increasing surface microroughness to contaminating adhesive structures with exfoliated crystals in order to capture and consume their insect prey. [40,41] A curious example of physical adhesion has developed in the Colorado potato beetle, Leptinotarsa decemlineata, to suit its copulation posture. [42] The tarsal microstructures on the beetles' legs exhibit sexual dimorphism-both male and female beetles have setae that terminate in points and spatulas, but only males have a third style of disk-shaped setae.…”