Micro-rough epicuticular (meaning ‘on top of the cuticle’) wax crystal coatings are found on leaves and stems throughout the plant kingdom, and usually render surfaces water-repellent and slippery for insects. In Nepenthes pitcher plants, they are commonly found on the interior trap walls above the digestive liquid level. The key role of wax crystals for successful prey capture has been known for over a century, but their detailed interaction with insect feet has only been investigated in recent decades, and it is still debated to some extent.
Scanning electron micrograph of a freeze-fractured inner pitcher wall surface of Nepenthes gracilis. A dense cover of delicate, upright-standing wax platelets drastically reduces the available contact area for insects’ adhesive footpads and renders the surface extremely slippery.
When we discovered the ‘springboard’ lid trapping mechanism of Nepenthes gracilis, we were surprised to find epicuticular wax crystals on the underside of the pitcher lid of this species; in particular, because these crystals looked nothing like ‘normal’ Nepenthes waxes! They not only looked different – they also were much less slippery. On the inner wall waxes, insects slip immediately and it is virtually impossible to measure any friction between their feet and the crystal surface. In contrast, the lid waxes don’t seem to affect insect adhesion and locomotion at all – at least at first glance. Ants and other insects can regularly be seen walking upside down and collecting generously secreted nectar underneath the pitcher lid.
Polyrhachis pruinosa ants harvesting nectar from the lid of a N. gracilis pitcher in Borneo.
Only when the lid is suddenly perturbed, e.g. by the impact of a rain drop, the function of the crystals becomes apparent: they reduce the adhesive forces of insects just enough for them to be shaken off and end up in the trap below! Fascinated by the occurrence of two morphologically and functionally so different wax crystal types in the same plant organ, we started investigating their chemical composition (in collaboration with Lucas Busta and Reinhard Jetter) which turned out to be as starkly different between both waxes as their crystal morphology. We are now looking at the development and biosynthesis pathways of different wax crystal types in order to understand how this extraordinary plasticity of surface topographies is achieved. In collaboration with Chester Zoo, we are conducting a comparative analysis of wax crystal surfaces and have since found an astounding diversity of crystal shapes, sizes and densities across the genus Nepenthes.
The wax crystals on the lower lid surface of N. gracilis look completely different to those on the inner pitcher wall. Instead of fine platelets, we find much larger, irregularly spaced blocky pillars. These provide more grip for insect feet, but still reduce friction and adhesion enough to dislodge visitors easily when a raindrop hits the lid from above.