RWTH-Aachen University, Department of Cellular Neurobionics, Kopernikusstrasse 16, 52056 Aachen, Germany.
J Exp Biol. 2010 Apr;213(Pt 7):1115-25. doi: 10.1242/jeb.035618.
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.
猪笼草属植物的叶子非常特殊，能够高效地捕获并困住昆虫猎物。除了具有光滑的叶边缘（可以防止昆虫附着），它们还在捕虫器的导水通道内壁上使用表皮蜡晶来阻碍昆虫通过粘性装置附着。有人提出，单个晶体的脱落以及由此产生的粘性器官的污染是捕捉昆虫的原因。然而，我们的结果提供了支持另一种机制的证据，主要基于蜡质表面的稳定性和粗糙度。首先，我们无法在从成熟猪笼草叶片上分离出来的昆虫附肢上检测到大量的晶体碎片。其次，通过聚焦离子束处理对捕虫器表面进行研究表明，蜡晶形成了一个紧密的 3D 结构。第三，对血小板状晶体的原子力显微镜观察表明，这些晶体在机械上是稳定的，昆虫附肢不太可能将其脱离。第四，蜡层的表面形貌参数与具有小粒径的抛光纸非常相似。通过测量昆虫在这种人工表面上的摩擦力，我们证明了仅微观粗糙度就足以最大限度地减少昆虫的附着。理论模型表明，在一定的尺度范围内，表面粗糙度对于粘性垫来说太粗糙而无法附着，但对于爪子来说又不够粗糙，因此可以防止附着。
