Composite structure of the crystalline epicuticular wax layer of the slippery zone in the pitchers of the carnivorous plant Nepenthes alata and its effect on insect attachment

Abstract

SUMMARY The slippery zone situated below the peristome inside pitchers of most carnivorous plants from the genus Nepenthes is covered with a thick layer of epicuticular wax. This slippery zone is reported to play a crucial role in animal trapping and prey retention. In N. alata, the wax coverage consists of two clearly distinguished layers. These layers differ in their structure, chemical composition and mechanical properties, and they reduce the insect attachment in different ways. The lower layer resembles foam, composed of interconnected membraneous platelets protruding from the surface at acute angles. The upper layer consists of densely placed separate irregular platelets, located perpendicular to the subjacent layer. Crystals of the upper layer bear small stalks, directed downwards and providing connections to the lower layer. These morphological distinctions correlate with differences in the chemical composition of waxes. The compound classes of alkanes, aldehydes, primary alcohols, free fatty acids, esters and triterpenoids occurred in extracts from both wax layers, but in different proportions. Chain length distributions in aliphatics were different in extracts from the lower and the upper wax layers. Waxes of the upper and lower layers exhibited different mechanical properties: wax of the lower layer is harder and stiffer than that of the upper layer. Moreover, crystals of the upper layer are brittle and may be easily exfoliated or broken to tiny pieces. Laboratory experiments using tethered insects showed that both wax layers reduce the attachment force of insects. It is assumed that a decrease in insect attachment on the two distinct wax layers is provided by the two different mechanisms: (1) crystals of the upper wax layer contaminate insects'adhesive pads; (2) the lower wax layer leads to a reduction of the real contact area of insects' feet with the plant surface.

Friction force reduction triggers feet grooming behaviour in beetles

“…(c) Force measurements A 10 g load cell force transducer (World Precision Instruments, Sarasota, FL, USA) was clamped to a holder perpendicular to the horizontal plane of the substrate tested (for further details see Gorb et al [17]). Prior to the experiment, each substrate was briefly cleaned with ethanol and distilled water and dried in a nitrogen jet.…”

“…That is why adhesive setae, bearing fluid drops on their tips, can be easily contaminated by environmental particles. Plants possessing self-cleaning surfaces [12] are covered with wax crystals, which in many cases can break when insects walk on them and contaminate attachment devices [3,11,[13][14][15][16][17] (figure 1). It has been previously demonstrated that attachment forces are strongly reduced on contaminated feet [11,17].…”

Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials

“…Structure-related properties of plant surfaces include the formation of low-adhesive surfaces, e.g. sliding of insects (Gorb et al 2005), and an increase in the reflection of visible light or absorption of harmful UV radiation (Barnes & Cardoso-Vilhena 1996;Pfündel et al 2006). Structures of plant surfaces are also relevant for surface wettability.…”

Harmless nectar source or deadly trap:Nepenthespitchers are activated by rain, condensation and nectar

“…First, trapping is thought to be based on a slippery wax bloom on the inner pitcher wall of many Nepenthes species (Knoll 1914;Lloyd 1942;Juniper & Burras 1962;Juniper et al 1989;Moran et al 1999;Gaume et al 2002). It is made up of microscopic, epicuticular wax crystal platelets which give rise to antiadhesive surface roughness and easily break off, thus contaminating attachment structures and causing arthropods to slip (Knoll 1914;Juniper & Burras 1962;Gorb et al 2005). Second, it has been suggested that anaesthesia by narcotic alkaloids causes prey capture in Nepenthes madagascariensis (Ratsirarson & Silander 1996).…”