Anti-Adhesive Layers Leave No Hope for Insects

Photo: Pitcher plant superposed by graphics

Carnivorous plants have evolved particular organs to catch their prey. The pitfall trap of the tropical pitcher plant Nepenthes belongs to the group with passive traps. Nepenthes pitchers are organised in a complex way, with a lid, a ring around the pitcher's entrance, and slippery and digestive zones, the latter containing a supply of digestive fluid.

The slippery zone is very important to successful trappings. It is covered by a layer of crystalline wax on which insects lose their footing and slide down into the digestive fluid.

Researchers from the Max Planck Institute for Metals Research and the University of Hohenheim have investigated the micromorphology, chemical composition, and mechanical characteristics of the wax and combined them with experiments concerning insect behaviour. The wax cover is made of two layers that are different in structure, chemical composition, hardness, and elasticity. The wax layers reduce the adhesive ability of insects in two very different ways.

The top layer is made of single, irregular, 30-50 nanometre thick platelets standing more or less perpendicular to the surface of the pitcher's wall. They are of somewhat random orientation without a clear pattern. The platelets bear a small "stalk" connected to the lower layer of wax.

The lower layer is similar to foam. It is made of connected membrane-like platelets, which stick out from the surface at sharp angles and do not show any clear orientation. The hardness and elasticity of both layers are different by more than an order of magnitude; the upper wax layer is much softer and more elastic than the lower one.

Laboratory experiments with lady-bird beetles Adalia bipunctata showed that the wax layers - compared to glass or de-waxed pitcher walls - significantly reduce the adhesive ability of the insects. The upper layer contaminates the insects' feet and makes them less adhesive. The lower layer reduces the contact area between the feet and the trap.

COMPAMED.de; Source: Max-Planck-Gesellschaft