Plants are able, using organic substances, to achieve
effects that we otherwise mostly know only from technical materials.
One example of this is the carnivorous pitcher plant, as researchers
from the Max Planck Institute for Metals Research and the University
of Hohenheim have shown. These plants catch insects and hold them
using traps with a double layer of crystalline wax. The upper layer
has crystalloids which contaminate the attachment organs that
insects use to adhere themselves to surfaces. The lower layer
additionally reduces the contact area between the insect feet and
plant surface. The insects thus slip into the pitcher-shaped traps,
where they are digested (The Journal of Experimental Biology,
December 2005). These results provide ideas for further developments
of technological anti-adhesive surfaces.
One highly specialised ecological group of plants is carnivores -
meat-eaters. In order to obtain nutrients such as nitrogen and
phosphorous, which may be lacking in the soil, carnivorous plants
catch and digest small animals, primarily insects. These plants have
evolved particular organs to catch their prey. Different types of
carnivores bear traps with different morphologies and
catch-mechanisms.
The pitfall trap of the tropical pitcher plant Nepenthes belongs
to the group with passive traps. In this group, the plant does not
move in order to catch animals. Although the origins of the pitchers
lie in leaf organs, the structures that originate from the leaves
are not leaf-like. Nepenthes pitchers are organised in a complex
way, with a lid, a peristome (a ring around the pitcher's entrance),
and slippery and digestive zones, the latter containing a supply of
digestive fluid. These pitchers draw in insects, hold them, and
finally digest them.
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. Until now, studies
have focused either on the wax structure or the behaviour of insects
in the trap.
The Stuttgart researchers 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.
Editor's Note: The original news release can be found here.