Ultrastructure and functional morphology of adhesive organs and anti-adhesive plant surfaces

Animals have evolved different adhesive structures on their legs to adhere on to a wide variety of substrates during locomotion. But some plant surfaces have evolved structures that hamper the attachment of animals. Knowledge of morphology, ultra-structure, surface structure and physical properties such as stiffness and elasticity are essential for understanding both of these two systems. The aim of this study was to characterize the surface structures and properties of adhesive organs and of slippery plant surfaces. In the first part of this study the morphology and ultra-structural setup of two different adhesive organs and of an anti-adhesive surface were characterised by using various microscopic techniques. Both animals under inverstigation, the Indian stick insect Carausius morosus and the Australian tree frog Litoria caerulea, possess adhesive organs classified as “smooth pads”. The adhesive organ of the stick insects, the arolium, revealed to consist of a layered setup. Underneath the surface with its fine longitudinal ridges the pads showed a construction of several layers. A thin epicuticular layer was situated on a loose fibrous structure consisting of stiff fibres embedded in a soft matrix. These fibres originated from thick principal rods within the endocuticle. They were oriented perpendicular to the surface within the thick procuticle and branch into finer fibres near the surface. The adhesive organ of the tree frog is macroscopically smooth but revealed to consist of flat-topped epithelial cells that form a hexagonal pattern at higher magnification. The hexagons are also structured by a tightly packed array of so called nanopillars. Those were found to be approximately as tall as wide and exhibit a characteristic dimple on the top. The cytoskeletal elements of the epithelia cells appeared as a sponge-like structure with smaller pore-diameters near the surface. The anti-adhesive surface of the pitcher plant Nepenthes alata is characterized by a continuous wax-layer that covers the inner surface of the conductive zone of the pitchers. Focused ion beam cuttings revealed a sponge-like appearance of the wax layer with cross-linkings of the wax crystals that, at the outermost part of the layer, are orientated perpendicular to the surface. The whole wax layer had a thickness of about 3 µm. In the second part of the study the mechanical properties of the adhesive organs and the plant surface were determined by using atomic force microscopy (AFM) techniques. The arolium consists of two layers with different mechanical properties. The soft (mean Eeff of 12 kPa) outermost layer is about 100 - 300 nm thick, while the subjacent layer is much stiffer (mean Eeff of 600 kPa). AFM contact mode imaging revealed that the cuticle is mechanically anisotropic. We propose that the described layered structure of smooth adhesive pads , consisting of materials decreasing in elasticity towards the outer surface, represents a superior design to conform and adhere to substrates with roughness at different length scales. The toe pad, also soft and easily deformable, shows a somehow “inverse” setup regarding its mechanical properties: the outermost layer (first 300 nm) has an effective Young’s modulus equivalent to silicon rubber (mean Eeff = 14.4 ± 20.9 MPa; median Eeff = 5.7 MPa). Former studies with indentation depths of 50 – 350 µm show a much lower E-modulus (4 – 20 kPa). The functions of this kind of cuticle is discussed in terms of maximising adhesive and frictional forces by conforming closely to surface irregularities at different length scales and maintaining an extremely thin fluid layer between pad and substrate. The anti-adhesive surface of N. alata turned out to withstand high mechanical loads without breaking. The platelet-shaped crystals of the wax-surface stayed stabile under a maximum mechanical load of FN=250 nN, which is a far higher load than an ant (as the natural prey) would generate even when hanging vertically with just one claws on one single crystal of the surface. The mechanical stability was underlined by slip-off experiments that resulted in only minor amounts of crystal fragments on the adhesive organs of insects slipped down on the conductive surface. In the third part of the study we characterised and compared the anti-adhesive properties of the surface of the conductive zone of a pitcher and surfaces of polishing paper with different grain sizes. We were able to show that the surface profile parameters of the wax-surface were comparable to those of P4000 polishing paper. Both surfaces were sufficient to inhibit the adhesion of insects. Based on those finding we developed a theoretical model that describes a surface with a roughness of a certain length scale that prevents attachment of insects. On the one hand the roughness of such a surface is not high enough for an insect to adhere using its claws. On the other hand the roughness is too high for the arolium to deform into the profile to maximize the contact area to gain sufficient adhesion. In the last part I characterized the chemical composition of the tarsal secretion of C. morosus, as it plays an important role for adhesion on rough surfaces. Using different analytical techniques as solid phase micro extraction, gas chromatography, mass spectroscopy and Fourier-transform-interference-spectroscopy several components of the secretion were identify. The results suggest that the secreted liquid mainly consists of long chained alkanes, fatty acids, hexadecanoic acid and water. The non-polar substances seemed to form micelles within the water which generally appear as small droplets in light microscopy. The ratio of polar and non-polar components within this emulsion could affect the physical properties of the liquid and therefore the effectiveness of its adhesive properties.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Scholz, Ingo

Contributors dc:contributor

• Baumgartner, Werner

• info:eu-repo/classification/ddc/570
• Dynamischer Elastizitätsmodul
• Insekten
• Tarsus <Gliederfüßer>
• Adhäsion
• Funktionsmorphologie
• Konfokale Mikroskopie
• Laser-Rastermikroskopie
• Froschlurche
• Kannenpflanze
• Carausius morosus
• Biomechanik
• Elektronenmikroskopie
• Haftorgan
• Biowissenschaften, Biologie
• Ultrastruktur
• Oberflächeneigenschaften
• ultrastructure
• biomechanics
• surface properties

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113815%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51530