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First published online December 2, 2005
Journal of Experimental Biology 208, 4651-4662 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01939

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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

E. Gorb^1,*, K. Haas², A. Henrich², S. Enders¹, N. Barbakadze¹ and S. Gorb¹

¹ Evolutionary Biomaterials Group, Department Arzt, Max Planck Institute for Metals Research, Heisenbergstrasse 3, 70569 Stuttgart, Germany
² Institute of Botany, University of Hohenheim, Garbenstrasse 30, 70593 Stuttgart, Germany

View larger version (148K): [in a new window]   Fig. 1. SEM micrographs of the waxy zone. (A) The upper wax layer (intact surface). (B) Wax crystals of the upper wax layer (intact surface). (C) The lower wax layer (surface after treatment with cold chloroform; the upper wax layer is removed). (D) The lower wax layer (surface after application of the dental wax; the upper wax layer is removed). (E) Waxy zone after complete wax removal (surface after treatment with hot chloroform).

View larger version (102K): [in a new window]   Fig. 2. TEM micrographs of the isolated wax crystals from the upper wax layer without coating (A) and after sputter-coating with carbon–platinum (B,C). Arrows in B and C show the direction of coating. Arrowheads in A and C indicate `stalks' of wax crystals.

View larger version (29K): [in a new window]   Fig. 3. Distributions of the aliphatic compound classes by chain length, obtained by successive extraction steps (first with cold chloroform, then with hot chloroform) from the waxy zone of Nepenthes alata pitchers.

View larger version (21K): [in a new window]   Fig. 4. Dependence of (A) hardness and (B) elasticity (E) modulus on indentation depth in the intact waxy surface containing both wax layers (filled triangles) and the surface treated with dental wax and bearing the lower layer only (open triangles). (C) The effect of contamination of the diamond tip. The curves in A and B represent mean values of 200 indentation tests performed on each sample type.

View larger version (24K): [in a new window]   Fig. 5. Dependence of (A) hardness and (B) elasticity (E) modulus on indentation depth in the intact waxy surface (filled triangles), the surface treated with the dental wax (open triangles), and samples covered with wax material extracted with cold (filled circles) and hot (open circles) chloroform. The curves represent mean values of 200 indentation tests performed on each sample type.

View larger version (87K): [in a new window]   Fig. 6. Attachment organs of the female of Adalia bipunctata beetle, visualised by SEM. (A) Ventral aspect of the tarsus. (B) Setal covering of the second tarsomere. (C) Setae of the second tarsomere. (D) Setal tips (spatulae). CW, claws; TA1–TA3, tarsomeres.

View larger version (18K): [in a new window]   Fig. 7. Maximal friction force generated by beetles Adalia bipunctata on test surfaces. (A) Comparison of raw data. (B) Comparison of normalised data. For each insect individual, the force produced on a plant substrate was compared with the force on glass, considered to be 100%. According to Dunn's test of multiple comparisons of means, performed after ANOVA, means with different letters differ significantly from each other.

View larger version (157K): [in a new window]   Fig. 8. Adhesive pads of Adalia bipunctata beetles after walking on different surfaces: waxy surface after treatment with hot chloroform (A); waxy surface after dental wax treatment (B); and the intact waxy surface of the pitcher (C–E).

View larger version (24K): [in a new window]   Fig. 9. Scheme explaining the role of the two wax layers in the reduction of the insect attachment force. (A–C) The intact surface with both wax layers. (D–F) The pitcher surface with the lower wax layer only.