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Functional Surfaces in Biology: Mechanisms and Applications 387

Figure 15.5 Dispensing system of the tenent seta in the syrphid fly Episyrphus balteatus. (From Gorb, S.N.
(2001) Attachment Devices of Insect Cuticle. Dordrecht, Boston, London: Kluwer Academic Publishers. With
permission of Springer Science þ Business Media B.V.) (a, b) SEM (a) and TEM (b) micrographs of the tenent
setae, (c) diagram of position of the seta on the substratum. Dotted area indicates lipid-containing secretion. Small
arrows indicate the route of secretion release. Large arrow indicates direction of pulling force. DL, dense layer; LU,
lumen; PL, end plate.

the flexibility of the material of the attachment structures, both mechanisms can maximize the
possible contact area with the substrate, regardless of their microsculpture (Figure 15.6a,b,e,f).
Tenent setae are relatively soft structures (Figure 15.6c,d). In Calliphora flies, their tips are usually
compressed, widened, and bent at an angle of 608 to the hair shaft (Bauchhenss and Renner, 1977).
When walking on smooth surfaces, these hairs in flies and beetles produce a secretion, which is
essential for attachment (Ishii, 1987; Gorb, 1998).
Different forces may contribute to the resulting attachment force: capillary adhesion and
intermolecular van der Waals forces. Geckos, which possess hairy attachment system, do not
produce any secretory fluid in contact area. Different authors have carried out force measurements
of gecko attachment system, at the global and local scales, and found evidences for contribution of
both van der Waals and capillary forces, generated by the layer of absorbed water, to the overall
adhesion (Hiller, 1968; Autumn et al., 2000; Autumn and Peattie, 2002; Huber et al., 2005). The
action of intermolecular forces is possible only at very close contact between surfaces. The forces
increase, when the contacting surfaces slide against each other. This may explain, why flies placed
on a smooth undersurface always move their legs in a lateral–medial direction (Wigglesworth,
1987; Niederegger and Gorb, 2003). During these movements, pulvilli slide over the surface
obtaining optimal contact. A contribution of intermolecular interaction to the overall adhesion
has been shown in experiments on the adherence of beetles (Stork, 1980) and beetle setae (Stork,
1983) on a glass surface. The presence of claws, decrease of air pressure, decrease of relative
humidity, or electrostatic forces do not influence beetle attachment on the smooth substrata. In the
beetle Chrysolina polita (Chrysomelidae), the resulting attachment force directly depends on the
number of single hairs contacting the surface. Recently, the contribution of intermolecular inter-
action and capillary force has been demonstrated for the fly, Calliphora vicina, in a nanoscale
experiment with the use of atomic force microscopy (Langer et al., 2004). Smooth systems are
composed of cuticles of unusual design (Figure 15.6g,h). The key property of smooth attachment
devices is deformability and the softness of the pad material. Viscoelastic properties have recently
been demonstrated (Gorb et al., 2000; Jiao et al., 1999).