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Functional Surfaces in Biology: Mechanisms and Applications 391

Figure 15.9 Plant antiwetting surfaces mediated by wax crystalloids. Tubules in Prunus domestica (a)
and Chelidonium majus (b), polygonal rodlets in Acer negundo (c), terete rodlets in Brassica oleracea (d).
(From Gorb, E.V. and S.N. Gorb (2002) Entomologia Experimentalis et Applicata 105: 13–28. With permission
of Springer Science þ Business Media B.V.) Inset shows the water droplet on the plant surface covered with
wax-crystallites.

to rolling water droplets. A series of experiments revealed that surfaces of such plants as Nelumbo,
Colocasia, and Brassica are richly covered by wax crystallites, which are responsible for the
cleaning effect of their surfaces (Barthlott and Neinhuis, 1997). Similar adaptations have also
been described for animal surfaces. Wings of insects (Odonata, Ephemeroptera, and Neuroptera)
are covered with wax crystallites with dimensions that are comparable to those found in plants.
These surfaces have been experimentally proven to be extremely nonwettable (Wagner et al., 1996).
Wax crystalloids on the flowering shoots of plants, such as Salix spp, Hypenia, and Eriope are
adaptations to prevent crawling insects from robbing nectar and other resources (Eigenbrode,
1996). The wax blooms of ant-plants from the genus Macaranga seem to be an ecological isolation
mechanism for the symbiotic ants. This mechanism is based exclusively on the influence of the ant
attachment abilities, but not on the repellent effects of the wax. The comparison of surfaces in
different species of the Macaranga revealed a high correspondence between the occurrence of wax
coverage and obligatory ant associations (Federle et al., 1997). To explain the anti-adhesive
properties of plant surfaces covered with waxes, several hypotheses are proposed: the roughness-
hypothesis, the contamination-hypothesis, the wax-dissolving-hypothesis, and the fluid-absorption-
hypothesis (Gorb and Gorb, 2002).
Many aquatic and semiaquatic arthropods have sculptured surfaces involved in holding air
underwater for respiration. Such surfaces, called plastrons usually contain fields of microtrichia
(Heckmann, 1983). These structures appear convergently in various arthropod taxa, as an adapta-
tion to aquatic environments: Collembola, Lepidoptera, Coleoptera, Heteroptera, Diptera, Araneae,
and Diplopoda (Thorpe and Crisp, 1947; Hinton, 1976; Messner, 1988). Some terrestrial insects,
such as Aphididae (Auchenorrhyncha), also bear similar structures in the form of bristles, mush-
room-like spines, or stigmal plates, which can protect their surfaces from moisture (Heie, 1987). In
water striders and some spiders, antiwetting surfaces of legs and ventral body side are involved in
the locomotion mechanism of walking on the water surface (Figure 15.3i,j).

15.5 OPTICS

Structural coloration, due to the presence of scales and bristles, is well-known in insects such as
butterflies (Ghiradella, 1989) and beetles (Schultz and Hadley, 1987). For example, scales of the
scarabaeid beetles from the genus Hoplia bear additional microtrichia on their surfaces responsible