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Functional Surfaces in Biology: Mechanisms and Applications 393

responsible for maintaining thermal balance (Karasev, 1989). An additional function of such
coverage is preventing water loss. This function was suggested for leaf-like bristles at the body
margins in Aphididae (Auchenorrhyncha) (Heie, 1987). Surfaces of desert plants are covered with
hydrophobic wax-crystalloids decreasing water loss through the cuticle.

15.7 SOUND GENERATION

Highly specialized areas of cuticle, responsible for sound generation, usually consist of patterns of
cuticular plates, seldom of microtrichia fields (Hinton, 1970). When these surfaces slide over each
other, sound is generated. Such structures have been previously described from elytra, abdomina,
and coxae in phylogenetically distantly related arthropod taxa: spiders (Aranei), crustaceans
Trizopagurus, bugs Cimicomorpha, Pentatomorpha, and beetles Geotrupes (Scarabaeoidea)
(Gogala, 1984; Starck, 1985; Field et al., 1987; Palestrini et al., 1987). The sound frequency
corresponds to the periodicity of these structures contacting functionally corresponding surfaces,
and the speed of sliding. These systems should be very wear-resistant. However, rigorous experi-
ments supporting or rejecting this statement are absent in the literature.

15.8 DEFENSE, GROOMING, SAMPLING, FILTRATING, GRINDING

Long, stiff, sclerotized cuticular spines are often used as defense mechanisms against predators.
These structures are particularly widespread in the marine arthropods, such as zoea of crabs
(Morgan, 1989). A similar function was described for long setae of certain beetle larvae from the
family Dermestidae (Nutting, 1963).
Grooming is a very important function for insects, which sometimes live in extremely dirty or
dusty environments. Their rich sensory equipment has to be kept clean in order to respond adequately
to external signals. Many Hymenoptera bear modified leg spines specialized for the cleaning of
antennae (Scho ¨nitzer and Lawitzky, 1987; Francouer and Loiselle, 1988). In aquatic environments,
the problem of grooming is even more important because of biofouling. Pinnoterid crabs (Decapoda,
Brachiura) use epipodite lobes covered by bristles for cleaning the gills (Pohle, 1989).
The collection of pollen grains and food sampling are functionally similar to grooming. They
require similar surface structures and similar motorics. In bees (Apoidea), systems responsible for
collecting pollen grains are usually equipped with urticating bristles (Pasteels and Pasteels, 1972;
Hesse, 1981). Mouthpart surfaces used for scratching food particles from substrates appeared
independently in the evolution of such phylogenetically far-related animal groups as insects (Figure
15.3h), crustaceans, molluscs and even some vertebrates (Arens, 1989). When food is sampled
from rough hard substrata, surfaces of sampling devices show traces of wear. Cuticular protuber-
ances that grind food items occur not only in the insect mouthparts, but also in one part of the
insect digestive system, called the proventriculus. Specialized spines on the inner surface of
the proventriculus have been reported in insects (Richards and Richards, 1969). Similar teeth are
found in crustacean stomachs.
Filtration systems are usually equipped with long bristles too. Such systems are well-known
from mouthparts of aquatic insect larvae. The filtering system of insect spiracles that keeps the
tracheae free of dirt particles is composed of branched acanthae (Figure 15.3g). However, in some
cases, completely different principles may be involved in the design of these systems. The labellum
of many flies (Brachycera, Diptera) bears so-called pseudotracheae (Gracham-Smith, 1930; Elzinga
and Broce, 1986). Outgrowths of the pseudotracheae have a complex material design. Labellum and
pseudotracheae are driven by muscles, resilin springs, and hydraulic pressure, which enable a
change in the diameter of the filtration sieve, depending on the size of the particles in the food
(Figure 15.3f).