Page 362 - Biomimetics : Biologically Inspired Technologies

Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c013 Final Proof page 348 21.9.2005 11:51pm

348 Biomimetics: Biologically Inspired Technologies

Figure 13.2 (See color insert following page 302) A cuttlefish (Sepia officinalis) can change its appearance
accordingto the background. Here the animal changes its body patternwhen movedfrom a sandyor gravel substrate
to one with shells. (Courtesy of Roger T. Hanlon, Senior Scientist, Marine Biological Laboratory, Woods Hole, MA.)

expanded. More remarkably, however, cephalopods can maintain countershading when they
become disorientated. The countershading reflex ensures that chromatophores on the ventral
surface of the entire body expand when the animal rolls over on its back: a half-roll elicits
expansion of the chromatophores only on the upper half of the ventral body. Such a response is,
of course, possible only in an animal whose chromatophores are neurally controlled (Ferguson et al.,
1994). When matching brightness, the chromatophores act like a half-tone screen; color matching is
achieved with the chromatophores, iridophores, and leucophores (Hanlon and Messenger, 1988).
On variegated backgrounds, a cuttlefish will adopt the disruptive body pattern, whose effect is
to break up the ‘‘wholeness’’ of the animal (Figure 13.2). Disruptive coloration is a concealment
technique widespread among animals. Octopus vulgaris has conspicuous frontal white spots;
loliginid squids show transverse dark bands around the mantle that probably render the animal
less conspicuous, and the harlequin octopuses have bold black-and-white stripes and spots.
Although many animals use patterning for concealment, it is nearly always a fixed pattern.
Because they control their chromatophores with nerves and muscles, cephalopods can select one of
several body patterns to use on a particular background.
Cephalopods also produce threatening or frightening displays. In its extreme form, the animal
spreads and flattens, becoming pale in the middle and dark around the edges, creating dark rings
around the eyes and dilating the pupil, and in sepioids and squids, creating large dark eyespots on
the mantle. This effect is extremely startling. The animal also seems to get bigger.
13.3.5 Translucent Camouflage

The best way to avoid being seen is to be invisible and so cast no shadow. The equivalent of
translucence is to present the observer with the scene which the object is blocking out. In a
technical world this can be done using a camera to film the scene that is blocked and presenting
it to the observer in front of the object.
Whole animals (e.g. pelagic marine organisms such as jelly fish, sea gooseberries, and many
larval forms) or parts of animals (e.g. the cornea of the eye) can be translucent and therefore nearly
invisible. To be translucent, reflection of incident light must be kept to a minimum and light must
be neither scattered nor absorbed as it passes through the body. Scattering is caused by variations
in refractive index. Animal tissue normally has many variations in refractive index (cells, fibres,
nuclei, nerves, and so on). The most important factors are the distribution and size of the
components; refractive index is less important; the shape of the components is least important.
For instance, if a cell requires a certain volume of fat to survive but must scatter as little light as