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Defense and Attack Strategies and Mechanisms in Biology 349

possible, it is best to divide the fat into many very small droplets. Slightly worse is to divide it into a
few large droplets, but the very worst is to divide it into drops about the size of the wavelength of
light (Johnsen, 2001).
Variations in refractive index do not always cause scattering. If the refractive indices vary by
less than half the wavelength of light, the scattered light is eliminated by destructive interference
and the light waves overlap in such a way that they cancel each another. This happens in the cornea
of the eye, which is constructed of an orthogonal array of collagen fibres.
Many organisms living in the deeper ocean, where there is little or no ambient light to be
reflected or by which camouflage color can be seen, produce their own light. The organs that do this
— photophores — can be mounted on mechanisms which rotate them so that they face the body and
are effectively obscured, hence can be modulated and switched on and off (Johnsen et al., 2004).

13.3.6 Reflecting Camouflage

If an object can simply reflect the color and pattern of its surroundings, then it will be adaptive. But
if it merely reflects the sky when looked at from above, or the ground when looked at from below,
this will be ineffective. The geometry of the reflecting surface is crucial. In deep water, the laterally
scattered light is equal in intensity from a range of angles. Looking up, one sees brightness; looking
down there is dim blue-green. A perfect mirror suspended vertically in the water would be invisible
since the light from the surface is reflected to a viewer below, making the mirror appear translucent.
Many fish have platelets of guanine in their scales arranged vertically, thus generating such a mirror
independently of the shape of the section of the body. The fish is also countershaded. Viewed
laterally the fish is a reflector and therefore invisible. Viewed from the top, it is dark like the depths
below it. Viewed from below it is silvery white like the surface.
The most difficult view to camouflage is that from directly below when the fish obscures light
from above. Many clupeids, such as the threadfin shag Dorosoma petense, are thin and come to a
sharp edge at the belly. This allows light from above to be reflected vertically downwards over the
entire outline (Johnsen, 2002).
Another form of reflecting camouflage is provided by the cuticle of some scarab beetles. The
cuticle is made of structures that look like liquid crystals, mainly nematic and cholesteric. Thus, of
the incident light on the cuticle, the right circularly polarized component can be reflected and the
left circularly polarized light can penetrate the helicoidally structured cuticle. However, at a certain
depth, there is a layer of nematic structure that acts as a half-wave plate, reversing the sense of
polarization of the light, which is then reflected when it reaches the next layer of helicoidal
structure, has its sense of polarization reversed again by the nematic layer, and continues back
out through the helicoidal cuticle with very little loss. The refractive index of the cuticle is
increased by the addition of uric acid. Thus the cuticle is an almost perfect reflector, making the
beetle appear the same green as its surroundings. This system will work only when the color and
light intensity are the same in all directions (Caveney, 1971).

13.3.7 Motion Camouflage

This is included here since it is a way of observing and approaching an object without making it
obvious to an observer or the object that it is being observed. The technique might have been
unintentionally deployed by attacking fighter aircraft, and is currently in development for disguis-
ing the intended target of guided missiles. An everyday equivalent, converted to the acoustic
environment, would be that if you are following someone closely, make sure that the noise of
your footfall is in synchrony with that of your quarry.
This is a stealth shadowing technique used by, for instance, the dragonfly approaching its prey
on the wing. The dragonfly follows a path such that it always lies on a line connecting itself and a
fixed point. Then the only visual cue to the dragonfly’s approach is its looming (i.e., the increase in