Structural Colors

There are many mechanisms by which structural colors can be produced. All depend directly or indirectly on the fact that a particular piece of material scatters or refracts different wavelengths of light to different degrees. This property of the material can be expressed in terms of its index of refraction, n, a measure of the degree to which a given wavelength of light entering the material is "retarded" or slowed down. For insect cuticle, n typically ranges from 1.5 for long-wave (red) light to 1.6 for short-wave (UV) light, although in special cases n less than 1.4 has been reported (for comparison, n for air is by definition 1). Structural colors described so far in biological systems fall into two general classes, scattering and interference.

Scattering

Scattering of light occurs when white light encounters a distributed cloud or array of molecules, particles, or other

FIGURE 3 Diffraction (in this example, from a grating). Light hitting an edge or discontinuity gets bent or refracted to different degrees, depending on its wavelength. When it is then reflected, which of the component wavelengths are reinforced varies with the position of the observer, so that from one angle shorter wave light (SW) predominates, whereas from another, longer wave (LW) light predominates.

FIGURE 3 Diffraction (in this example, from a grating). Light hitting an edge or discontinuity gets bent or refracted to different degrees, depending on its wavelength. When it is then reflected, which of the component wavelengths are reinforced varies with the position of the observer, so that from one angle shorter wave light (SW) predominates, whereas from another, longer wave (LW) light predominates.

structures (Fig. 2). At least some of the component wavelengths of the beam will be reflected in random directions, including toward the observer. If the scattering agents are relatively large (700 nm or more), all visible wavelengths are scattered, and the resulting color is a matte white (the color of whole milk is an example of such scattering). If the particles are smaller (in the 400 nm range), the short wavelengths are scattered to a much greater degree than the long ones, which tend to pass on through the system and not reach the eye of the observer. The resulting color, Tyndall blue, is commonly seen in blue eyes and bluejay feathers; in insects it occurs in blue dragonflies and in some blue butterflies. Often, the blue structure is underlaid by a layer of ommochrome pigments, which, as mentioned above, deepen and intensify the color by absorbing stray light. Lacking such pigment backing, the blue is a dilute "powder" blue.

Interference

The general category of interference includes those situations in which the rays of a beam of white light are temporarily separated and then brought back together in such a manner that some have traveled a longer path than others. Depending on geometry, when the rays recombine, certain wavelengths are in phase and reinforced ("constructive interference"), whereas others are out of phase and cancel each other ("destructive interference"). The results are the brilliant, shimmering colors we call "iridescent." There are many ways of producing iridescence; this article considers only those of known importance in insects.

FIGURE 4 Two forms of interference from layers. (a) Thin film. A thin film can be described in terms of its optical thickness, its index of refraction, n, times its actual thickness, d. When white light encounters such a film, part of the light reflects from the top surface and part from the bottom. When these two beams recombine, those wavelengths four times the optical thickness of the film are constructively reinforced and the others not. If many films are stacked, light not reflected by the first film may be so by the others; if the films are alternated with others of equal optical thickness but of a different refractive index (so that n1d1 = n2d2), the stack reflects essentially all light of the reinforced wavelength. (b) Lattice. A lattice of points, spheres, or other structures reflects light in a manner analogous to that of a crystal. Each plane reflects part of a beam and transmits the rest (transmitted light not diagramed here). If the planes are evenly spaced, they reflect light the wavelength of which is twice the spacing, i.e., they will form a half-wave reflector. As in the case of thin films, with enough reflective planes, essentially all the light of the reinforced wavelength will be reflected.

diffraction Diffraction occurs when light strikes the edge of a slit, groove, or ridge. Different wavelengths bend around the edge to different degrees and the spectrum fans out into its components. If many such grooves or ridges occur in a regularly spaced array (for example, a "diffraction grating" such as that in Fig. 3) light of different wavelengths is reinforced at different angles so that the colors change with the position of the viewer (e.g., consider iridescent bumper stickers and other shimmering plastic labels). Many insect cuticles have fine gratings etched into them; these and the ridge and crossrib structures (see later) of some lepidopteran scales and bristles produce diffraction colors.

thin-film interference Thin-film interference involves, as the name implies, the interaction of light with ultrathin films of a material (e.g., iridescence from soap bubbles and oil slicks). Light reflecting from the top surface of such a film interacts with that reflecting from the bottom surface (Fig. 4a) and depending on the optical thickness of the film (its index of refraction, n, times its actual thickness, d), some wavelengths are reinforced and others not. Because the wavelengths of the reinforced light are four times the optical thickness of the film (i.e., a film of 100 nm optical thickness results in reflected light of wavelength 400 nm), such films are commonly called "quarter-wave interference reflectors" or "quarter-wave films." Because a slanted beam of light has to penetrate a greater thickness of film, thereby changing the effective optical geometry, thin-film colors shift toward the shorter wavelengths when the films are tilted with respect to the light source (e.g., the familiar blue of the morpho butterflies becomes more violet).

Of course any film thin enough to act as a quarter-wave reflector can catch and reflect only a portion of the incident light; the rest passes through. The presence of other films below the first increases the likelihood that light will be reflected, and in fact the most efficient of these reflector systems are stacks of thin films of the material in question, separated by other films with a different refractive index or by air (n = 1), so that the light is reflected from layers of alternating high and low n. If all the films are equivalent in nd, their optical thickness, the emerging colors are relatively pure, whereas varied spacing produces a less intense but broader range of reflection. As in all these systems, there may be behind the "mirror" a layer of pigment that intensifies the color by eliminating stray light that would otherwise interfere with the efficiency of the interference and thereby dilute the color.

lattices Many iridescent colors are produced, not by thin films per se but by thin-film analogues, systems that achieve similar effects without actual discrete films. One such mechanism is the "Bragg" or space lattice (Fig. 4b), a highly regular array of spheres or other units. Light entering such a lattice is reflected from the various layers, and the beams interfere in a manner analogous to that in thin-film stacks. In

FIGURE 5 This beetle shows the metallic coloration typical of many beetles and flies. The colors have at least two possible origins: they may be caused by a thin-film stack in the exocuticle (or sometimes the endocuticle) (Fig. 6d) or they may be the result of a helicoidal arrangement of chitin fibrils in the exocuticle (Fig. 6e). The latter effect is analogous to that produced by certain types of liquid crystal in common technological use. The red and black coloration in the eyes, on the other hand, is almost certainly pigmentary.

FIGURE 5 This beetle shows the metallic coloration typical of many beetles and flies. The colors have at least two possible origins: they may be caused by a thin-film stack in the exocuticle (or sometimes the endocuticle) (Fig. 6d) or they may be the result of a helicoidal arrangement of chitin fibrils in the exocuticle (Fig. 6e). The latter effect is analogous to that produced by certain types of liquid crystal in common technological use. The red and black coloration in the eyes, on the other hand, is almost certainly pigmentary.

this example, the wavelength reinforced is twice that of the spacing between the layers of the lattice, which therefore acts as a half-wave reflector. The familiar brilliance of the mineral opal is an example of this type of interference, caused in this instance by a lattice of tiny silica spheres. These lattices are very common in the biological world; those described so far in insects are "reverse" lattices, consisting of spheres of air in a matrix of cuticle.

helicoids The metallically colored cuticles of many beetles and flies (Fig. 5) either are thin film (Fig. 6d) or owe their iridescence to yet another mechanism, one analogous to that shown by the familiar and brightly colored liquid crystal displays in our electronic world. Cuticle is of course a composite of chitin fibrils in a complex matrix that is laid down sequentially in what can be considered a series of layers. If the fibrils in a particular layer are lined up in the same direction, the layer exhibits form birefringence, i.e., different indices of refraction parallel to and normal to the fibrils. In many cuticles, the layers precess, that is, each is laid down slightly rotated relative to the previous one (Fig. 6e). In essence, the structure can be considered a helicoid, and like all helical structures, it repeats itself with a certain spacing (called a "pitch"). As the layers precess, so does the difference in refractive index, so that viewed from a given direction a helicoidal array displays what are essentially layers of alternate high and low n, reminiscent of those in thin films. (Unlike thin films, helicoids also circularly polarize light, which insects may be able to see and which may therefore carry additional information to them.) If the spacing is regular and the pitch is appropriate, the helicoid behaves like a half-wave interference reflector, i.e., it reflects light of wavelengths twice the pitch. In the typical metallic cuticles,

FIGURE 6 How to make an interference color. A block of hard insect cuticle (bottom center) typically consists of a relatively thin epicuticle (here represented as a featureless covering layer) and an inner procuticle, which in turn consists of a distal exocuticle and an inner endocuticle (this diagram also shows the attendant epithelial cells). The layering of the procuticle is common in most (but not all) cuticles and is the visible manifestation of the helicoidal architecture of the chitin fibrils. Such a block of cuticle may be modified in any of several ways to manipulate light: (a) The surface investiture (the scales and/or bristles) may be modified to produce scattering or iridescent colors (see Fig. 7). (b, c) The cuticle surface may be sculpted into fine protuberances that serve as an antiglare coating (see Figs. 8 and 9) or into fine parallel grooves that act as diffraction gratings. (d) Part of the procuticle may be elaborated into a quarter-wave thin-film reflector stack. (e) The chitin fibrils of the exocuticle may be arranged in a "helicoidal" array, analogous to that in a liquid crystal and producing color by a similar mechanism. (The apparent parabolic bending of the fibers is an optical illusion.)

FIGURE 6 How to make an interference color. A block of hard insect cuticle (bottom center) typically consists of a relatively thin epicuticle (here represented as a featureless covering layer) and an inner procuticle, which in turn consists of a distal exocuticle and an inner endocuticle (this diagram also shows the attendant epithelial cells). The layering of the procuticle is common in most (but not all) cuticles and is the visible manifestation of the helicoidal architecture of the chitin fibrils. Such a block of cuticle may be modified in any of several ways to manipulate light: (a) The surface investiture (the scales and/or bristles) may be modified to produce scattering or iridescent colors (see Fig. 7). (b, c) The cuticle surface may be sculpted into fine protuberances that serve as an antiglare coating (see Figs. 8 and 9) or into fine parallel grooves that act as diffraction gratings. (d) Part of the procuticle may be elaborated into a quarter-wave thin-film reflector stack. (e) The chitin fibrils of the exocuticle may be arranged in a "helicoidal" array, analogous to that in a liquid crystal and producing color by a similar mechanism. (The apparent parabolic bending of the fibers is an optical illusion.)

the helicoids of the exocuticle are so tuned, and because the helicoidal arrangements of their fibrils resemble that of the molecules in one iridescent class of liquid crystals, they are often referred to as "liquid crystal analogs." Some insects intensify the effect by doping the cuticle with uric acid, which increases its birefringence.

0 0

Post a comment