FIGURE 7 Closer look at the investiture (scales and bristles), which in the Lepidoptera typically carries the color. Scales and bristles are complex cuticular structures each elaborated by a single cell, and they are often both pigmentarily and structurally colored. (a) As in Fig. 6a, a patch of cuticle surface showing several overlapping scales and one empty socket. (b) Diagrammatic view of a small fragment of a more or less typical unspecialized scale. The scale may be thought of as a flattened sac, the two surfaces of which are joined by fine pillars. (A bristle would be cylindrical, rather than flattened, but it is essentially the same type of structure.) The upper surface is a rectangular grid made up of longitudinal ridges (R) joined at regular intervals by transverse crossribs from which, in some species, hang pigment granules (arrows) (in other insects, pigment is incorporated into the cuticle itself). Ridges and crossribs together frame a series of windows opening into the interior of the scale. Virtually any part of this basic scale may be elaborated into a reflective structure. In the following examples, scales have been fractured to show their interior structures; lines indicate which basic scale structures have been elaborated to produce each structural color. [Modified from Ghiradella, H. (1998). Hairs, bristles and scales. In "Insecta." (M. Locke, ed.), vol. 11A of "Microscopic Anatomy of Invertebrates" (F. W. Harrison, ed.) pp. 257-287. Wiley, New York. Copyright 1998 John Wiley & Sons. Reprinted by permission of John Wiley & Sons.] (c) Papilio zalmoxis, fragment of upper scale surface. The ridges are low and unornamented, but the crossribs have "filled in" the windows with a network of "alveolae" that scatter light to produce a Tyndall blue color (compare Fig. 2). Bar, 1 jlm. (d) Morpho menelaus, fragment of deep blue iridescent scale, fractured longitudinally to show a side view of a ridge (R), together with the pillars that join it to the bottom layer of the scale. The ridge (and those behind it) has been elaborated into stacks of slanting thin films that reflect the characteristic blue of this butterfly. Bar, 1 jlm. (e) Urania riphaeus, fractured green iridescent scale (see Fig. 1). The ridges and crossribs are not particularly elaborate, but the interior of the scale is filled with a stack of thin films that produce the color. Bar, 1 jlm. (f) Teinopalpus sp., fractured green iridescent scale. The scale interior is filled with a space lattice that produces the color. Bar, 1 jlm.
FIGURE 8 Effects of an antiglare coating. Even though the wing of this clearwing moth is somewhat wrinkled and parts of it would therefore be expected to reflect light, its matte surface (Fig. 9) allows the text to be read through it with minimal loss.
transverse crossribs (Fig. 7b). Fine flutings or microribs line the sides of the ridges and sometimes run out across the crossribs. Slender pillars join top and bottom surfaces. Pigments in some groups (typically the Pieridae) may exist in discrete granules, whereas in other insects they are laid into the scale cuticle itself.
Virtually any part of this basic scale can be elaborated to produce a structural color. The spacing of the ridges and/or crossribs may be appropriate to produce diffraction colors. The crossribs and microribs may extend to fill in the windows with a network of "alveolae" that reflects Tyndall blue (Fig. 7c). The scale ridges may bear stacks of thin films (examples known so far reflect green, blue, or ultraviolet) (Fig. 7d). The interior of the scale may be filled with stacks of thin films tuned to produce green or blue (Fig. 7e), or it may contain a space lattice that reflects iridescent green (Fig. 7f). And, as mentioned earlier, these structural colors may be combined with pigments to give yet additional colors and effects.
More detailed study of some of these systems is revealing yet more complicated and sophisticated optical effects. For example, in blue Morpho butterflies, the deep blue iridescent scales (whose color comes from thin-film iridescence on the ridges—Fig. 7d) are overlaid by a layer of "glass scales," which, though otherwise transparent, do have iridescent ridges. The apparent function of the glass scales is to broaden the effective angle of reflection (see Vukusic et al., 1999). The iridescent scales of Papilio palinurus have stacks of internal thin films, but rather than being flat, the stacks are puckered into shallow cup-shaped depressions whose bottoms reflect yellow light, whereas the sides reflect blue, giving the human observer the sensation of green (see Vukusic et al., 2000)—it is not known why these animals have developed this mechanism to produce green scales when other iridescent greens are produced by more conventional thin films or by lattices. There are other intriguing scale and bristle types whose optics are now being studied, and from these insect systems new and sophisticated insights into the effective control of light can be expected.
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