Bases Of Structural Colors In Insects

As mentioned above, because scattering colors (whites and Tyndall blues) can be produced by granules or droplets as well as hard structures, these may come from the epidermis and internal tissues, as well as from the integument. Interference colors, which require stable structures to produce them, are limited to the cuticle and its investiture. Figure 6 shows diagrammatically a patch of cuticle with its two basic layers, the thin outer epicuticle and the inner procuticle. The procuticle commonly shows the helicoidal arrangement described above, which results in a banded or layered appearance in section. In hard or stiffened cuticle, the procuticle is commonly further subdivided into a cross-linked, more tightly woven distal exocuticle and a basal, more loosely structured endocuticle.

The cuticular surface and the exocuticle are most likely to be modified to produce structural colors, although in some insects the endocuticle may be as well. Several possibilities exist (Fig. 6). For example, the surface may be invested with layers of scales and/or bristles (Fig. 6a), which carry the color, especially in the Lepidoptera (see later). Alternatively, it can be sculpted into a series of nipple-like protuberances (Fig. 6b—more about this later) or into the fine grooves that characterize diffraction gratings (Fig. 6c). In the exocuticle (and sometimes the endocuticle) metallic colors can be produced by stacks of thin films of alternating refractive indices (n = 1.58 alternating with n = 1.38 has been measured in one of these systems) (Fig. 6d) or by appropriately tuned helical rotation of the chitin fibrils (Fig. 6e). (As yet another example of insect command of light, in many corneas, the helicoidal architecture of the cuticle is tailored not to produce structural colors but to control refractive index, so that incoming light is appropriately focused as it enters the ommatidia.)

Scales and bristles are particularly impressive in the variety and complexity of their architecture (Fig. 7). They commonly exist in two and sometimes three layers on the body or wing surface (Fig. 7a), and each layer may be modified in shape and color. A typical scale consists of a flattened sleeve of cuticle whose lower surface (that toward the wing) is relatively featureless, whereas the upper surface is elaborated into a reticular network of longitudinal ridges joined at intervals by

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