An easy question is why such arrays of color? Insects share the same challenges as humans and so they use color and patterning for species and mate recognition, camouflage, startling potential predators, and mimicry. Energy is also almost certainly a factor: dark colors absorb more heat, and butterflies, for example, may use pigments and possibly interference mechanisms to increase the absorption of infrared. It can only be speculated as to why structural colors predominate at the short end of the visible spectrum. As a biological material, cuticle is assumed to have a limited range of refractive indices and if so, only shorter wavelengths may be refracted and scattered effectively enough to produce the needed effects. It could also be that short-wave structural colors are metabolically "cheaper" (i.e., require less energy to produce) or easier to make than short-wave pigments, which do seem relatively rare in biological systems. Further study may enlighten both biologists and engineers.
How are these structures and color patterns made? On one time scale the question is developmental: how can an animal transform its genetic information into the complicated structures observed? This is the general question of pattern formation, the nested series of instructions that must be carried out by a developing organism on many levels at once. A developing butterfly must specify, for example, the general shape of its wing, the precise venation pattern, the distribution of scale and/or bristle types on both sides and on all edges of the wing, the distribution of pigment(s), and finally whether scales are to be structurally colored and if so, what type of structure they are to have. Nijhout has presented a compact and authoritative review of pattern formation in butterfly wing systems; many other researchers are currently studying the molecular and genetic mechanisms underlying pigment formation and deployment. Common themes are emerging, but much still remains to be done, especially on the role of physical forces that almost certainly work along with the biochemical ones to bring forth the final form.
The formation of the microarchitecture underlying structural color systems is less well understood. Ghiradella in 1998 reviewed what was known about development of structural colors in scale systems, and Neville in 1993 presented a comprehensive review of the formation of helicoidal and other fibrous composite systems. However, despite their value as potential models for human research and development, particularly of optical systems, very little is known about these systems. There surely are lessons to be learned here. For example, Bragg lattices are of interest to engineers seeking more efficient transmission of information along optical fibers, and scale optics is becoming of interest to the photonics research community, which is seeking to develop structures and materials that can control light for purposes of communication, paints, surface coatings, electronic displays, etc. Again, the insect systems have a lot to teach us, especially since their structures are made at room temperature and without toxic solvents.
On the longer time scale, how did these systems evolve? In some examples there are grounds for speculation. As mentioned above, many pigments may have originally been metabolic by-products that, because of actual or potential ability to absorb some wavelengths of light, were somehow co-opted for purposes of display. The helicoidal arrangement of chitin fibrils in cuticle is part of a larger structural adaptation of cuticle as a building material. As in all skeletons, fibril orientation in cuticle is tailored to local challenges. Helicoidal arrangements, with their multidirectional fibril orientation, are well equipped to to provide toughness and strength in the face of multidirectional stresses and are common in areas exposed to such stresses. Having evolved such a helicoidal arrangement to confer a particular type of strength, the animals needed only to make the pitches of the helices regular and to tune them to have fine iridescent reflectors at the same time.
The evolution of the thin-film, diffraction, and other systems is at present a very open question. They appear to be of great antiquity. Parker reported diffraction and antiglare structures in Burgess shale fossils and suggested that the emergence at the beginning of the Cambrian period of image-forming eyes (to quote Parker, "...the lights were effectively turned on.") may have produced extreme selection pressure for potential prey animals to develop rigid armor (with its inherent potential for forming structural colors) and at the same time a need for, and an opportunity to develop, camouflage, recognition patterns, and all the other common uses and expressions of biological color. While there is no question about the utility of these color mechanisms, it is still hard to imagine how so much can have been accomplished, even with millions of years of research and development in a competitive and presumably highly selective world.
To this point coloration has been considered in terms of passive and static displays on the surfaces of insects. But in living insects, the color-producing structures are situated on a moving body with moving appendages, and so the displays are modulated over time. The resulting signals are four dimensional, which adds to them a richness of information that we cannot begin to appreciate, especially because the true capabilities of the insect eye (which is much "faster" than that of the human) in its processing of either color or movement are not known.
The subject of insect mastery of light must also include bioluminescence. Lantern types and flash patterns come in a variety of forms; superficially, the mechanisms by which they are produced seem to differ radically from those already discussed. But here too, the insect displays mastery of architecture—in the design of the lantern cells themselves— and of chemistry to create light signals that can be controlled in space and time, but at those times of day when sunlight is not available to power the display. In doing so, the insects have truly made "the lights come on," replacing in their signaling the warmth of sunlight with their own cold light. As researchers continue to learn about these systems, they are exploring worlds within worlds of complexity and can only gain in appreciation of the enormous capabilities of biological systems in their communication with their environments . and with each other.
See Also the Following Articles
Aposematic Coloration • Bioluminescence • Cuticle •
Eyes and Vision • Industrial Melanism • Integument
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