The wings of a Morpho butterfly from Brazil shimmer a strange and potent blue, creating an impression that the insect sheaths itself in metallic satin. The wings' optical verve reportedly can be seen from aircraft flying above the forest. Equally dazzling are many other curiously brilliant colors in nature. The rippling from gold to green of a Japanese beetle's wing covers. The blue glow of a neon tetra in a tropical fish tank. The pearly pinks and blues of an abalone shell. The deep purple of a raven's back or the red of a ruby-throated hummingbird. Even the gleaming rainbow of an oil slick in a wet gutter. These are examples of one of nature's most spectacular optical tricks -- creating color without pigment. It's a natural light show in which waves of ordinary light are separated, combined, reflected and otherwise manipulated to create a phenomenon called iridescence. The word comes from Iris, the Greek goddess of the rainbow. Light comes in many wavelengths, each corresponding to a specific color. In the range of hues visible to the human eye, violet results from the shortest wavelengths and red from the longest. Ordinary light, sometimes called white light, is a mixture of different colors. Thus, when an object appears a certain color, it is because it absorbs most wavelengths and reflects only some to our eyes to produce the color we see. Iridescent objects do something very different. They unmix the brew of colors in white light by quelling some and intensifying others and then send the result to our eyes. The result is a brilliance, an intense, penetrating color that no simple reflection of light can achieve. Three centuries ago, the great English scientist Isaac Newton, thinking about the iridescent colors of peacock feathers, surmised that they were not produced by colored substances. He speculated that such colorful effects come from the way light interacts with tiny, invisible shapes of the objects. Physicists now know that is exactly what happens. The colors are produced by the way certain kinds of surfaces reflect light. There are two kinds of surfaces that can produce iridescence -- thin films such as those of a soap bubble and microscopic wrinkles or grooves such as those built into the scales of a butterfly. The diagrams on this page explain how they work. The resulting hues are called physical colors to set them apart from colors caused by chemical pigments such as those in paint. As it happens, some everyday objects, such as a compact disc, inadvertently create the same effect. Newton had good intuition about peacock feathers. But before researchers could understand the phenomenon better, it took a new understanding of the wave-nature of light and the evolution of the scanning electron microscope (SEM) in this century. What has unfolded before biologists' eyes is a world whose structural beauty matches the dazzle of its colorful optical effects. Consider the enormous biological order Lepidoptera, which includes butterflies such as the iridescent Morpho. Most butterfly colors such as the yellows and rusts of the monarch do come from chemical pigments. The same goes for some greens and blues, but many of these colors and the dazzling and metallic optical effects of iridescence come from exquisitely tiny and detailed architecture of the surfaces of the scales covering the wings. No one has studied and appreciated these natural masterworks more than Helen Ghiradella, a biologist at the State University of New York at Albany. For more than 20 years there, she has been training electron microscopes and other research tools onto butterfly and moth scales, which she rates as "marvels of biological construction." Ghiradella's low-magnification SEM image of the wing of a Morpho menelaus shows that it is covered with overlapping rows of scales, each no wider than the thickness of a human hair. Each scale looks like a shovel blade plugged into the wing membrane by a slim handle. At higher magnification, each scale reveals a far more elaborate structure. This is what makes the Morpho such a looker. From the relatively flat surface of each tiny scale rises a series of parallel ridges, each made of stacks of yet finer layers called lamellae. Between each adjacent pair of lamellae, in turn, are sandwiched a series of still tinier "microribs" roughly perpendicular to the lamellae stacks. Within the width of a scale can be 50 microribs. This is rococo architecture writ tiny. Such stacked, semitransparent structures produce one of the two types of iridescence, the one caused by "thin film interference." In this phenomenon, incoming light partially penetrates and partially reflects from each layer. The distances between the layers determines which colors are reflected and intensified and which are suppressed. Besides ridges, lamellae and microribs, Ghiradella and the few other researchers in this field have found other remarkable microstructures. The bristle scales of the Astraptes azul butterfly, for example, are made of rolled-up layers like a sheet of paper rolled into a tube. The distance between the layers is roughly one-fourth the wavelength of green light. So this part of the wing looks iridescent green. The bristle is a biological version of what physicists call "quarter-wave interference mirrors," which are at the heart of many solid-state lasers made from multiple-layer sandwiches of semiconductor crystals. In both cases, specific wavelengths from a source of light emerge more brilliant in a process by which their many reflections within the mirror interfere constructively and add to each other while reflections of other wavelengths (colors) interfere destructively, becoming dimmer or disappearing. What is the effect in Astraptes? "Bristles of fire" is how Ghiradella describes the insect's scales when she views them under modest magnification. The beautiful green of the scales of yet another butterfly, Parides sesostris from Peru, derive from yet another intricate structure. In addition to ridges and microridges on the scale surface, the scale's interior harbors a honeycomb-like "diffraction lattice" whose cavity-to-cavity distance is about 0.26 microns, just right for yielding the insect's green color. What amazes Ghiradella most is that each scale develops from a single epidermal cell. Like human skin cells programmed to die and form a tough protective coating, butterfly epidermal cells also perish to fulfill their role. As the cell dies, it dries out and shrivels, its surface buckling, folding, creasing and crenolating in a precisely predictable way. "Each scale or bristle is made by a single epidermal cell, which must lay out the basic architecture, specify the pattern and its details and secrete and shape the cuticle which makes up the final structure," Ghiradella writes in one of her articles. Because "even the simplest scales are highly structured, scale formation is a virtuoso exercise in biological pattern formation at the cellular level." Beyond understanding of the source of iridescent color, Ghiradella is trying to help establish a deeper realm of knowledge. One of the fundamental mysteries of biology is how cells determine their overall architecture -- how, in other words, a red blood cell "knows" to be round and a nerve cell "knows" to be long and spindly. Biologists know that cells of all species are fundamentally alike and that if a mechanism can be understood in cells of one species, it is likely to play a role in all. In butterfly scales, the scenario starts during the insect's pupal stage, the time when it undergoes metamorphosis from a caterpillar into a winged adult. First epidermal cells secrete a covering, or cuticle, made mostly of chitin, the same carbohydrate-based polymer that makes crab shells. Then, as the cell dies and dries, forces of contraction cause the cuticle to buckle and curl, even to fracture in predictable ways, creating the final microarchitecture. Equally remarkable feats of microsculpture are responsible for the metallic green color of tiger beetles, the multicolored sheens of pigeon feathers, the polished-brass look of some scarab beetles, the greenish luster of annoying June beetles and even the iridescence of a strange species of blue fruit known as Elaeocarpus angustifolius. In every case, a close look at the structures responsible for the colors have awed researchers with nature's microarchitectural prowess. One of the most striking examples is manifest in the pupa stage of the tropical butterfly subfamily known as Danainae. In their natural habitat, these pupae look like lima bean-shaped nuggets of polished gold hanging from the undersides of leaves. According to two German researchers who investigated their optical properties in the 1980s, "the shining golden luster of these pupae is so perfect and covers the entire body surface except for a few pigment spots and lines that even biologists are often misled to believe in specimens artificially coated with gold for examination in a scanning electron microscope." Gold coating is a standard laboratory procedure. The researchers discovered that the pupa's surface consists of as many as 520 alternating layers of chitin-based cuticle and water with the distances between succeeding layers changing continuously. The result is a so-called "broad-band interference reflector" that happens to reflect the same range of colors with as much efficiency as a polished gold surface. Unlike the real metallic McCoy, the luster of the pupae disappears in remarkable series of color changes as their surfaces dry out. In another amazing example of metallic fakery, a scarab beetle species, Plusiotis resplendens, has, in the words of physicist George W. Kattawar of Texas A&M University, "the appearance of highly polished brass." What might be the biological significance of all of these special effects? There are plenty of ideas, but no one really knows. It is possible that the metallic reflection of the Danaines pupae and many silvery fish helps to make the organisms less visible to predators since the image they project might be perceived as a mirror of their surroundings. Tom Eisner, a chemical ecology expert at Cornell University, has speculated that the more brilliant blues and greens, along with satiny and iridescent effects, could serve as loud reminders to predators of the foul taste and poison contained within many of the colorful organisms. For the fruit, biologist David Lee of Florida International University and Fairchild Tropical Garden suspects that the dazzle enhances the likelihood that the fruit will be noticed and eaten and its seeds dispersed. On the other hand, many scientists concede that in some cases the optical magic of these organisms may be an inadvertent sideshow with no important biological role. They may serve no more purpose than the optical accident of rainbows in an oil slick in a gutter. CAPTION: Iridescent colors shimmer in soap bubbles, created by the interplay of ordinary light and thin films of colorless water. CAPTION: The most diverse exhibitors of iridescence are insects. This is just a small sample. CAPTION: Scales of a Morpho butterfly's wing, each about 0.0002 inch wide. In other words, it would take 5,000 of them side by side to cover one inch. CAPTION: Ridges and microribs in this piece of one scale from a Papilio zalmoxis interact with white light to produce a Tyndall blue color, which is like sky blue. CAPTION: Part of the surface of one satiny scale of a butterfly called Caligo memnon, magnified even more than in picture above. Pictures were taken with a scanning electron microscope. CAPTION: Cross section of a single scale cell in the process of forming during metamorphosis. Black dots are structures within the still-living cell called ribosomes. CAPTION: Many an observer has been fooled by the iridescent gold pupa of a butterfly from Sri Lanka, called Euploea core. The pupa stage of metamorphosis is also called chrysalis, from the Greek chrysos for gold. This one hangs from a laurel leaf. CAPTION: Iridescence can be sexy. Peacocks, for example, display plumes like these to attract peahens. The curiously intense colors of peacock feathers suggested to Isaac Newton that the cause was more than mere pigmentation. CAPTION: THIN FILM IRIDESCENCE (Soap, bubbles, oil slicks, butterfly wings) A. Constructive interference: Wave fronts are in step, color appear stronger B. Destructive interference: Wave fonts are out out step. Each wave cancels out the other. Color is absent. DIFFRACTION GRATING IRIDESCENCE (Butterfly wings, beetles, bird feathers) A. Constructive interference: Wave fronts are in step, color appears stronger B. Destructive interference: Wave fronts are out of step. Each wave cancels out the other. Color is absent.