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Insect inscriptions: hunting for human symbols in gently fluttering wings.

Insect Inscriptions

With its misty cascades, Iguassu Falls punctuates the already steamy border between Paraguay, Argentina and Brazil.

"There, for the first time, I saw butterflies with the number 89 on their wings," recalls nature photographer Kjell B. Sandved. "Some of them even have more than one number."

They're easy to photograph, too, he says. "That's because when you go there, you always sweat. They smell you and love it, and they alight right on your hand."

Before Sandved began his search for fluttering numerals, he had already found a variety of letters on the wings of Lepidoptera -- the diverse taxonomic order that includes the world's 200,000 or so species of butterflies and moths. The alphabet project sprang from a chance discovery made nearly 30 years ago, when Sandved was working as a volunteer at the Smithsonian Institution's National Museum of Natural History in Washington, D.C.

"I went up into a dusty attic of the museum," Sandved recounts. There he found boxes and boxes of uncatalogued Lepidoptera. "I opened one of these boxes and saw a beautiful letter F."

That was in 1961. From there, he says, "I started looking around for more letters"--and not just in boxes. Over the next 15 years, Sandved tracked down and photographed thousands of butterflies and moths in a quest that took him from the Peruvian Andes to the rain forests of New Guinea to the highlands of Malaysia. In the mid-1970s, he published his first series of photos featuring colorful pointillist letters emerging in the breathtaking arrangements of dust-speck lepidopteran scales.

Today, Sandved's office in Washington, D.C., is crammed from floor to ceiling with his photos, posters and slides. He has managed to assemble the entire alphabet several times over. He has also photographed wing versions of the digits zero through nine, ampersands and question marks, human-like faces and blinking eyes, Greek letters like [Omega] and [pi], the Scandanivian [Phi] and the Germanic umlaut vowels. He has even amassed a series of animal images from the wings of butterflies and moths, which he keeps in a black portfolio labeled "Noah's Ark."

"I have wings looking like algae, looking like shells and fishes and cacti," he says -- not to mention some remarkably detailed images of entire spiders and beetles, all discernible in the patterns of lepidopteran scales.

Through people tend to perceive Sandved's typographical discoveries in an anthropomorphic context, the wing patterns evolved to convey distinctly nonhuman messages. Sandved and other lepidopterologists, suspect, for instance, that the circles or O's seen on so many moths and butterflies evolved to appear as eyes. Many animals, including fish and birds, sport similar circular motifs. A striking example is the male peacock, which spreads its tail plume to reveal a profusion of vivid eyespots. A potential assailant might think twice before messing with those glaring eyes.

Large, dramatic eyespots on butterflies and moths may likewise discourage hungry birds and other predators. The spots can appear eerily lifelike; Sandved says he has found moths with eye designs that actually seem to blink when a hind wing eclipses a front wing.

On the other hand, small spots on wing edges might serve to divert a predator's attention from more critical tissues such as the insect's head and body, Sandved suggests. In such cases, he speculates, the lepidopteran logic goes as follows: "If my primary wing designs haven't concealed me from you or frightened you away, and you're determined to bite me, then at least bite here, on the edge of my wing, where I will be injured the least." He has photographed a butterfly with spidermimicking designs on the edges of its wings, where the unusual patterns might offer a similar last-ditch line of defense.

Some wing designs spill over to other parts of the anatomy. In Venezuela, Sandved found a moth with an L on two of its wings. During the day, when this moth needs to conceal itself, the Ls connect with lines on its other two wings and outstretched legs to give the insect the appearance of a shriveled leaf complete with "veins," he says. At night, the leaf comes alive, flitting about in search of food and mates.

Color has its own significance in the lepidopteran realm, and is one obvious attribute separating the moths from the butterflies. Moths, whose species vastly outnumber those of butterflies, rely mainly on the cloak of night for defense against predators. The pressure to conceal has driven their evolution, Sandved says. Rather than advertise their presence with bright colors, moths tend to blend into their surroundings with subtle earth tones. And because moths communicate with each other primarily by smell, their plain wrappers pose no handicap in courtship.

Butterflies, in contrast, communicate visually and are active during the day. "Butterflies are very visual creatures," says Thomas Eisner, a chemical ecologist and longtime butterfly researcher at Cornell University. "Their ability to spot one another from a distance is crucial."

The broad palette of visible colors tells only part of the story, he adds. About 20 years ago, Eisner and his colleagues showed that the intricate layered structures of scales in butterfly wings can produce intense ultraviolet reflections. These reflections, while invisible to people, provide important cues to other butterflies during courtship, he says.

Eisner has begun floating a new evolutionary rationale for butterfly colors. Butterflies distinguish themselves from other flying insects in part by their erratic flying styles, "and that makes them extremely difficult to catch," he notes. "Butterflies, I would argue, are colorful as a collective advertisement of their being hard to catch."

From the butterfly's point of view, Eisner fleshes out the logic behind this defense tactic: "I know you spotted me from 50 yards, but if you take off after me, you'll have a hard time catching me. And even if you grab me, I'm slippery because my scales slide off easily. You'll find that I'm all candy wrapper and no candy."

The thin, scaly wings don't make delicious or nutritious eating, Eisner says. Bright colors may help predators learn quickly that this erratic, slippery snack just isn't worth the effort.

Every exquisite pattern on a quivering wing reflects the special properties of the scales that combine, like pixels on a television screen, to create the overall design. Zooming down to the microscopic level of individual scales--each roughly 50 by 100 microns--reveals a breathtaking view of functional bio-architecture.

On a square centimeter of a typical wing, tens of thousands of scales attach with tiny stems and overlap like cedar shakes to form unbroken coverage. During the insect's pupa stage, individual wing cells metamorphose into "ornate hollow shells containing struts, pillars, pigment granules and/or more elaborate constructions," says Helen Ghiradella, a specialist in butterfly scale morphology at the State University of New York at Albany. A day or two before the adult emerges from the pupa, the watery interior of each scale cell disappears, leaving behind a tough, nonliving, light-manipulating microarchitecture. "The astonishing thing is that a single cell is doing all of this," Ghiradella says.

Each scale's color arises from an interplay of pigments or physical structures that sculpt incoming light into fantastic visual forms by some combination of absorption, reflection, diffraction, scattering and interference effects.

Browns and blacks come most often from melanin pigments, which are diffusely distributed throughout the scale, says research entomologist Donald Davis of the National Museum of Natural History. both the melanins and the pteridine compounds that generate the reds and yellows are by-products of metabolic activity that took place when the cells were still alive in the late pupa stage. Pteridines often reside in "little bags [pigment granules] hanging down inside the scale that you can see through little transparent windows," Davis says. The "windows" are the spaces between the struts and other structural elements that make up the scale's solid framework.

Some of the most breath-taking butterfly colors -- the brilliant and iridescent blues and greens -- emerge from the way light diffracts, refracts and scatters from the layers, lattices and ribbed walls of the scales, Ghiradella points out. For example, thin chitin (structural polysaccharide) layers stacked within the bodies of some scales cause incoming light waves to interfere with each other and produce iridescent colors like those seen in soap bubbles or oil films. Latticeworks that look like densely packed bubbles surrounded by a chitinous solid use both interference and scattering to produce an opal-like iridescence.

Ghiradella says she has found both types of structures in different scales of the same butterfly. The distances between individual layers and "bubbles" usually fall in the 300-to-700-nanometer range--the same range of electromagnetic wavelengths that humans see as colors. For some effects, such as reflecting shorter-wavelength ultraviolet light, the distances are even smaller.

Davis has examined multilayered scales in which chitinous layers alternate with "air blankets" at specific distances that determine which wavelengths, or colors, of incoming light will get reflected. These thin, chitin-air layers act "like stacked oil slicks," Ghiradella adds. In some cases, they function as prisms, refracting different colors at different angles so that the wings assume different hues depending on the observer's viewing point.

Besides serving as pixels for wing designs, scales may act as solar collectors to heat wing muscles, Davis suggests. "These insects have to raise their body temperature a certain amount before they can get their engines started in the morning," he says. That may explain why butterflies often appear to bask in the sun before flitting into the air.

The scales also serve a sacrificial role that enables butterflies and moths to get out of dangerous tangles. "When the insect hits a spiderweb, because its scales are loose, it can break free by shedding those scales," Eisner notes.

Whether admired for their architectural intricacies or for their many varieties of chromatic splendor, lepidopteran wings evoke fascination and delight. As Eisner puts it, "They give us a sense of the beautiful."

For Sandved, the sense of beauty deepens with each new discovery of a human symbol in these wild but gentle beings. He laments that the rapid disappearance of many butterfly habitats will bar researchers from retracing the evolutionary paths that have led to such a spectacular diversity of designs.

For Ghiradella, the microstructural elegance of the scales demonstrates that lepidopteran beauty is more than a mere cover. And for anybody strolling in a mountain meadow, hiking through an Appalachian forest or sweltering in a tropical jungle, the fluttering creatures serve as public announcements that art abounds in nature.
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Author:Amato, Ivan
Publication:Science News
Date:Jun 16, 1990
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