Dragonflies Are More Interesting


August 29, 1991|By BARBARA TUFTY

Washington -- Another truck crashes over the highway and toxic chemicals splash into the river; another tanker collides and spills black, viscous oil over coastal rocks and wildlife; recent heat-wave records further define global warming; drug wars shatter the neighborhood. With such daily assaults on the environment, it's hard to find a quiet place to admire the diffraction grating that gives a butterfly wing its splendid colors.

But it's summer -- the season of light -- and it's hard not to notice the whirling and buzzing and humming and snapping and

chewing of insects all around us. It's their season of life; they thrive in the expanse of heat and light.

A dazzling blue dragonfly zips the air in front of me, then hovers a moment before coming to rest on a day-lily leaf, radiating azure light waves over me and the garden. Yet that brilliant iridescent blue of its needle-thin body is an illusion. It is not a pigment like the color of a blue gentian or forget-me-not. It is really not there, for it is only a fragmentation of light produced by microscopic parallel ridges and grooves in the tissues of the dragonfly's body.

Colors of insects are produced by two basic methods, explains Ross Hutchins in his book ''Insects.'' One is the presence of actual pigments; the other is the microscopic structure of surface tissues -- fine lines or striations. Physicists call these striations diffraction gratings. They work much like glass prisms in the laboratory or like raindrops in a rainbow -- breaking up rays of white light into different components of colored light waves of red, orange, yellow, blue, green, violet. Depending on the animal and angle at which the light hits the groove (or the prism or the raindrop) and bounces into our eyes, the light rays emit different colors -- short for the blues, long for the red end of the spectrum.

Butterflies are some of the most beautiful examples of structural diffraction. These members of the Lepidoptera family have transparent membrane wings that are completely covered with tiny colored scales made of paper-thin cuticle that overlap like tiles on a roof. Some of these scales are structured with grooves that transmit iridescent color when white sunlight rays strike them and become fractured into different wave lengths. In addition to the microscopic striations, laminations or layers within the scales also break up light rays. These layers are very thin, about 0.001 millimeter. The combined effect of the striations and layers of butterfly scales produces a wide range of colors.

Some of the world's most spectacular butterflies are the great Morpho butterflies of the Amazon tropics, with their intense blue wings. Part of the laser-light intensity is due to the large number of striations: more than 1,000 per millimeter.

Some colors of butterfly wings result from actual pigments within the scales -- a mosaic of tiny pieces that produce the yellow and blue of swallowtails and the orange and black of monarchs.

Many insects, especially the beetle family, the Coleoptera, reflect metallic colors from extremely thin layers or laminations on their backs: mainly iridescent blues and greens, and metallic colors of bronze and gold.

You may be startled by the flashing emerald green of the shy tiger beetle skittering off the roadside to hide under a leaf. You may curse the changing glints of purple, bronze, green of the Japanese beetle as you pluck it off your rose bush. Another obnoxious insect, the horsefly, flashes iridescent rainbows from its large eyes. Do such startlingly beautiful colors make up for their ugly habits?

Watch the throat and back of that aggressive nuisance grackle as it struts across the sidewalk, or the iridescent neck of the pigeon cajoling for a piece of your sandwich -- rose, purple, green, rippling as they bob their silly heads. The throat of a hummingbird flashes brilliant ruby one moment, then turns green the next as it moves its head. These colors, like those of the insects, are tricks of science to produce colors of the spectrum as sunlight breaks into different rays on the feathers.

*** The dragonfly lifts off like a helicopter, and I am left with the tenuous remembrance of a magical moment of unbelievable color and beauty.

Now back to the news: Heat ignites truck's toxic waste, halting traffic along the interstate beltway and paralyzing motorists for five hot hours under a broiling sun.

I'd rather be watching dragonflies.

Barbara Tufty is conservation editor for the Audubon Society.

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