Of Butterfly Wings and Caterpillar Brains
Duke biology professor Fred Nijhout received the international Kowalevsky Medal for his insights into insect development and evolution. Photo by Megan Mendenhall, Duke Photography.
When it comes to bright colors and bold patterns, the fashion industry can’t hold a candle to butterflies.
Their wings come in a dizzying array of designs and hues, from the iridescent blue bands of the morpho butterfly and the red dots of the ruby-spotted swallowtail, to the orange, black and white warning colors of the monarch.
No two of the world’s more than 12,000 butterfly species look quite alike. To most people, the spectacular diversity of wing patterns is a chaotic riot of color, dots and squiggles. But to Duke biology professor Fred Nijhout, they’re mostly variations on the same basic theme.
Nijhout has spent four decades puzzling out how butterflies and other insects develop from a larva to a winged adult -- how a maggot transforms into a fly, or a caterpillar becomes a butterfly. They’re the “extreme makeovers” of the animal world.
His work on wing pattern development and other questions has earned him the international Kowalevsky Medal, awarded each year by the St. Petersburg Society of Naturalists for extraordinary achievements in evolutionary developmental biology and comparative zoology.
Nijhout became interested in butterfly wing patterns in the 1970s while a graduate student at Harvard, when he happened upon papers published half a century earlier by a Russian entomologist named Boris Schwanwitsch.
In the 1920s, Schwanwitsch and a German researcher named Fritz Suffert both proposed that most butterfly color patterns can be derived from the same ground plan, consisting of three parallel bands of pigment that run from the top to the bottom of each wing.
Inspired by their idea, Nijhout set out to figure out how it would work.
Seen through a microscope, the intricate designs on butterfly wings are produced by thousands of tiny overlapping scales arranged in different patterns. Each scale is a dot of color, like a pointillist painting. Photo by Anatoly Mikhaltsov, Wikimedia Commons.
Butterfly wings are divided into wedge-shaped compartments by the branching black veins that traverse each wing. Nijhout observed that each wing compartment contains the same basic set of spots and dashes, repeated over and over again with slight variations across the wing to make up the overall pattern.
“Very often you don’t need to understand the whole wing; you just need to understand what’s happening in one of these wing cells,” Nijhout said. “Once you see that, then all of these patterns that looked noisy before become understandable.”
Nijhout has done most of his color pattern work in the buckeye butterfly.
The North American buckeye is a master of deception. Males and females have tawny brown and orange wings with bold yellow-rimmed iridescent blue and violet spots along their scalloped edges, which draw attention away from the butterfly’s head and body. A hungry bird or lizard gets a mouthful of wing, but at least the butterfly escapes with its vital organs unharmed.
In the hallway outside the Nijhout lab are a pair of walk-in insect nurseries where spiny orange-and-black buckeye butterfly caterpillars munch on plantain mush.
Nijhout found that the position and shape of the spots and other designs on the future wings are established here, when the insect is still a caterpillar.
In some of his earliest experiments, he showed that when he damaged or cut out small patches of cells in a pupa’s developing wings, after the caterpillar retreats into its chrysalis to change into a butterfly, specific spots in one wing compartment failed to develop, but the pattern elsewhere on the wing was unaffected.
Nijhout deduced that the dots and dashes within each compartment can be controlled independently, and that the dazzling variety of different wing patterns can all be derived from the same ground plan by evolutionary changes in the position, shape, size or color of a spot or stripe in one compartment but not others.
Nijhout created flash-animated cartoons of several butterflies that show the process compressed to a few seconds.
When its wings are closed, the Kallima inachus butterfly of Asia bears a striking resemblance to a dry brown leaf. When you choose Kallima and click “play,” the spots and stripes of the basic butterfly ground plan begin to move, fuse, change color or disappear until they resemble the dark leaf veins and fungus spots of a dead leaf.
If you click on Precis coenia, the buckeye butterfly, some of the same starting elements expand and others vanish to form the bold bullseye wing spots of the buckeye.
Buckeye butterflies can also develop different wing colors depending on the season they experience when they’re growing up.
Caterpillars that hatch in the summer develop into adults with light tan wings, while those that hatch in the fall become a rich reddish brown -- presumably to help them hide against dead and drying leaves.
Nijhout’s color pattern research led to an interest in how such environmental changes prompt genetically identical individuals to develop into different-looking adults -- a phenomenon called polyphenism.
He focuses on how the brain senses changes in day length, temperature or other environmental cues, and releases hormones that set individuals on one path or another.
The two-dimensional quality of butterfly wings makes them easier to study than three-dimensional structures like legs or antennae. By studying how color patterns form in butterfly wings, researchers hope to better understand how complex patterns form in other animals and tissues as well.
Advances in developmental genetics in recent decades, for example, revealed that spot patterns in butterflies and limb position in fruits flies depend on some of the same genes.
“Through his now classic work Fred really defined the most important biological questions one can ask using wing patterns as a model,” said Robert Reed, associate professor at Cornell. “There is now a whole community of butterfly wing pattern researchers, including myself, who spend their lives simply trying to answer the remarkable, insightful questions that Fred posed.”
Reed and other researchers are now building on Nijhout’s work to identify the genetic changes responsible for turning different wing colors on and off, or controlling how butterflies and other animals respond to changes in the seasons.
Nijhout has always preferred performing experiments with simple do-it-yourself equipment. Once, to test a theory about how caterpillars know when to stop growing and start metamorphosis, he built a handmade centrifuge that took up half his garage.
Butterflies were his first love. But over the years Nijhout’s lab has also housed ants, moths, bugs and other insects in various stages of development, including several species of dung flies and beetles that required regular feedings of cowpies.
Over his 40-year career, Nijhout’s lab has housed butterflies, moths, ants, bugs and other insects, such as these tobacco hornworms. Nijhout and former student Yuichiro Suzuki reared these caterpillars to be black or green depending on the temperature. Photo by Megan Mendenhall, Duke Photography.
The Kowalevsky Medal was established in the 1910s in honor of Alexander Kowalevsky, a professor of biology at St. Petersburg University. It sank into oblivion in the wake of World War I and the Russian Civil War, but was reinstated in 2000 in honor of the 100th anniversary of Kowalesky’s death.
Nijhout was selected from nominations from around the world for the award, which included a diploma and bronze medal presented at an April gala at St. Petersburg State University.
Nijhout has written or edited three books and more than 160 scientific papers. He is best known for his work on insects, but for the last 15 years he has also been building mathematical models of metabolic pathways critical to health and disease.
With his collaborator Mike Reed, math professor at Duke, he has been investigating questions such as how dopamine and serotonin interact in the brains of Parkinson’s patients, or how the body attempts to cleanse itself of arsenic-laced drinking water or excessive doses of acetaminophen painkillers.
Back in Nijhout’s lab, a dozen termites await inspection under the microscope, and tobacco hawkmoth pupae lie motionless in their glossy brown shells, like a tray of dried dates.
Scattered among butterfly nets and petri plates are bits and pieces of another of Nijhout’s longtime passions: pottery. Just like the mind-boggling diversity of butterfly patterns are derived from a simple set of common elements, a staggering variety of glazes can be made from a handful of ingredients.
By mixing and testing different recipes in his home studio, Nijhout discovered he could make any number of surfaces for his raku-fired pots, from metallic blues and crusty yellows to speckled greens and crack-linked reds.
“I enjoy mucking around trying to make new glazes,” Nijhout said. “Every time you do it you get a different effect.”