The blue-green–streaked wings of the swallowtail butterfly harbor an intricate optical system with a design reminiscent of the latest in light-emitting diode technology, researchers now report.
Swallowtails of the group Princeps nireus are among the few types of butterfly with fluorescent wings. The colored scales on the wings’ top surfaces contain a fluorescent pigment that absorbs violet and ultraviolet light and reemits it at a longer wavelength.
Physicist Pete Vukusic of Exeter University in England describes shining an ultraviolet light on swallowtail wings: “It was like someone had switched on a blue-green light, the fluorescence was so bright,” he says. “We wanted to understand what was going on.”
Hundreds of thousands of scales made of cuticle, which is similar to human fingernails, cover butterfly wings much as tiles lie atop a roof. With scanning and transmission-electron microscopes, Vukusic’s group imaged individual scales.
The new, nanoscale pictures reveal that each colored scale has three distinct tiers. The bottom tier is made up of three layers—cuticle, then air, then cuticle—each roughly 90 nanometers (nm) thick. The middle tier is an air space, about 1.5 micrometers (µm) thick, studded with columns of cuticle.
In the top tier, which is about 2 µm thick, the cuticle forms an imperfect honeycomb pattern and contains thousands of air-filled cylinders, each roughly 240 nm in diameter. The fluorescent pigment resides within the walls of those cylinders.
The structure enhances the wings’ brightness in two ways, Vukusic’s team reports in the Nov. 18 Science. First, it ensures that all the blue-green fluorescent light reflects out of the top of the wing. Like a mirror, the bottom tier reflects any light that reaches it. And the cylinders of the top tier prevent light from escaping sideways.
Second, the bottom tier adds blue-green light to that emitted by the fluorescent pigment in the top tier. The blue-green light in incoming sunlight passes unaltered through the top tier and reaches the bottom tier, which reflects it. The bright blue-green color that people see on swallowtail wings is the sum of this light with the light produced via fluorescence, Vukusic says.
Light-emitting diodes that include two-dimensional, photonic crystals have a similar structure, he notes. Such a diode has a patterned, hole-filled crystal above a multilayered mirror. These electronic devices put out six times as much light as standard light-emitting diodes do.
The butterflies’ system is “so analogous in terms of light extraction, it’s slap-in-the-face amazing,” says Vukusic.
“We had no idea that [the wing structures] were going to be so precise and so refined,” says biologist Helen T. Ghiradella of the State University of New York at Albany.