What does it mean to be blue? The wings of
a morpho butterfly are some of the most beautiful structures in nature, and yet they contain
no blue pigment. Artists use paint, dyes and ink to make their creations.
Nature draws on a palette of biological pigments, also known as biochromes. But there’s another way of producing color.
It’s called “structural color.” And it harnesses the physics of light at the nanoscale. Nipam Patel’s lab at UC Berkeley is studying
how morpho butterflies form the special structures that cover their wings — scales — while
still inside the pupae. Morphos live mostly in the tropics. When resting,
they fold their wings up, showing their dark earth-toned undersides. The brown, yellow and black colors are generated
by pigments. But the other side is all about structural
color. It gives their wings a vibrant, iridescent blue hue. Each scale is like a pixel, a tiny tile in
a larger mosaic in layers of overlapping rows. Researchers want to see how structural color
takes shape on the wings, but normally this happens inside the pupae, which is opaque.
So they’ve figured out how to remove the morpho wings from the pupae and grow them
in a Petri dish. Just like a developing photograph, patterns
and colors slowly appear on the ghostly white wings as each scale’s surface transforms
over time. Ridges on the scales’ surface are a key
component that affect how the wing spreads — or refracts — light, similar to a prism. When light hits these ridges, a phenomena
called constructive interference comes into play. The spacing within the ridges — which look like
little Christmas trees — perfectly reinforces specific wavelengths while canceling out others.
This is why your eyes perceive that shimmering blue. Scientists aren’t sure why, but vertebrates
and plants rarely produce blue as a pigment. For some reason, it’s a pigment you don’t
see much of in nature. So think of structural color as an evolutionary
work-around — a way of producing brilliant blues at a nano level. Not just on butterfly
wings, but on feathers, beetles — even our own bodies.