How physicists revealed the unexpected color of a 47-million-year-old moth

Though fossils can reveal many things about the paleolithic world, they can also lie. Case in point: The blue-tinged fossils of an ancient ancestor of the forester moth (pictured). For years, scientists believed these fossils meant the 47 million-year-old moths had blue wings. But a new analysis by physicists has revealed that the truth is a lot stranger. Find out how the moths' true colors were revealed.

The ancestors of forester moths were flapping around millions of years ago. We know about them because some of them blundered into the right kind of sediment, and were fossilized. Some of the fossils were incredibly well-preserved, showing scientists the wing structure and venation, and came with a bluish chemical. What luck, thought scientists. Not only do we know the shape of the moths, we know the color as well. It was assumed that these early moths were blue.

In fact, the chemicals in the wings only turned blue during the fossilization process. These moths didn't have chemical color, they had structural color. Structural color is what's happening in the iridescent feathers of a peacock, and the metallic surface of some beetles. Instead of absorbing and re-emitting certain light waves, the way dye does, a neutral colored material is formed into a kind of grating. Light waves bounce off the various levels in the grating and interact so that the overall light to escape the grating is of a certain color. It's a little like waves in water bouncing off different rock shelves. Five big waves per minute may go into an area, but they're chopped up and reflected off of so many surfaces that ten smaller waves per minute come out. The difference in frequency becomes a difference in color. And the color is built into the material.

How physicists revealed the unexpected color of a 47-million-year-old moth

To find the tiny structures in a moth's wings takes some doing. To find it in the fossil of an extinct moth takes the GrubeMessel oil shale of Germany. The Messel Pit was a coal mine in the early days of the Industrial Revolution, and then was mined for oil shale. Even as it was being dug up it was known for its incredibly detailed fossils. When it was no long economical to mine, it was almost turned into a landfill, before local resistance (thank you, local German citizens!) stopped the process and turned it into a World Heritage site. Scientists took scanning electron micrographs of fossil moths and amazingly found the structurePLoS Biology of scales on the moth's wings. An analysis of the light coming off such a structure showed that instead of a deep blue, the center of the wings would have been a bright yellow-green.

Via PLoS Biology.

Top Image of Forester Moth: Fritz Geller-Grimm and Felix Grimm

Second Image: PLoS Biology