When a cockroach is made of silver

Chitin makes cockroaches blend in with pantry shelves and dark linoleum, and yet, without mixing in any new elements, it makes the gold and silver beetles of Costa Rica shine like jewelry. Find out how changing shape can change color.

We are used to thinking of color in terms of pigment, a chemical element or compound that absorbs certain wavelengths of light and reflects other ones. We see the sum total of the colors it reflects. Cutting a substance with pigment in half - assuming the pigment is even throughout - may lessen the concentration of the pigment that we see, but it doesn't change the actual color of the object. Pigmentation is the way we usually get desired colors, but there's another way. Color can be built into structure, not just chemical components.

Two brilliant examples of this are the gold and silver beetles of Costa Rica. Each have outer wings that shine like polished metal - but neither has any metallic components mixed in with its structure. It has chitin, the same thing cockroaches have to help them look like dirty floors. The beetle can't add metal or pigment, so it re-shapes the interior of its wings. The wings are made up of seventy layers of semi-reflective chitin. Each successive layer gets thinner. Each layer reflects a sliver of the light coming in.

The light reflected from each layer forms different waves that interfere with each other. When peaks of light meet, they build to higher peaks, and the intensity of the light amplifies. It's this amplified light - and the corresponding troughs that make the beetle dark at different angles to the light - that hits the eye and makes the shell look like it's coated in gold and silver. The silver beetle reflects light of all different wavelengths, which makes the shine a white-silver. The gold beetle only reflects light with wavelengths over 525 nanometers, eliminating the high-frequency blues and greens and making it shine gold.

A cynic might ask how this beetle could possibly blend in with its natural environment, unless its natural environment was the bumper of a 1950s Chevy. Scientists believe that the shine of the beetles helps them blend in with dew drops or morning light off of wet leaves. Which isn't to say they aren't on chrome bumpers or skittering around pirate chests full of coins. It's not like we would have seen them.

Via Optics Info Base and Scientific American.

Photograph by Dan Rosenberg