It’s all in the structure

Arid Lands Tiger Beetle (Cicindela marutha)

In response to my tiger beetle post last week, TR expressed admiration for the beautiful metallic green color of Cicindela marutha, the arid-lands tiger beetle and asked

What chemistry creates those patterns and colors?

It’s a fun question, because the beautiful reds and greens of tiger beetles aren’t created by chemistry at all. Color in the natural world can be segregated into two main varieties: pigment and structural. Pigments are chemical compounds. It’s possible to grind up, for example, monarch wings and isolate orange colors from them. In contrast, you can’t isolate a blue chemical compound from a Morpho butterfly wing.

Morpho granadensis
Photo: Jyoti

Structural color is caused by some aspect of the physical structure of, say, a wing or a shell that causes interference or diffraction of the light striking the object. Often the structure has tiny ridges or is composed of multiple layers. A nice technical presentation of structural color in Morpho butterfly scales can be found here (pdf file). If you would describe a color using words like metallic, iridescent, opalescent or even glittering you are probably describing structural color. Colors that change depending on the angle that you view them at are also structural.

Cairns Birdwing (Ornithoptera priamus). Structural green.
Photo: Jyoti

In general, greens blues or purples are structural colors. The obvious exceptions are green plants, which derive their color from the well-known pigment chlorophyll. Blue, in particular is very rare as a pigment in nature. Nearly ever time you see blue in the natural world, be it a butterfly wing, a blue jay feather, or a mussel shell, you are seeing structural color. One of my favorite butterflies in Butterfly Haven, the blue-banded shoemaker (Nessaea aglaura) is noteworthy because it contains a very rare example of a naturally occurring blue pigment. If you look at the butterfly in the photo below, you can notice that the blue band on the butterfly’s wings does not have a shiny, metallic appearance. I think it looks a bit like a stripe of tempera paint that was applied to the wings.

Blue-banded Shoemaker (Nessaea aglaura)
A very rare example of a natural blue pigment.
Photo: Jyoti

Pigments are much more likely to result in reds, oranges and yellows, as well as browns and black. These colors can also, at times, be produced structurally. Consider two tiger beetle below. The shiny metallic cast to the red colors is an indication that it is structural rather than pigmented.

White-lined Tiger Beetle (Cicindela lemniscata)
Structural red

Finally, consider white, a color that is also often the result of structure rather than pigment. This chalk-fronted corporal dragonfly has white on the body and wings. This color is the result of a colorless waxy compound that is secreted as the dragonfly ages. The color is referred to as pruinescence. The most common white associated with nature- although not biological- is also structural. If you melt snow, you don’t get a white pigment, but clear water. The white is simply light scattering off of the irregular surface.

Chalk-fronted Corporal (Libellula julia)

Structural white

Special thanks to Jyoti for letting me use her excellent photos from Butterfly Haven.