Iridescence has evolved on Earth several times, as in the nacre of mollusc shells, the feathers of birds, and the shells of insects like beetles.
They all share a common structure: nanoscale molecules that refract different wavelengths of light depending on the angle that the light reflects.
It is very hard to draw.
Conceptualizing and Representing
The way I’ve started conceptualizing it is that there are two “light sources”. One is the natural light source that we all learn about in drawing class. It lights the side closest to the light, then casts increasing shadows as the subject’s surfaces turn away from the source. It leaves a specular highlight if the surface is shiny, reflecting onto the eyes of the observer.
The other “light source” is conditional: it’s the eye of the viewer, itself. Instead of casting single color of light, it casts a spectrum. Instead of turning from the pure light color to black, it goes through a spectrum determined by the precise nanostructure of the surface. That tends to be a slow rotation around the color wheel, followed by an abrupt shift to a contrasting color at a certain angle, followed by an even more abrupt shift to black as the nanostructure stops reflecting toward the viewer at all.
Because iridescence has evolved so many times, we have to wonder what benefit it confers on the genes that produce it.
As far as I’m able to determine:
- It provides active camouflage. You might think that such a visually spectacular feature would make a creature stand out, but, like a zebra or tiger’s stripes, it dissociates the shape of the individual from the light reflecting off of it. Iridescence, like transparency, makes it hard to discern the edges of the creature. As in the zebra and tiger examples, profile-breaking camouflage is beneficial to predators who hunt vision-using prey and prey that are avoiding vision-using predators.
- It’s hydrophobic. One of the most central spacification of a creature’s metabolism is its operating temperature. Some creatures, like mammals and dinosaurs, have a rich evolutionary history of homeostatic processes, like the air-cooling bones of sauropods and our own ability to shiver and sweat. Insects, however, can’t do that. The colder they get, the slower their metabolic processes become, making them vulnerable to faster, less energy-efficient predators like us. They tend to be small (though there were once cat-sized insects on Earth when conditions were quite different), which means their surface area:volume ratio is very low, so they cool down fast. The ability to shed water keeps them from critically losing their body heat every time it rains.
- It’s visually stunning. Bird feathers have no pigments or dyes in them. If you grind up bird features, you’ll find a pale, grey dust that looks like skin flakes. Instead, birds (and probably other kinds of dinosaurs) use(d) iridescence to enable each other to recognize potential mates, as in the case of the cardinals outside my window right now. It also allows them to recognize other members of their own species so they can flock with individuals who probably won’t eat them. (No guarantees. Birds can be real jerks.) That cardinal’s sexual display aspect, though, is where iridescence gets really crazy for Dinosauria: just look at a peacock or a parrot or starling. Those glittering colors are an indicator of health. It means that the nanostructures aren’t clogged by dirt or damaged in some other way. For a bird, that is sexy.
I’ve still got a way to go before I understand completely how to represent iridescence, but I’m pretty sure it’s going to come up more, now that I’m getting a handle on it.
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