When we think of rainbows in nature, we think of spring showers or misty waterfalls or maybe even a prism.
But this week we’re going to explore the ROY G. BIV of crawlies! All types of crawlies from the smallest wasps to big old wild turkeys rock a rainbow of colors on their wings, feathers, scales, exoskeletons and even their eyes.
How, and more importantly, why do so many crawlies positively sparkle in the full spectrum?
We’ll start with a “why” that applies to several crawlies: Rainbow refraction can act as camouflage.
Iridescent targets are more difficult for birds (a main predator of many arthropods) to spot. Humans (another big killer of arthros) also have a harder time spotting iridescent critters. It becomes even more difficult if the arthropod is on a shiny leaf.
This colorful camo can also help our shimmery common garter snake, as they are both a predator of birds and prey of others.
How do insect wings and snake scales produce a rainbow of colors? For insects, it’s like the way a soap bubble shines; the thinness and transparency of the wings. Insect wings also have specialized membranes and hairs, microstructures that help refract a rainbow.
Our snake’s shimmer is produced by structures called iridophores. They are on the top of the snake’s scales, a kind of gloss over the layer of melanin which gives the snake its dominant color.
The iridophores are made up of tiny crystal-like cells stacked on top of each other. Light refracts through the layers with different light waves interfering with each other to produce different colors.
When it comes to our brilliant buprestid beetle, the “why” is quite the opposite of our winged and scaled crawlies. Iridescent beetles use their shifting colors to startle and confuse potential predators, especially birds. Studies have shown birds are wary of shimmery beetle elytra, even when they are baited with extra yummy snacks.
For fancy-eyed arthropods like our green lacewing their metallic-colored eyes have layers and layers of lenses that reflect light, lending an iridescent quality. The different colors help the lacewings, and other arthropods with colored eyes, narrow down the visual information being processed by each of the lenses allowing them to hone in on objects that are important to their particular survival.
The how and why of bird rainbow coloration are unique to our winged crawlies. They refract the rainbow to get as much attention as possible. Their array of colors virtually screams, “Look at me!”
Our male bufflehead duck tries to literally outshine the other males to woo the ladies. Meanwhile, our Anna’s hummingbird uses his vibrant colors both to attract a mate and warn off other males who may enter his turf.
As to the how: Feathers produce structural color (versus pigment color, the dull kind our skin has). The iridescent gorget feathers have barbules (teeny filaments attached to feathers) and each of those is packed with platelets of different thicknesses and filled with air bubbles!
The showy colors come from light having to pass through all those layers, then bouncing back the way they came. Different colors have different wavelengths, so some colors get mixed up on the journey through the layers and lost on the way out. When that happens, we can’t see those colors at all. Meanwhile, other colors – like that fiery orange blazing on the center Rufous – have wavelengths which perfectly match the bird’s particular feathers and they are then intensified by the passage and return.
Possibly the most amazing part of all this is how precise the thicknesses of each part of each feather must be to produce a specific color for us to see.