Why So Blue? Tarantula's Cool Color Is Still a Mystery

A blue tarantula. — A greenbottle blue tarantula (C. cyaneopubescens) on a branch. Despite its name, saturated, bright green color rarely occurs in tarantulas.

(Image credit: Michael Kern, www.thegardensofeden.org)

It's eight-legged, furry and a very cool shade of cobalt blue. What is it? A tarantula, of course!

While tarantulas aren't normally associated with the color blue, many of these critters have a distinct cobalt hue, which is produced by tiny structures located on the animals' hairy bodies and appendages. Known as photonic nanostructures, the itsy-bitsy structures reflect blue light, turning a creepy-crawly arachnid into something resembling an eight-legged Cookie Monster.

Scientists have known about the tarantula'slight-scattering hairs for some time, but a recent study took a closer look at the nanostructures that make so many spiders in the family Theraphosidae appear blue. The study found that the blue-reflecting nanostructures are unlikely to have evolved as a result of sexual selection, which is often responsible for the bright colors that distinguish closely related species. (The vividly colored peacock spiders provide an excellent example.) [Goliath Birdeater: Images of a Colossal Spider]

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To back up this hypothesis, the new study notes the presence of blue-reflecting nanostructures across many species of tarantulas that are not closely related. And these nanostructures are quite distinct from one another (i.e., they don't look the same under a microscope) in various genera of tarantula, a finding that suggests the structures developed independently many times over the course of the tarantula's long evolution, said study lead author Bill Hsiung, a postgraduate student in biology at the University of Akron in Ohio.

To learn more about the tarantula's coloring, Hsiung and his colleagues first looked at digital images of the critters to create a phylogenetic tree— a chart that shows the evolutionary relationships between related species. They charted the evolutionary history of tarantulas from 53 genera (the family Theraphosidae contains more than 100 genera in total) and found that at least 40 of these genera had almost the exact same blue coloration. By contrast, only 12 genera of tarantulas were found to have green coloring.

But why blue, specifically? If a tarantula can use its nanostructures to appear blue, then presumably it could also use similar nanostructures to appear to be a different color — like yellow or green, the scientists said. Yet green, in particular, is not this critter's color of choice. Blue-reflecting nanostructures, on the other hand, evolved independently at least eight times in different species, the researchers found.

After sorting dozens of tarantula images, Hsiung and his colleagues decided to get their hands on a few live specimens. The researchers obtained eight blue tarantulas that are indigenous to distinct geographic locations, including Singapore, India, Chile and Brazil. They examined the creatures' light-scattering photonic nanostructures, or "structure colors," using high-powered microscopes. What they found surprised them. [Photos: The World's Creepiest Spiders]

Structure colors like those of the blue-hued tarantulas are not uncommon in nature. Many species of birds and insects also get their colors from nanostructures, rather than the pigments that color the hair and skin of many animals (like humans). However, the tarantula's structure color differs from those of birds and bugs in an important way — it isn't iridescent. That is, the spider's blue color doesn't seem to change when you look at it from different angles.

"Structure colors are usually brighter, and [they] won't fade over time as long as their nanostructures are still intact. These are advantages that people want to use to make color displays for phones, or pigments you can use in your cosmetics or in your clothes. Iridescence is a big constraint in those applications because we usually don't want color to change when we change our viewing angle," Hsiung said.

To produce different structure colors, researchers just need to change the spacing between one nanostructure and the next (which changes the way the structures scatter and absorb light). Producing different pigments is a totally different process in which an entirely new material must be made for every color you want to create, Hsiung said. And sometimes, the materials used to make the pigments that color fabrics and other materials are toxic, he added.