Creatures of the night that stalk their prey underground are rarely clad in bright colors. But a surprising number of tarantulas—ground-dwelling, nocturnal predators—carry hairs that are a vivid shade of blue. Last year, researchers found that this hue was surprisingly common; it actually had evolved multiple times in different lineages and converged on remarkably similar shades of blue across 40 of 53 tarantula groups (1).

Several types of tarantulas, including this green bottle blue tarantula, have evolved to display a vivid shade of blue that results not from pigments, but from a phenomenon known as structural color. Image courtesy of Shutterstock/Cathy Keifer.

Like many shades of blues in the natural world, tarantulas’ tints are not formed by colored pigments. Instead, they’re a result of nanostructure patterns that interfere with specific wavelengths of light, a phenomenon known as structural color. Unlike pigments, which are colored because excited electrons absorb certain wavelengths of light and emit others, structural colors are purely physical arrangements, often created by colorless materials. Figuring out the precise patterns that create tarantulas’ brilliant hues may not only offer up insights into animal adaptations, but could point to means for creating colors from new, less toxic materials: for example, longer lasting, brighter paints. One of the keys to such advances could be a tool most wouldn’t consider for the study of animal hair: 3D printing.

All Shapes and Sizes The nanostructures in tarantulas take myriad shapes: some are smooth cylinders, some bear blade-like edges, and others form bulbous floral patterns, all of which make vivid blues. Because structural color stems from the angle at which a beam of light strikes these nanostructures, one might suspect that the tarantulas’ tones are iridescent, like tin foil shining in sunlight. But they’re not. Graduate student Bor-Kai Hsiung of the University of Akron in Ohio and his colleagues, with the aid of 3D printing, designed five photonic structures that closely resemble the shapes seen in real tarantula hairs. Simulating these patterns’ optical properties, Hsiung and his colleagues discovered that within each hair, the nanoscale patterns had an additional, higher-level arrangement of rotational symmetry that strongly reduced iridescence (2). “Their study tells us that even with these periodic nanostructures, if you introduce this rotational symmetry on top of these structures you can still get noniridescent colors,” says Hao Jiang, a photonics researcher at Simon Fraser University in Canada. “To figure out the underlying design principles opens up a new direction for people to think about structural color. From now on, you can’t just look at a periodic structure and assume it’s iridescent.” Researchers used 3D printing to create a structure (Left) that mimics the cross-section pattern of a blue tarantula hair (Right). Left image reproduced from ref. 2, and Right image courtesy of Dimitri Deheyn (Scripps Institution of Oceanography, La Jolla, CA).

Formed for Function From dinosaur fossils to ∼47-million-year-old beetles, traces of structural colors have persisted across “Ordered materials tend to be stronger, so it's possible that these nanostructures evolved for mechanical functions.” —Matthew Shawkey millennia (3). Nearly all life forms—microbes, plants, birds, and insects—form these hues in similar ways, using layers of a hard material, such as chitin, interspersed with minuscule pockets of air. The colors, which are usually iridescent, typically serve to attract pollinators or the opposite sex or warn predators away. But color-making nanostructures also pop up in unexpected places, such as the hairs of African golden moles, which are blind. Here, researchers suspect, they’re meant for more than a bright display (4). “Ordered materials tend to be stronger, so it’s possible that these nanostructures evolved for mechanical functions—maybe it makes the hairs more water-repellent or toughens them up so the moles can live under the dirt,” says Matthew Shawkey, a biologist at Ghent University in Belgium, who coauthored the new study. Or maybe the color has no particular evolutionary purpose at all. “It’s possible that the colors are just a byproduct of the structure,” Shawkey notes. Understanding the physical shapes and nanoscale features that produce color—or other properties—can yield insight into why these hues have persisted across evolution in tarantulas, moles, and other species. Traditionally, researchers have relied on microscopy followed by simulations to predict how these patterns might interact with light. Moving beyond those simulations, the team turned to 3D printing at a tiny scale to produce structures that mimicked their designs, and found that the printed designs did indeed result in the same blues seen in tarantulas. “[Three-dimensional] printing really opens up the possibility of empirically testing a model such as this one,” says Shawkey. “It was very exciting to see that it worked at a scale small enough to print the nanostructures that make these colors.” The 3D printer modeling could prove crucial to figuring out more than just the wavelength of light a nanostructure will scatter. Now, the team plans to use similar tools to study other animals’ colors. “In many cases just modeling the nanostructure will predict the color. But if we want to look at other optical effects, like gloss or iridescence, we need to look at other factors as well,” Shawkey adds.