Looking at the fire engine red and psychedelic blue body of the Australian peacock spider, Maratus splendens (pictured), is like peeking through the hole of a kaleidoscope. The bright red scales that adorn the arachnid are colored by pigments. But a new study shows that the glowing blue, a color rarely found in animal pigments, is a structural color. A bit like optical illusions, structural colors depend on two things: the physical design of the animal, and the way the light interacts with that design to produce a particular color. Using scanning electron microscopy to see the peacock spider’s design up close, researchers discovered that tiny threadlike fibers sandwiched in between two layers of scales coat the spider’s exterior—a design unique to this spider. The arrangement slightly alters the way light bounces between the scales, producing the distinct blue color. This structural color is similar in concept to that seen in the spider’s namesake, the peacock, whose feathers sport miniscule indentions for scattering light. But the simple insertion of fibers that produce a subtle color change is an invention unique to the peacock spider, the researchers report today in the Journal of the Royal Society Interface .