Abstract:Colors in organisms can be produced either chemically by pigments or physically by the constructive interference of light scattered by biophotonic nanostructures and sometimes as a combination of both. Fade-proof, saturated structural colors that have evolved over millions of years of selective optimization are an ideal source to look for natural solutions to our current technological challenges in optics, and sensing. However, given that the underlying nanostructures are overwhelmingly diverse in form and function, their characterization has suffered for over a century. I have pioneered the use of synchrotron Small Angle X-ray Scattering (SAXS) as a high throughput technique to structurally and optically characterize integumentary photonic nanostructures from hundreds of species in a comparative fashion. This led to the discovery of the first single gyroid crystals in biology within the iridescent green wing scales of certain butterflies whose self-assembly beautifully pre-empts our current engineering approaches and more recently, within the feathers of some leafbirds. The latter is the first directly phase-separated single gyroid known to science and at the hard to achieve visible optical length scales. But broadly, this wealth of structural knowledge has led to the realization that these diverse, mesoscale nanostructures share a unifying theme – they appear to be self-assembled within cells by bottom-up and directed processes. I will broadly summarize our current state of knowledge about the structure, function, development and evolution of organismal structural colors in birds, butterflies and beetles, as well as discuss some future directions on how understanding the intracellular development of biophotonic nanostructures can lead to novel, facile biomimetic routes to sustainable mesoscale synthesis for advanced functional applications from sensors, photonics, energy harvesting to catalysis