Many spiders exhibit vivid colors that are not produced by pigments, but rather by optical interference, diffraction, and scattering — structural colors. Traditionally, structural color research in nature focused on birds, butterflies and beetles. But the long evolutionary history and extreme diversity of spiders provide fruitful new territory. The repeated evolution of blue in large, nearly blind tarantulas and the diversification of sexual display colors in tiny peacock spiders provide two striking examples. Here, we show how tarantula blue is produced using specialized hairs with complex hierarchical structure that greatly reduces iridescence — which has been a key obstacle to the production of synthetic structural colorants without the shimmering effects. On the other hand, the strikingly iridescent scales of the rainbow peacock spider (Maratus robinsoni) can produce every color of the rainbow, and may hold the secrets for future optical device miniaturization. We used an interdisciplinary biomimetic approach to investigate both questions by including techniques such as: morphological characterization (SEM/TEM), phylogenetic analysis, spectrophotometry, optical simulation, and rapid prototyping by 3D nano-printing. Particularly with the rapid prototyping capability, we can create engineering models to test biological hypotheses in a controlled manner that may not be feasible with the living systems. Hence, biomimicry is not only taking what we learned from natural systems to practical human applications, but it is also providing insightful feedbacks and ideas to deepen our understanding of the biological system subject matter during the process.
Structural color is produced when nanostructures called schemochromes alter light reflected from a surface through different optic principles, in contrast with other types of colors that are produced when pigments selectively absorb certain wavelengths of light. Research on biogenic photonic nanostructures has focused primarily on bird feathers, butterfly wings and beetle elytra, ignoring other diverse groups such as spiders. We argue that spiders are a good model system to study the functions and evolution of colors in nature for the following reasons. First, these colors clearly function in spiders such as the tarantulas outside of sexual selection, which is likely the dominant driver of the evolution of structural colors in birds and butterflies. Second, within more than 44,000 currently known spider species, colors are used in every possible way based on the same sets of relatively simple materials. Using spiders, we can study how colors evolve to serve different functions under a variety of combinations of driving forces, and how those colors are produced within a relatively simple system. Here, we first review the different color-producing materials and mechanisms (i.e., light absorbing, reflecting and emitting) in birds, butterflies and beetles, the interactions between these different elements, and the functions of colors in different organisms. We then summarize the current state of knowledge of spider colors and compare it with that of birds and insects. We then raise questions including: 1. Could spiders use fluorescence as a mechanism to protect themselves from UV radiation, if they do not have the biosynthetic pathways to produce melanins? 2. What functions could color serve for nearly blind tarantulas? 3. Why are only multilayer nanostructures (thus far) found in spiders, while birds and butterflies use many diverse nanostructures? And, does this limit the diversity of structural colors found in spiders? Answering any of these questions in the future will bring spiders to the forefront of the study of structural colors in nature.