Structurally assisted super black in colorful peacock spiders

Male peacock spiders (Maratus, Salticidae) compete to attract female mates using elaborate, sexually-selected displays. They evolved both brilliant color and velvety black. Here we use scanning electron microscopy (SEM), hyperspectral imaging, and finite-difference time-domain (FDTD) optical modeling to investigate the deep black surfaces of peacock spiders. We found that super black regions reflect <0.5% of light (for a 30° collection angle) in Maratus speciosus (0.44%) and Maratus karrie (0.35%) due to microscale structures. Both species evolved unusually high, tightly packed cuticular bumps (microlens arrays), and M. karrie has an additional dense covering of black brush-like scales atop the cuticle. Our optical models show that the radius and height of spider microlenses achieve a balance between (i) decreased surface reflectance and (ii) enhanced melanin absorption (through multiple scattering, diffraction out of the acceptance cone of female eyes, and increased path length of light through absorbing melanin pigments). The birds-of-paradise (Paradiseidae), ecological analogues of peacock spiders, also evolved super black near bright color patches. Super black locally eliminates white specular highlights, reference points used to calibrate color perception, making nearby colors appear brighter, even luminous, to vertebrates. We propose that this pre-existing, qualitative sensory experience—“sensory bias”—is also found in spiders, leading to the convergent evolution of super black for mating displays in jumping spiders.