Peacock spiders’ superblack spots reflect just 0.5 percent of light

Male peacock spiders know how to put on a show for potential mates, with dancing and a bit of optical trickery.

Microscopic bumps on the arachnids’ exoskeletons make velvety black areas look darker than a typical black by manipulating light. This architecture reflects less than 0.5 percent of light, researchers report May 15 in the Proceedings of the Royal Society B.

The ultradark spots, found near vivid colors on the spiders’ abdomens, create an “optical illusion that the colors are so bright … they're practically glowing,” says Harvard University evolutionary biologist Dakota McCoy.

Male peacock spiders swing and shake their brilliantly colored abdomens during elaborate mating displays. Pigments produce the red and yellow hues, but blues and purples come from light interacting with hairlike scales (SN: 09/17/16, p. 32).

Black areas on the spiders contain pigment, too. But scanning electron microscopy also revealed a landscape of tiny bumps in superblack patches on Maratus speciosus and M. karrie peacock spiders. In contrast, all-black, closely related Cylistella spiders have a smooth texture.

Bumps galore

Scanning electron microscope images show bumps, which manipulate incoming light, in superblack patches on the abdomens of two species of peacock spiders. While Maratus speciosus (left) has only bumps, M. karrie (middle) also sports spiky scales that limit reflection by scattering and absorbing light. A Cylistella spider (right) has a smoother surface, which results in an ordinary black appearance.

Using simulations, McCoy and colleagues showed that the bumps make dark spots appear even darker in several ways. Curved surfaces bounce light around, so less is reflected, and diffract light in a way that it evades the field of view of an onlooking female. And the bumps are microlenses — angling entering light so that it takes a longer path and spends more time interacting with light-absorbing black melanin pigment than it would if the surface was flat.

The spiders’ luxe looks resemble that of birds of paradise, which also use tiny structures to create ultrablack feathers (SN: 2/3/18, p. 32). But the animals evolved their abilities separately, the researchers say. And the phenomenon may not be so rare. There is evidence for superblack shades in snakes, a type of beetle and a variety of other birds, McCoy says.