Some jumping spiders are incredibly flashy and colorful, yet they only have two types of color-sensitive cone cells in their eyes. It seems like such a waste for the males to have such flamboyant colors if the females can’t see them. Well, as it turns out, these spiders can see extra colors, thanks to a filter that covers part of their retina, according to a new study published in Current Biology this week.
Previous work revealed that the jumping spider family can see in remarkably high resolution. In fact, these centimeter-long masters of miniature vision have achieved higher spatial resolution relative to body size than any other animal. However, their eyes only have two types of color-sensitive pigments, Los Angeles Times reports: One is sensitive to ultraviolet wavelengths, the other to green wavelengths.
Their color sensitivity seems limited. Yet, in this video, you can see how Habronattus pyrrithrix males display green, cream, orange, and red ornaments to drab females (one is pictured below) during complex courtship sequences. To understand how jumping spiders in the colorful genus Habronattus see in color, a team led by Daniel Zurek and Nathan Morehouse from University of Pittsburgh examined the structures in their eyes.
Video Credit: Zurek et al./Current Biology 2015
These spiders have four pairs of eyes that pick up on different aspects of their surroundings. Their principle eyes, the team found, can see in three color channels -- red, green, and ultraviolet -- thanks to a filter that converts some of the green-sensitive cells into red-sensitive ones, like a pair of sunglasses. A thin layer of red-pigmented cells covers a small spot in center of their retinas, Science explains, allowing only red light to pass through and activate the retinal cells below.
"The eyes of jumping spiders could not be more different from those of butterflies or birds, and yet all three tune the color sensitivities using pigments that filter light," Morehouse says in a news release. "It's actually a pretty clever, simple solution with a big payoff.” This visual strategy is called "spectral filtering," and it’s never been described in any spider before, making this a very cool example of evolutionary convergence.
Humans also see in three color channels: red, green, and blue. And even though the spiders seem to have "true" color vision, they don’t see the world in exactly the same way we do. "One fascinating thing about the trichromatic area in these spiders' retinas is that it is very restricted in field of view,” Zurek explains, “which means they'd have to scan scenes 'line by line' to accumulate color information.”
Next, the team plans to explore the role that color vision had in generating the diversity of the genus Habronattus over evolutionary time.
Images: Daniel Zurek