By Jack Tamisiea

The dark depths of southwestern China’s cave systems are patrolled by pale spiders smaller than a Tic Tac. Like many other cave creatures, these arachnids sport noticeably undeveloped eyes. Some lack eyes altogether.

But new research shows that these cave spiders don’t turn a blind eye to light. In a study published on Wednesday in Science Advances, researchers found that even cave-dwelling spiders that lack eyes entirely can still sense light. This vestigial “vision” may help the spiders, which thrive in damp conditions, steer clear of bright cave openings that lack moisture.

Spiders are widespread predators in most cave systems: more than 1,000 species of the leggy carnivores reside in caves. And few are more distinct than those in the genus Leptonetela. Whereas most spiders have eight eyes, these tiny hunters sport only six at most. Their two front middle eyes, which most spiders use to pinpoint prey, have been lost to evolution.

Several Leptonetela species take eye loss even further. “These spiders generally exhibit varying degrees of eye reduction, including high reduction or complete eyelessness,” says study co-author Jie Liu, an evolutionary biologist who studies spiders at China’s Hubei University. Leptonetela spiders that live closer to cave entrances often possess a full set of six eyes, but those found in the darker depths sport simple eyespots or lack eyes altogether.

The eye loss is linked to the difficulty of maintaining that energetically expensive body feature in nutrient-poor environments such as caves. Several other cave critters, including olm salamanders and cave fish, have also lost their sight. Yet some sightless species have retained the ability to sense light.

“A lot of organisms can sense light even without eyes,” says Elke Buschbeck, a researcher studying arthropod visual systems at the University of Cincinnati, who was not involved in the new study. For example, some eyeless amphipod crustaceans and blind fish have been observed to actively avoid light to stay safe in their dark environments. But these cave creatures are not exactly visualizing what’s around them. “There is an important distinction between sensing changes in ambient light levels and being able to actually see structure in our environment,” Buschbeck says.

To see if eyeless cave spiders possess an ability to sense light, Liu and his colleagues collected 10 species of Leptonetela from caves in China’s Yunnan-Guizhou Plateau. Five of the species they collected lived in the lighter “twilight zone” near the mouth of a cave, and the rest came from the cave’s ink-black inner chambers. Two of the species in this latter group possessed reduced eyes, while the other three had no eyes.

The researchers conducted a behavioral test to see how each species of spider responded to light. They placed the arachnids in a container divided into a light half and a dark half and recorded each species’ preference. No matter how degraded or nonexistent the spiders’ eyes were, the majority of individuals of all 10 species preferred to stay on the dark side of the container when placed inside the cave near its entrance. The researchers say this reveals that even eyeless Leptonetela spiders can sense and avoid light. To back up the results, the researchers also analyzed the genetic codes of each spider species. They found that the eyeless arachnids retained nearly a full complement of genes coding for photoreceptive capabilities that help them sense changes in light levels despite lacking visual pigments or eyes altogether.

To determine why sensing light was so important for spiders that live in perpetual darkness, the team placed individual spiders from two cave-dwelling and two entrance-dwelling Leptonetela species at a cave entrance. Without additional water, the cave-dwelling spiders quickly succumbed to the arid conditions at the entrance. Liu and his co-authors hypothesize that avoiding light helps these spiders stay in the damp depths of their caves.

While Buschbeck is not shocked that these eyeless cave spiders can sense changes in light levels, she thinks the findings emphasize how fundamental it is to sense light. “It fits the general idea that the basic mechanism of light detection is ancient and well conserved,” she says. No matter how long cave critters have resided in the darkness, it always helps to “see” the light.