Here’s how spiders that go overboard use light to find land

Biologist Brian Gall was flinging stowaway spiders out of his kayak when he noticed an interesting pattern: After landing on the water’s surface, the arachnids quickly darted to the nearest shoreline, no matter how far he paddled from dry land. 

The passengers, elongate stilt spiders (Tetragnatha elongata), spin their webs on the edges of ponds to catch prey. When the spiders tumble into the water, which happens often, they rely on surface tension to evade predators and skitter to shore. But just how the stilt spiders navigate the water’s surface has been unclear — until now. The arachnids appear to use light reflected off the water to pinpoint the less-reflective shoreline, Gall and his team at Hanover College in Indiana report in the December Zoology. 

Scientists have studied the navigation skills of only a handful of the approximately 51,000 known species of arachnids. Spiders have been shown to rely on sound, vibrations, chemical signals and, of course, their eight eyes (SN: 10/29/20). Some species can see and use polarized light, which can occur naturally when light waves flatten as they reflect off a surface such as water.

“Spider vision is completely different than ours,” says Sidney Goedeker, who worked with Gall as an undergraduate and is currently a research technologist at the University of Louisville in Kentucky. “And it’s not something that we can perceive because we don’t have what they have.”

Perhaps, Gall thought, the stowaways could offer a way to study the elongate stilt spider’s homing senses. His team built test arenas in an outdoor tank and a natural pond in Gall’s backyard, using a film suspended over the water to polarize incoming sunlight before it hit the surface, creating areas without glare that mimicked what land might look like to a spider. Then, the researchers dropped 68 spiders into the arenas and recorded their movements.

Spiders in the tank overwhelmingly raced to covered areas. In the pond, spiders dropped between the shore and the covered areas chose both options with equal frequency, making circular passes until they found land. In both arenas, spiders that chose covered areas would often circle in and out of the film’s shadow, searching for the promised shoreline.

In previous pond experiments without the film, elongate stilt spiders unerringly zipped toward land, Gall says. Taken together with the new findings, the species probably perceives polarized light and uses it as a “not-land” landmark, the team says.

“It’s hard to overstate how crazy these results are,” Gall says. “I have dropped probably one thousand spiders onto the water’s surface, and I’ve almost never seen them make the wrong choice.”

There is still much to learn about the navigation skills of other similar spiders, says Eileen Hebets, an arachnologist at the University of Nebraska–Lincoln, who was not involved in the study. “An orb-weaving spider with evidence of using visual cues opens up lots of new questions about other groups,” she says. “We tend not to think of vision as being very important for web-building spiders.”

Gall is delighted that a day spent lobbing spiders from his kayak led to this discovery. “It just highlights how watching and asking really basic questions can lead to really cool stuff.”