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

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Experiments indicate that spiders are thrown off a kayak head and land in locations that don’t reflect polarized light.

When biologist Brian Gall was throwing stowaway spiders out of his kayak, he noticed a curious pattern: no matter how far he paddled from dry land, the arachnids swiftly scurried to the nearest shoreline after landing on the water’s surface.

In order to capture prey, the passengers—elongate stilt spiders, or Tetragnatha elongata—spin webs on the margins of ponds. The spiders use surface tension to their advantage when they fall into the water, which happens frequently, in order to avoid predators and quickly swim to land. So yet, it’s unknown how exactly stilt spiders move across the water’s surface. As reported in the December Zoology, Gall and his colleagues at Hanover College in Indiana have discovered that the arachnids seem to use light reflected off the water to locate the less reflecting shoreline.

Of the approximately 51,000 species of arachnids known to science, only a small number have had their navigational abilities examined by scientists. Research has demonstrated that in addition to their eight eyes, spiders also depend on sound, vibrations, and chemical cues (SN: 10/29/20). Polarized light, which naturally arises as light waves flatten when they reflect off a surface like water, is visible to and useful to certain species.

“Our vision is not at all like Spider’s,” remarks Sidney Goedeker, a research technologist at the University of Louisville in Kentucky and a former student collaborator of Gall’s. “And since we lack what they have, it’s not something we can perceive.”

Gall speculated that studying the stowaways would provide insight into the elongate stilt spider’s homing senses. Using a film hung over the water to polarize incoming sunlight before it struck the surface, his team created test venues in an outdoor tank and a naturally occurring pond in Gall’s backyard. This created sections free of glare that resembled what land could appear like to a spider. After that, the researchers observed the motions of 68 spiders as they were dropped into the arenas.

In the tank, the majority of the spiders ran for cover. Spiders plummeted between the covered regions and the shoreline in the pond with equal regularity, moving in circles until they found land. Spiders that preferred shaded spots would frequently circle in and out of the shadows in both arenas as they looked for the beach that was supposed to be there.

According to Gall, in earlier pond studies conducted without the film, elongate stilt spiders consistently flew toward the land. When combined with the recent discoveries, the team concludes that the animal likely recognizes polarized light and uses it as a “not-land” landmark.

“The absurdity of these findings cannot be overstated,” remarks Gall. “I’ve probably dropped a thousand spider’s into the water, and I’ve seen almost none of them make the wrong decision.”

Experts like Eileen Hebets, an arachnologist at the University of Nebraska–Lincoln who was not involved in the study, believe that much more needs to be discovered about the navigational abilities of other similar spiders. According to the author, a spider that can weave orbs and appears to be using visual cues raises a lot of new issues about other groups. “We don’t usually consider vision to be very important for spiders that build webs.”

That Gall found this after spending a day lobbing spiders from his kayak makes him very happy. It merely illustrates how basic observation and inquiry may result in amazing things.

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