How Did That Spiderweb Cross The Road?

Short answer: It had help.

Arachnid Aloft

Jason Schneider

With spiders, the age-old question isn’t why they crossed the road but how. Naturalists long believed spiders would fasten a line to a plant and carry the other end away on foot. But in 1889, spider expert Henry Christopher McCook offered an alternative theory based on webs he observed near a New Jersey boat landing, cast from shrub to shrub across open water. “How were these snares built?” he wrote. “We are constrained to call in the aeronautic habit and the air.”

The silk of the Darwin's bark spider stretches across lakes and rivers more than 80 feet wide.

McCook never found proof of how wind-facilitated travel works. Almost a century later, William Eberhard, a scientist at the Smithsonian Tropical Research Institute in Panama, captured spiders of 65 different species and studied their strategies. He saw that some release a "bridging line" into the breeze and wait for it to snag on a branch or stone. Once the line catches, the spider reels in the slack until the line is taut enough to creep across. McCook measured a strand that spanned 26 feet, clear across a country road. The record holder, Caerostris darwini, or Darwin's bark spider, lives in Madagascar. Its silk stretches across lakes and rivers more than 80 feet wide.

Then there’s the “balloon line.” Spiders using this strategy release a strand into the air and hang onto the end. The wind catches and carries the spider and the line like Mary Poppins with her umbrella. “There are anecdotal records of spiders ballooning many, many miles,” says Todd Blackledge, a spider biologist at the University of Akron. “Some are captured a thousand feet up in the atmosphere or on ships in the middle of the ocean.”

But with spider locomotion, every answer begets a new question. “Can a spider, as it’s flying, decide, ‘I’m going down here’?” Eberhard asks. “That’s a really interesting question nobody’s touched.”

This article originally appeared in the December 2014 issue of Popular Science, under the title "How Do Spiders Cast Silk Strands Across Roads?".