As if spiders aren’t unnerving enough, did you know that some of them use an electrostatic charge to leap into the air and fly for miles? They’re probably coming to your house.Silk flows from a spiny-backed spider. Kunkel Microscopy, Inc./Visuals Unlimited, Inc./Corbis
The flight of the gossamer spider was a curiosity even to Charles Darwin, who noted that his boat, the HMS Beagle, was “inundated by ballooning spiders on a relatively calm, clear day,” according to a new paper by Peter Gorham of the University of Hawaii that’s posted on the arXiv website. (Also see “Darwin the Buddhist? Empathy Writings Reveal Parallels.”)
Darwin watched two species of spider, one smaller, one larger. The first raised its abdomen, released a thread and launched itself horizontally with “unaccountable” speed. A larger species released several threads more than a yard long, which he described as undulating “like films of silk blown by the wind.” The spider then let go of the post it had been perched on, and flew away —an arachnid paraglider in action.
Darwin thought thermal air currents could be the secret of the spiders’ aerial abilities, but that doesn’t explain things like why the threads fanned out and how even fairly heavy spiders launched so quickly when the air was relatively still. Also, as the Physics arXiv blog notes, these spiders have been found at altitudes as high as 2.5 miles (4 kilometers) and are not likely to have gotten there by hot air alone.
Darwin and others also theorized that “electrostatic repulsion” played a role in the fanning of the threads. Lo these many years later, Gorham says that indeed, electrostatic forces could determine the spiders’ flight. (Watch a video of the world’s largest spider.)
“There are thus a wide and plausible range of processes by which the strands can acquire initial charge,” Gorham writes. One of these is the charging of the strands by the earth’s atmosphere during spinning in a process called “flow electrification.”
From the arXiv blog:
“There must be a source for this charge, of course. Gorham thinks a likely origin is the Earth itself, which has a negative charge density of about 6 nanoCoulombs per square metre on average. That’s more than enough to give the silk a healthy boost and spiders may well be able to pick out prominences where the charge density is much higher.”
All this explains the spider’s launch power in still air, why large spiders can get such a lift and why the silk strands fan out: “because their negative charges repel.” (Also see “Tarantulas Shoot Silk From Feet, Spider-Man Style.”)
The arXiv blog notes that Gorham’s theory still needs to be tested by some “enterprising biologist.” For that individual and for Gorham, how remarkable it must be to work with ideas blown in on gossamer silk all the way from Darwin.