6. Something wondrous
Spiders can set sails of silk that they ride for hundreds of kilometers. This so-called “ballooning” behavior, which can take them 2.5 miles up in the air, opens the world to these wingless arthropods, Alison Snyder writes.
From the decks of the HMS Beagle more than 180 years ago, Charles Darwin described spiders ballooning on and off the ship as it sat off the coast of Argentina waiting for the wind.
What's new: We've long known light winds enable spiders to perform such physical feats. It turns out, though, that another hypothesis is also likely true: Spiders can ride Earth's own subtle ever-present electrical fields.
What they did: Erica Morley and Daniel Robert from the University of Bristol in the U.K. put adult Linyphiid spiders in a box within a Faraday cage, in which all electric fields are isolated and there is no wind. A metal plate at the bottom of the box served as the ground and on top Morley applied different voltages corresponding to those in the atmosphere. She filmed the spiders’ behavior.
As she increased the strength of the electric field, the spiders lofted. They would raft — in which they drop a line of silk and dangle while releasing silk to balloon on. Or, they stood on their tiptoes while sticking their abdomen in the air and releasing silk. This is an act done exclusively for ballooning.
As she turned the electric field on and off, the ballooning spiders would rise and fall.
They also measured how the trichobothria — a hair on each of the front pair of legs — moved in response to the electric field. “They are probably the electroreceptors but we can’t say that definitively,” Morley says.
What it means: The electric field in Earth's atmosphere and how it varies as the weather changes from fair to stormy have been observed and described. “But it is not widely known or thought about in terms of biology,” Morley says. Bees, and now spiders, seem to be sensitive to it. Morley and her co-author suggest studying Earth’s atmospheric electricity could help to predict the migration of spiders and potentially other organisms.
Peter Gorham, a physicist at the University of Hawaii who was not involved in the research but has modeled how spiders might use electrostatic forces to balloon, says: “If Darwin could see this paper, he would be thrilled.”