Since the 1800s, scientists have marveled at how spiders can take flight using their webbing. Charles Darwin remarked on the behavior when tiny spiders landed on the HMS Beagle, trailing lines of silk. He thought the arachnids might be using heat-generated updrafts to take to the sky, but new research shows a totally different cause may be at play.
Erica Morley and Daniel Robert from the University of Bristol in England were interested in exploring a second explanation for the spiders' ability. They thought spiders might sense and use electrostatic fields in the air.
"There have been several studies looking at how air movement and wind can get spiders airborne, but the electrostatic hypothesis was never tested," Morley told VOA.
Some observers suggested electrostatic fields might be the reason the multiple draglines some spiders use to float don't get tangled with each other. Biologist Kimberley Sheldon from the University of Tennessee at Knoxville, who was not involved in the new research, pointed out that "though these spiders will have five or six draglines, those strands of silk do not get entangled. So we've known for a while that electrostatics probably [are] at least interacting with the spider, with the silk lines themselves, to keep them from getting tangled."
Morley and Robert created a box with a grounded metal plate on the bottom and a plate on the top that they could pass an electrical current through. The scientists placed spiders in the box and turned on the voltage, watching as the creatures reacted to the electric field.
Reaction to current
When the electric field was on, the spiders lifted their abdomens into the air and started tiptoeing by raising up on the very ends of their legs. Morley told VOA that spiders only tiptoe right before they release silk draglines to fly away, in a process called ballooning.
And when the spiders did balloon and rise into the air, turning off the electric current caused them to drop.
Sheldon compared it to taking a balloon and rubbing it against your clothing. "If you hold the balloon [near your head], your hair stands on end. That's kind of what's happening with the spider silk."
Clearly the spiders were able to sense the local electrostatic field and respond appropriately by releasing silk, but Morley and Robert wanted to know how.
"As a sensory biologist, I was keen to understand what sensory system they might use to detect electric fields," said Morley. "We know that they have very sensitive hairs that are displaced by air movements or even sound. So I thought that it's possible that they might be using these same hairs to detect electric fields."
This was exactly what she observed. The small hairs along the spiders' legs react not only to physical experiences like a breeze but also to the electric field. In nature, it makes sense for spiders to sense both the electrostatic field around them as well as wind conditions. Spiders probably use both when taking off and navigating the skies.
Mathematician Longhua Zhao from Case Western Reserve University in Cleveland has made computer models of how spiders balloon. She told VOA, "I think that both the electrical field and the fluid mechanics [of air flow] are important. They definitely play very important roles. However, we don't know at this point which is the dominant factor."
Lead researcher Morley pointed out that spiders aren't the only invertebrates to balloon. "Caterpillars and spider mites, which are arachnids but not spiders, balloon as well." Morley hopes to see others follow up her research to see if these other animals respond in a way similar to the spiders.
The study is published in Current Biology.