Scientists have long wondered how spiders can use the webbing they make to fly.
The 19th century English naturalist Charles Darwin commented on their behavior when very small spiders landed on his ship, the HMS Beagle. He noted that they were following lines of smooth silk. He thought the creatures might have been using warm air to take to the sky. However, new research shows a totally different cause may be involved.
Erica Morley and Daniel Robert are with the University of Bristol in Britain. They were interested in exploring another explanation for the spiders' ability. They thought spiders might sense and use electrostatic fields in the air.
Morley told VOA: "There have been several studies looking at how air movement and wind can get spiders airborne, but the electrostatic hypothesis was never tested."
But some observers suggested electrostatic fields might be the reason the many draglines some spiders use to float never get tangled.
Kimberley Sheldon is a biologist with the University of Tennessee, Knoxville, in the United States. She said that although "these spiders will have 5 or 6 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."
Sheldon was not involved in the latest study.
Morley and Robert created a box with a grounded metal plate on the bottom and a plate on the top. It was designed so they could send an electrical current through the box. They placed spiders inside the box and turned on the electricity, then watched as the spiders reacted to the electric field.
When the electric field was on, the spiders lifted their abdomens into the air and started raising up on the very ends of their legs. Morley told VOA that spiders only move this way before they release silk draglines to fly away, in a process called ballooning.
When the spiders did balloon and rise into the air, turning off the electric current caused them to drop.
Sheldon compares this to someone taking a balloon and rubbing it against their 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 react by releasing silk, but Morley and Robert wanted to know how.
Morley noted that as a sensory biologist, she was interested in understanding what sensory system spiders might use to sense electric fields.
"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," she said.
This was exactly what she observed. The small hairs along the spiders' legs react not only to physical experiences like air movement, 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 moving through the sky.
Mathematician Longhua Zhao is with Case Western Reserve University in Cleveland, Ohio. She has made computer models of how spiders balloon. She told VOA that both the electrical field and the air flow are important.
"They definitely play very important roles; however, we don't know at this point, which is the dominant factor," she added.
Lead researcher Morley pointed out that spiders are not the only creatures to balloon. "Caterpillars and spider mites, which are arachnids but not spiders, balloon as well," she noted.
Arachnids have eight legs and a body made of two parts.
Invertebrates have no back bone.
Morley hopes other researchers will follow up her study to see if these other creatures react in much the same way as spiders.
I'm Pete Musto.
Sadie Witkowski reported this story for VOANews. Phil Dierking adapted her story for Learning English. George Grow was the editor.
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