Alongside its many other amazing abilities, spider silk has been seen to carry the arachnids thousands of miles through the air, like balloons. It's long been thought that this phenomenon was simply a case of the wind picking them up, but a new study has found that the creatures are actually making use of atmospheric electric fields instead.
At a glance, the wind theory seems like the common-sense way that spiders balloon, but it has a few holes. For one, the silk strands themselves form a fan shape, which don't seem able to catch air effectively enough to lift the creatures up. On top of that, they've been observed getting great airtime even on calm days.
"Current theories fail to predict patterns in spider ballooning using wind alone as the driver," says Erica Morley, lead researcher on the study. "Why is it that some days there are large numbers that take to the air, while other days no spiders will attempt to balloon at all? We wanted to find out whether there were other external forces as well as aerodynamic drag that could trigger ballooning and what sensory system they might use to detect this stimulus."
An alternative idea that's been floated (pun intended) is that spiders are tapping into electric fields (e-fields). These e-fields exist around all matter, and even surround the Earth in the form of the Atmospheric Potential Gradient (APG). But while insects like bees can detect them, spiders aren't known to.
Strangely, the Bristol researchers say, no one has ever tested whether e-fields play a role in spider ballooning, or if the animals can even detect them. To find out, the team placed Linyphiid spiders in a lab setting, free of any external wind or natural e-fields. Then, they created their own electric fields in the room, at the same strength as those found in the atmosphere.
The team noticed that in response to the e-fields, the spiders would enter a "tiptoe" stance, raising their bodies higher and pointing their abdomen upwards, which seems to indicate that the creature was trying to begin ballooning. The tiny hairs on the spiders' legs also stood on end in response to the e-field, which the team inferred was how they sensed the charge.
As the e-field got stronger, the spiders took to the air, and were lowered back down when the scientists turned the field off again. That shows that spiders can use electrostatic forces to lift off, and once airborne, they might tap into the wind to travel long distances.
"Previously, drag forces from wind or thermals were thought responsible for this mode of dispersal, but we show that electric fields, at strengths found in the atmosphere, can trigger ballooning and provide lift in the absence of any air movement," says Morley. "This means that electric fields as well as drag could provide the forces needed for spider ballooning dispersal in nature."
The team plans to look into whether other animals might be detecting and using electric fields for similar behaviors.
The research was published in the journal Current Biology, and the team describes the study in the video below.
Source: University of Bristol
