When one thinks of airborne organisms, spiders do not usually come to mind. However, these wingless creatures have been found 2.5 miles (4 km) up in the sky, dispersing hundreds of miles. To disperse, they ‘balloon,’ whereby they climb to the top of a prominence, let out silk, and float away. Wind was considered to be the trigger and driving force for this behavior, but a duo of biologists at the University of Bristol, UK, shows that spiders can balloon without wind if there is a vertical electric field present.
Ballooning spider showing a tiptoe stance on a daisy. Image credit: Michael Hutchinson.
“Many spiders balloon using multiple strands of silk that splay out in a fan-like shape, which suggests that there must be a repelling electrostatic force involved,” said lead author Dr. Erica Morley, an expert in sensory biophysics in the School of Biological Sciences at the University of Bristol.
“Current theories fail to predict patterns in spider ballooning using wind alone as the driver. 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.”
The solution to the mystery could lie in the Atmospheric Potential Gradient (APG), a global electric circuit that is always present in the atmosphere.
APGs and the electric fields surrounding all matter can be detected by insects. For example, bumblebees can detect electric fields arising between themselves and flowers, and honeybees can use their charge to communicate with the hive.
Spider silk has long been known as an effective electric insulator, but until now, it wasn’t known that spiders could detect and respond to electric fields in a similar way to bees.
In the new study, Dr. Morley and her colleague, Professor Daniel Robert from the School of Biological Sciences at the University of Bristol, exposed Linyphiid spiders to lab-controlled electric fields that were quantitatively equivalent to those found in the atmosphere.
They noticed that switching the electric field on and off caused the spider to move upwards (on) or downwards (off), proving that spiders can become airborne in the absence of wind when subjected to electric fields.
“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,” Dr. Morley said.
“This means that electric fields as well as drag could provide the forces needed for spider ballooning dispersal in nature.”
“The next step will involve looking to see whether other animals also detect and use electric fields in ballooning,” she added.
“We also hope to carry out further investigations into the physical properties of ballooning silk and carry out ballooning studies in the field.”
The findings appear this week in the journal Current Biology.
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Erica L. Morley & Daniel Robert. Electric Fields Elicit Ballooning in Spiders. Current Biology, published online July 5, 2018; doi: 10.1016/j.cub.2018.05.057
