Spiders can travel many hundreds of miles through the air by releasing silk and floating away. Researchers had thought that ballooning behavior required drag forces from wind or thermals. But, now researchers reporting in the journal Current Biology on July 5 show that electric fields at strengths found in nature not only trigger ballooning, but also provide lift, even in the absence of any air movement.
"We don't yet know whether electric fields are required to allow spider ballooning," says Erica Morley from the University of Bristol. "We do, however, know that they are sufficient."
Morley and her colleague Daniel Robert had been intrigued by a theory of electrostatic ballooning by spiders presented by another researcher in 2013. In fact, the notion that atmospheric electric fields might play a role had first been floated in the early 1800s, but it had been long ago dismissed despite never being tested.
The atmospheric potential gradient (APG)--an electric circuit between the Earth and the ionosphere that is maintained by thunderstorms--is ever present around the world, Morley explains. But the strength of the APG also varies; on a calm day with clear skies, the APG may reach 100 V/m. On a stormy day or in the presence of charged clouds, the APG can soar to 10 kV/m. Empirical measurements were needed to see whether spiders actually responded to electric fields and to variation in those fields.
To find out, Morley and Robert carried out experiments in the lab with Linyphiid spiders (Erigone). The laboratory environment allowed them to remove other stimuli, such as air movement, and provide a uniform electric field for the spiders.
Their experiments showed a significant increase in ballooning when electric fields were switched on. That change in the spiders' behavior confirmed that spiders can indeed detect APG-like electric fields and that they respond to those electric fields by ballooning. Once the spiders were airborne, switching the electric field on and off led them to move upward or downward, respectively.
Their studies also show that sensory hairs called trichobothria found on the surface of the spiders' exoskeletons move in response to electric fields. The researchers suggest that those tiny hairs allow detection of electric field stimuli by spiders.
There are days when many thousands of spiders take to the air in mass ballooning events and days when none disperse at all. Predicting those dispersal patterns had proven difficult, the researchers say. The new findings suggest that variation in the APG might explain those behaviors and help to predict when they will occur, not only in spiders, but also in other ballooning animals, including caterpillars and spider mites.
Morley says they'd now like to examine the physical properties of the silk used for ballooning. They also hope to tease apart the contributions of both wind and electric fields to ballooning behavior in nature.
The researchers were supported by grants from the UK BBSRC and the European Research Commission.
Current Biology, Morley and Robert: "Electric Fields Elicit Ballooning in Spiders" https://www.cell.com/current-biology/fulltext/S0960-9822(18)30693-6
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