Flying spiders that stumped Darwin use electrical waves
You might assume that evolution gave Charles Darwin enough to ponder during his five-year voyage on The Beagle.
But of all the phenomena the naturalist encountered circumnavigating the globe, it was the flight of spiders which continued to puzzle him.
Darwin noticed that hundreds of spiders would inexplicably land on the Beagle even on a calm day, only to soar away again, as if magically plucked up by an unseen hand into the skies, a technique dubbed ‘‘ballooning’’.
Now biologists at Bristol University believe they have the answer.
The spiders take advantage of electricity in the atmosphere to lift off.
Shooting strands of silk into the air to create a kind of sail, the arthropods are drawn up by the natural electrostatic charge, in the same way that hair rises up when rubbed with a balloon. ‘‘Many spiders balloon using multiple strands of silk that splay out in a fan-like shape,’’ said Dr Erica Morley, the lead researcher. ‘‘Previously, drag forces from wind or thermals were thought responsible . . . 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.’’
Aerial dispersal is crucial for many insects, giving species the best chance of surviving and spreading over a wide area. Spiders have been known to use their silk sails to travel hundreds of miles, but it had been thought they caught thermals or used slight breezes.
Baffled by the flying spiders, Darwin wrote: ‘‘Inundated by ballooning spiders on a relatively, calm, clear day,’’ later noting that the spiders took off again with ‘‘unaccountable speed’’.
The team at Bristol wanted to test whether spiders could be using the Atmospheric Potential Gradient (APG), a global electric circuit that is always present in the atmosphere.
To test whether the natural electricity could be helping, scientists exposed linyphiid spiders to similar electric fields in the lab. They found that when they switched on the field, the spiders moved on to their ‘‘tiptoes’’ as if getting ready to balloon. They sank down again when the field was turned off.
Dr Morley added: ‘‘The next step will involve looking to see whether other animals detect and use electric fields in ballooning. We also hope to carry out further investigations into the physical properties of ballooning silk and carry out ballooning studies in the field.’’
The research is published in the journal .–