Our worst fears are confirmed: Spiders can fly - and now we know why!
Charles Darwin's greatest mystery has been solved, and all we can think about is when can we live on Mars
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 without any wind to blow them on board.
Similarly, they would suddenly soar away again, as if magically plucked up by an unseen hand into the serene 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 lead researcher Dr Erica Morley, an expert in sensory biophysics.
“Current theories fail to predict patterns in spider ballooning using wind alone. 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.”
Aerial dispersal is a crucial process for many insects, and often occurs soon after birth to give 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 kilometres, but it was previously supposed they caught thermals or used slight breezes to waft themselves onwards.
Baffled by the sight, Darwin wrote in his HMS Beagle diary: “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.
Insects are already known to pick up and use this current. For example, bumblebees can detect e-fields arising between themselves and flowers, and honeybees can use their charge to communicate with the hive.
To test whether the natural electricity in the atmosphere 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 tip-toes as if getting ready to balloon. They sank down again when the field was turned off.
Morley added: “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.
“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. 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 Current Biology.
– © The Daily Telegraph