GARDEN ROUTE NEWS - In 1832 Darwin’s curiosity was piqued when he found thousands of spiders on the deck of the HMS Beagle, on which he was travelling around the world.
The ship was 100 kilometres offshore, so how could these wingless spiders fly so far? Darwin concluded that the spiders must have floated over from the Argentinian mainland.
Ballooning
This became known as ballooning, a process by which spiders move through the air by releasing silk threads to catch the wind, causing them to become airborne. Spiders have been found kilometres up in the air and thousands of kilometres out at sea. A spider climbs to an exposed point, raises its abdomens to the sky, extrudes strands of silk, and floats away.
It was generally believed that ballooning was the result of the spider’s silk floating on the wind. But Darwin found the rapidity of the spiders’ flight to be “quite unaccountable” and its cause to be “inexplicable”.
Spiders can sense the earth’s electric field
Recently Erica Morley and Daniel Robert from the University of Bristol figured out how ballooning works. They have shown that spiders can sense the earth’s electric field and use it to launch themselves into the air. The electric fields can even provide them with a lift without a breeze.
Earth’s atmosphere is essentially a giant electrical circuit due to the 40 000 thunderstorms that are generated around the world every day. These thunderstorms behave like a giant atmospheric battery, charging and maintaining the electric fields around the planet. Even on sunny, cloudless days, the air still carries around 100 volts for every meter above the ground. The highest reaches of Earth’s atmosphere (ionosphere) have a positive charge while the planet’s surface has a negative one.
Ballooning spiders operate within this planetary electric field. The moment their silk leaves their bodies, it picks up a negative charge. The similar negative charges are repelled on the surfaces on which the spiders sit, generating enough force to lift them into the air.
Testing the hypothesis
The concept that ballooning behaviour was caused by electrostatic repulsion was first proposed in the 1800s, but was dismissed because it hadn’t been tested. In 2013 the idea was revisited by physicist, Peter Gorham, who showed that it was mathematically feasible. Now Morley and Robert have tested this hypothesis with spiders.
They put spiders on vertical strips of cardboard in the middle of a polycarbonate box and then generated electric fields similar to what the spiders would experience in nature. The fields caused tiny sensory hairs (trichobothria) on the spiders’ feet to respond. “It’s like when you rub a balloon and hold it up to your hair,” Morley said.
Once the spiders’ trichobothria were ruffled they performed a set of movements called tiptoeing where they stand on the ends of their legs and protrude their abdomens in the air. This behaviour is only seen during ballooning. In spite of being in closed boxes with no airflow, many of the spiders took off. When the electric fields in the boxes were turned off, the spiders dropped.
