Sometimes, on a rainy day, when a predator is at bay, or if they want to migrate away, spiders whip out their silk, and balloon away. “Ballooning” has been a mystery for over a century already and now scientists are finally beginning to understand it.
Charles Darwin started being curious about how these wingless spiders can fly so far when he found thousands of them on the deck of the HMS Beagle ship on October 31, 1832. The ship was 60 miles offshore, which means the spiders must have floated over from the Argentinian mainland. These tiny red spiders, each a millimeter wide, were spotted all over the ship. “All the ropes were coated and fringed with gossamer web,” wrote Darwin.
What Is Ballooning?
Even though spiders have no wings they can still take to the air and fly away. Spiders have even been found two and a half miles up in the air and 1,000 miles out to sea. How do they do this? By ballooning. Ballooning is the behavior in which a spider climbs to an exposed point, raise their abdomens to the sky, extrude strands of silk, and float away.
It was believed that ballooning worked because the silk catches wind, dragging the spider with it. Although since spiders only balloon during light winds, that doesn’t make much sense. Some spiders are quite large, therefore it seems unlikely that such a light breeze could be strong enough to carry them aloft, or to generate the high accelerations of arachnid takeoff. Darwin found the speed of the spiders’ travel to be “quite unaccountable” and its cause to be “inexplicable.”
How does Ballooning work?
A duo from the University of Bristol, Erica Morley and Daniel Robert, have figured out the mystery of how ballooning really works. They discovered that spiders can actually sense the Earth’s electric field, and use it to launch themselves into the air. What’s impressive, is that the electric fields can even provide them with a lift without the slightest breeze.
Where Does The Electricity Come From?
The Earth’s atmosphere is basically a giant electrical circuit due to the 40,000 thunderstorms that crackle around the world every single day. These thunderstorms act like a giant battery for the atmosphere, charging up and maintaining the electric fields. Even on sunny, cloudless days, the air still carries around 100 volts for every meter above the ground. On stormy, foggy days, that number rises to tens of thousands of volts per meter. The highest reaches of the Earth’s atmosphere (ionosphere) have a positive charge while the planet’s surface has a negative one.
The 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. Spiders can increase those forces by crawling onto leaves, twigs or even grass. How do they increase those forces by just crawling on plants? Plants have the same negative charge as the ground, but they protrude into the positively charged air causing substantial electric fields between the air around them and the tips of their leaves and branches.
Testing The Spiders
The idea of ballooning behavior caused by electrostatic repulsion was first proposed in the 1800s but was dismissed without being tested. Then in 2013 the idea was brought back to life by a physicist, Peter Gorham, who showed that it was mathematically plausible. Now most recently, Morley and Robert were interested to see if the spiders actually responded to the electric fields and their fluctuations, so they tested it with actual spiders.
In order to show that the spiders can detect electric fields they put them on vertical strips of cardboard in the center of a plastic box, then generated electric fields between the floor and ceiling. They generated similar strengths of electricity to what the spiders would naturally experience outdoors. The fields caused tiny sensory hairs on the spiders’ feet to ruffle up. These sensory hairs are called trichobothria, which the researchers believe is what the spiders use to detect electricity. “It’s like when you rub a balloon and hold it up to your hairs,” Morley said.
Once the spiders’ trichobothria were ruffled they performed a set of movements called tiptoeing. Tiptoeing is when the spider stands on the end of their legs and stick their abdomens in the air, which is a behavior only ever seen when ballooning. Despite being in closed boxes with no airflow, many of the spiders managed to take off. But once Morley turned off the electric fields within the boxes, the spiders dropped.
Conclusion
The same hairs that allow spiders to sense electric fields also help them to detect wind speed or direction, so it’s possible that air currents might also play a role in ballooning. Nonetheless, Morley and Robert’s study reveals that electrostatic forces are, on their own, enough to propel spiders into the air.
The researchers published this study in Current Biology.
