“Scientists are finally starting to understand the centuries-old mystery of ‘ballooning.'”
From The Atlantic:
Spiders have no wings, but they can take to the air nonetheless. They’ll climb to an exposed point, raise their abdomens to the sky, extrude strands of silk, and float away. This behavior is called ballooning. It might carry spiders away from predators and competitors, or toward new lands with abundant resources. But whatever the reason for it, it’s clearly an effective means of travel. Spiders have been found two-and-a-half miles up in the air, and 1,000 miles out to sea.
It is commonly believed that ballooning works because the silk catches on the wind, dragging the spider with it. But that doesn’t entirely make sense, especially since spiders only balloon during light winds. Spiders don’t shoot silk from their abdomens, and it seems unlikely that such gentle breezes could be strong enough to yank the threads out — let alone to carry the largest species aloft, or to generate the high accelerations of arachnid takeoff. Darwin himself found the rapidity of the spiders’ flight to be “quite unaccountable” and its cause to be “inexplicable.”
But Erica Morley and Daniel Robert have an explanation. The duo, who work at the University of Bristol, has shown that spiders can sense the Earth’s electric field, and use it to launch themselves into the air.
Every day, around 40,000 thunderstorms crackle around the world, collectively turning Earth’s atmosphere into a giant electrical circuit. The upper reaches of the atmosphere have a positive charge, and the planet’s surface has a negative one. Even on sunny days with cloudless skies, the air carries a voltage of around 100 volts for every meter above the ground. In foggy or stormy conditions, that gradient might increase to tens of thousands of volts per meter.
Ballooning spiders operate within this planetary electric field. When their silk leaves their bodies, it typically picks up a negative charge. This repels the similar negative charges on the surfaces on which the spiders sit, creating enough force to lift them into the air. And spiders can increase those forces by climbing onto twigs, leaves, or blades of grass. Plants, being earthed, have the same negative charge as the ground that they grow upon, but they protrude into the positively charged air. This creates substantial electric fields between the air around them and the tips of their leaves and branches — and the spiders ballooning from those tips.