Scientists are finally starting to understand the centuries-old mystery of “ballooning.”
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.”
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.
This idea—flight by electrostatic repulsion—was first proposed in the early 1800s, around the time of Darwin’s voyage. Peter Gorham, a physicist, resurrected the idea in 2013, and showed that it was mathematically plausible. And now, Morley and Robert have tested it with actual spiders.
First, they showed that spiders can detect electric fields. They put the arachnids on vertical strips of cardboard in the center of a plastic box, and then generated electric fields between the floor and ceiling of similar strengths to what the spiders would experience outdoors. These fields ruffled tiny sensory hairs on the spiders’ feet, known as trichobothria. “It’s like when you rub a balloon and hold it up to your hairs,” Morley says.
In response, the spiders performed a set of movements called tiptoeing—they stood on the ends of their legs and stuck their abdomens in the air. “That behavior is only ever seen before ballooning,” says Morley. Many of the spiders actually managed to take off, despite being in closed boxes with no airflow within them. And when Morley turned off the electric fields inside the boxes, the ballooning spiders dropped.
It’s especially important, says Angela Chuang, from the University of Tennessee, to know that spiders can physically detect electrostatic changes in their surroundings. “[That’s] the foundation for lots of interesting research questions,” she says. “How do various electric-field strengths affect the physics of takeoff, flight, and landing? Do spiders use information on atmospheric conditions to make decisions about when to break down their webs, or create new ones?”
“I think Charles Darwin would be as thrilled to read it as I was,” he adds.