Spiders, creatures often associated with webs and scurrying legs, might not seem like candidates for flight. Yet, surprisingly, spiders possess a remarkable ability to travel vast distances through the air, even across entire oceans. This phenomenon, known as “ballooning,” has intrigued scientists for centuries. For a long time, wind was considered the primary force behind this aerial dispersal, propelling spiders on silken threads high into the atmosphere, sometimes reaching jet stream altitudes. However, recent groundbreaking research has unveiled another fascinating factor at play: Earth’s electric field. This discovery adds a new dimension to our understanding of spider flight, revealing that these eight-legged adventurers are not merely at the mercy of the wind, but can also harness electrostatic forces to take to the skies.
A new study published in Current Biology has shed light on the role of electric fields in spider ballooning. Researchers found that spiders placed in a controlled environment, devoid of wind but with a weak electric field, exhibited behavior indicative of flight preparation. They even managed to take off. Intriguingly, the study also revealed that the fine sensory hairs covering a spider’s body respond to electric fields, much like human hair reacts to static electricity. This “spidey sense” for electric fields could be the key to how spiders detect and utilize these forces for aerial voyages.
This revelation positions spiders as only the second known arthropod species, after bees, to possess the ability to sense and respond to electric fields. Erica Morley, the lead author of the study, emphasizes that the significance of Earth’s electric field in biological processes is often underestimated because humans are unable to perceive it.
When spiders sense an electric field, they extend their spinnerets into the air to release silk, a behavior known as 'tiptoeing,' signaling their readiness for takeoff.
The inspiration for Morley’s research traces back five years to an unexpected source: astrophysicist Peter Gorham. Gorham, while reading Charles Darwin’s accounts of mass spider ballooning observed from a ship at sea, was struck by Darwin’s speculation about electrostatics playing a role in spider takeoff. Intrigued, Gorham explored this idea through a physics lens.
“When I worked through the numbers it looked quite compelling,” Gorham noted. “This was a plausible explanation for not all of the flight but at least for some of it.”
Gorham’s theoretical work, published on the open-access platform arXiv, piqued the interest of Morley, a sensory biologist at the University of Bristol. Recognizing an opportunity to investigate this intriguing hypothesis experimentally, Morley embarked on her groundbreaking study.
While the influence of wind currents in carrying spiders to great heights and distances has been recognized for some time, explaining how larger spiders achieve loft even on calm days remained a puzzle. These “ballooning” events facilitate spider dispersal across hundreds of miles, enabling them to colonize new ecosystems.
However, the notion of electric fields contributing to spider flight had been dismissed two centuries prior. Morley explained, “In the early 1800s, there were arguments that spiders might be using electric fields to balloon, but then there were also people arguing that it was wind. And the argument for wind won over probably because it’s more obvious.”
Since then, scientists have established the existence of a naturally occurring global electric field, situated between the Earth’s negatively charged surface and the positively charged ionosphere, extending 50 to 600 miles above. Yet, the potential impact of this electric field on spider ballooning remained largely unexplored until Morley’s recent investigation.
Building a Flight Simulator for Spiders
To meticulously study spider behavior under controlled conditions, Morley constructed a specialized “arena” roughly the size of a mini-refrigerator. This setup was designed to shield spiders from both air currents and external electric fields. To replicate natural electric field conditions, Morley installed charged metal plates – electrodes – at the top and bottom of a transparent plastic enclosure.
Upon activating the electric field, Morley observed a significant change in spider behavior. Spiders began exhibiting “tiptoeing” on a vertical cardboard strip, mimicking their pre-flight preparations on branches or leaves in natural settings.
Fritz Vollrath, an evolutionary and behavioral biologist at the University of Oxford (unaffiliated with the study), described this behavior: “The spider goes somewhere, sticks his bum in the air, lets out some silk, and then waits to take off.”
Morley documented a dramatic increase in this “tiptoeing” behavior when the electric field was switched on. Remarkably, in some instances, spiders even commenced flight within the experimental box.
Further demonstrating the influence of the electric field, Morley was able to manipulate a spider’s vertical movement during flight by modulating the electric field. “I was really excited when that happened,” Morley recounted, highlighting the direct control she had over spider flight through electric field manipulation.
Morley hypothesized that spiders’ numerous sensory hairs, known to detect sound and subtle air currents, might also be sensitive to electric fields. To test this, she directed a focused laser beam onto individual hairs to observe their movement in response to electric field activation.
Once airborne, spiders can travel short distances of a few feet or embark on epic journeys spanning thousands of miles, even crossing oceans to colonize new continents.
By analyzing the reflected laser light, Morley could detect minute hair movements. She discovered that a specific type of sensory hair, called trichobothria, exhibited movement in response to the electric field, while other types of hairs remained unaffected.
Understanding spider migration patterns is crucial, as these top insect predators play a vital role in maintaining ecological balance, according to Vollrath. Monitoring fluctuations in natural electric fields caused by weather patterns could potentially enable scientists to predict mass ballooning events, where thousands of spiders take flight simultaneously. Such events can have significant impacts on insect populations across entire continents.
One potential practical application of this research lies in pest control. Spiders are natural predators of many agricultural pests, such as fruit flies. Vollrath playfully suggested that artificially generated electric fields might be used to attract spiders to croplands, leveraging their pest-control capabilities.
Gorham expressed his admiration for the robust statistical rigor of Morley’s experiments, emphasizing the extremely low probability of the observed spider “tiptoeing” behavior being mere coincidence.
“These authors have done exactly the right kind of experiment,” Gorham concluded. “I think Charles Darwin would be thrilled.”
