Soaring spiders may get cues from electric charges in the air

Spiders may lack wings, but they aren’t confined to the ground. Under the right conditions, some spider species will climb to a high point, release silk strands to form a parachute, and float away on the breeze. Buoyed by air currents, they’ve been known to drift kilometers above Earth’s surface, and even to cross oceans to reach new habitats (SN: 2/4/17, p. 12).

Now, new research suggests air isn’t the only force behind this flight, called ballooning. Spiders can sense electrical charges in Earth’s atmosphere, and the forces exerted by these charges might be a cue for them to alight, researchers suggest July 5 in Current Biology. That invisible signal could help explain why spiders’ take-off timing seem a bit, well, flighty. Some days, arachnids balloon en masse; other days, they remain firmly grounded despite similar weather conditions.
Spiders with atmospheric aspirations do need a gentle breeze with speeds below around 11 kilometers per hour, past studies have shown. But those speeds alone shouldn’t be strong enough to get some of the larger species of ballooning spiders off the ground, says Erica Morley, a sensory biologist at the University of Bristol in England.
So scientists have long wondered if some other force might be involved: Perhaps electrical charges in Earth’s atmosphere push against the silken threads of airborne spiders’ silk streamers to help them stay fanned out in a parachute. These electric charges form an electric field that attracts or repels other charged objects or particles. It varies in strength, becoming stronger around objects such as leaves and branches on trees, and also fluctuating with meteorological conditions.
In the first experimental test of whether spiders can sense these electric charges, Morley and Bristol sensory biologist Daniel Robert blocked out naturally occurring electric fields in a lab. They then created an artificial one mimicking what would-be arthropod aerialists would experience, and placed teeny-tiny spiders from the Linyphiidae family into that faux field. Under the electric field, even with no breeze, the spiders perched on the tips of their legs, a ballerina-like behavior that precedes ballooning. When the researchers switched off the artificial electric field, the behavior (which scientists call the “tiptoe stance”) subsided.
Tiny hairs on the spiders’ bodies react to both moving air and an electric field’s presence, but differently, Morley found. The hairs stood on end as long as air was blowing on them. But when faced with an electric field, they stood on end most dramatically when the field was switched on and then gradually deflated to their resting position over about 30 seconds.

The study links the pre-ballooning tiptoeing behavior to the presence of an electric field, but actually taking off might require something more, suggests Moonsung Cho, an aerodynamics researcher at the Technical University of Berlin who wasn’t involved in the study. While some spiders in the study did incidentally float away, that liftoff behavior wasn’t actually measured.

And responding to electric fields probably isn’t the full story when it comes take-off timing: A different genus of spider, Xysticus, or ground crab spiders, appears to sense wind speed with its legs before going aloft, wiggling one spindly appendage around to sense moving air and determine whether wind conditions are favorable for lift-off, Cho’s team reported June 14 in PLOS Biology.