A “shocking” new study suggests that spiders can fly by tapping into the air’s natural electricity.
University of Bristol biologist Erica Morley published a study this week in Current Biology on how electric fields trigger ballooning in spiders.
Ballooning is a common behaviour in most small spiders, particularly newborns and the orb-weavers and hacklemesh varieties, said Royal Alberta Museum bug guy Peter Heule. Almost every Alberta spider balloons at some point, but what prompts them to do so is little understood.
When a spider wants to disperse to new lands, you’ll see it stop and raise one or two legs up into the air, kind of like how people wet their fingers and hold them up to tell which way the wind is blowing, Heule explained. If conditions feel right, the spider will stand on its tiptoes, raise its abdomen, appear to shoot strands of webbing from its butt and thwip! – zip skyward.
“These strands can be several metres long but crazy thin,” Heule said, noting that the strands are not shot but somehow pulled from the spider’s spinnerets.
“They’re actually like 320 nanometres thick, which means they’re thinner than an ultraviolet light wavelength!”
Once airborne, ballooning spiders can travel up to four kilometres up and far out to sea before they land, Heule said. Naturalist Charles Darwin actually saw spiders land on his ship The Beagle back in the 1800s on calm days.
While researchers have long thought that spiders used the wind to power their balloon trips, tests suggested that this couldn’t be the whole story, Morley said in an email. Spiders accelerate at takeoff far faster than the wind alone should allow, and can take off in breezes far too small for their body size. Their silk lines also splay out in a fan-like shape instead of tangling, suggesting a repulsive force between them.
Morley and her team sought to test the theory that electrostatic forces could be involved in ballooning – a theory that had been around since the 1800s, but has never been tested. Electric fields are naturally present in the air, and can range from 120 V/m on clear days to several kV/m during thunderstorms.
Morley’s team took sheet weaver spiders and placed them on vertical strips of cardboard inside clear plastic boxes in a room isolated from sound and electric fields. They then filmed the spiders to see if they became more or less likely to stand on tiptoes (indicating they were about to balloon) when exposed to three levels of electric field (0 V, 1.25 kV, and 6.25 kV).
The team found that the spiders were significantly more likely to go tiptoe the stronger they made the field in the box. Once airborne, the spiders flew upwards when the team turned the field on and down when they turned it off. By using a laser in a way similar to a photo-radar gun, the team determined that tiny hairs on the spiders’ bodies moved in response to electric fields, suggesting that they could sense fields through the hairs.
Heule said he’s not surprised that spiders could use their many hairs to detect electric fields – humans can use their arm hairs to detect thunderstorms. He also noted that this study suggests that it’s the electric charge of the air itself that pulls silk from spider bottoms.
Morley said this research suggests that spiders can use electric fields to know when to balloon and to actually take off. It’s unclear if the fields are actually required for flight, or if spiders can use them to navigate.
“To see that electric fields trigger ballooning behaviour is fascinating and surprising,” Morley said, particularly in untrained spiders.
Morley now hopes to study the silk used in ballooning and figure out the role of wind and electric fields in ballooning. This research could also help scientists figure out how caterpillars and other ballooning species disperse.
Researchers could potentially direct spiders to pest-filled areas if they could figure out the triggers for ballooning, Heule said.
“It just adds one more factor to how amazing spiders are.”
The study can be found at bit.ly/2NuzZXt.
