Exploring the mystery of invasion by ballooning
How do spiders float through the air? Scientists have explained this by demonstrating that spiders detect and react to electric charges around them
Reading about all that Hurricane Florence wrought in the US in September, at one point, a butterfly came to mind. I realized that’s a little incongruous—putting it kindly—given the vast destruction Florence caused and the lives it took. But it’s true. Because Florence churned across North Carolina’s Outer Banks islands on the way to inundating much of that lovely state, I thought of a particular butterfly.
The Outer Banks are long, narrow barrier islands that lie off the coast of North Carolina. They were made famous by the Wright Brothers, who in 1903 flew their “Flyer” at Kitty Hawk, in the northern reaches of the Outer Banks. When not being assaulted by hurricanes, the islands are a charm to wander through, with their dunes, long grass swaying in the breeze and the views of the sea on both sides.
The southernmost of the islands is Ocracoke, and one way to get there is by a ferry from a point on the mainland known as Cedar Island. The ferry takes a couple of hours to make the trip, which gives you an idea of just how far offshore Ocracoke is. Once that I took it, I remember standing at the railing and looking out at the shimmering sea and the cloudless sky, luxuriating in the breeze. No land was visible anywhere.
And then I saw the butterfly.
One lone butterfly, fluttering along several feet above the water, being steadily overtaken by our ferry. One lone butterfly, several miles from land in any direction. I was flabbergasted. Where did this little creature come from? Where was it going? I didn’t have answers, of course. So I put it down to one of those wondrous mysteries of nature and forgot about it till Florence beat up on the Outer Banks last month.
But I thought of that butterfly again a few days later, when I ran across an article about spiders in The Atlantic. Here are the lines in question: “On October 31, 1832, a young naturalist named Charles Darwin walked onto the deck of the HMS Beagle and realized that the ship had been boarded by thousands of intruders. Tiny red spiders, each a millimeter wide, were everywhere. The ship was 60 miles offshore, so the creatures must have floated over from the Argentinian mainland.”
I savour possibly the sole parallel between the great naturalist and me. Him on the Beagle, me on the Ocracoke ferry—both of us flabbergasted. How did a host of tiny spiders “float” nearly 60 miles (around 100km) across open sea? If the magnitude of that feat doesn’t immediately strike you, let me give it some perspective. A 100km is about 100,000,000 (that’s one hundred million) times the size of the little red fellows. So their journey to the Beagle was as if you or I journeyed 100,000km. How far is that? Two-and-a-half times around our planet. Or one-third of the way to the moon. Not your average weekend jaunt.
Why would these minuscule spiders attempt a journey on this epic scale? How did they accomplish it? I’m not so sure about the “why”. For the “how”, there’s one word: “ballooning”. No, I don’t mean that they hitch a ride on some convenient hot-air balloons. With spiders, that term refers to a peculiar behaviour. From The Atlantic again: “They’ll climb to an exposed point, raise their abdomens to the sky, extrude strands of silk, and float away.”
They are light enough to float through the air while being suspended from silk strands like that. And sometimes, hundreds and thousands of spiders will simultaneously start floating—as Darwin found they had done that day in 1832.
Yet, here’s a kicker: Despite what you might think, it’s not wind that makes them take to the air this way. Not wind that carries them across stretches of ocean. We know this because spiders never behave this way during a storm, or even a gale, for example. Meaning, windy conditions are not what spiders are looking for when they want to fly. Instead, they take off only when there are light, weak breezes (less than about 10kmph). In fact, even in conditions that you and I might describe as “still”, spiders can leave the ground and sail along. In fact, some larger spiders are known to balloon like this as well, and certainly that gentle a breeze would not be enough to carry heavier creatures like them into the air.
So, if it is not the wind that carries spiders along, and often pretty swiftly, what is it? The question baffled Darwin himself: “I repeatedly observed [this kind of] small spider,” he writes in his Voyage of the Beagle, “either when placed or having crawled on some little eminence, elevate its abdomen, send forth a thread, and then sail away horizontally, but with a rapidity which was quite unaccountable.”
There’s a hint of an answer in the silk that the animals extrude. Each spider sends out several strands. You might think that this simply helps them fly on a wind, but it’s more interesting than that: The strands spread out like a fan, much as if each strand has a magnetic or electric charge, and is repelled by the other strands.
Two scientists, Erica Morley and Daniel Robert, of the University of Bristol, recently demonstrated that spiders are attuned to the electric field that surrounds our earth. You didn’t know about this field? Well, next time you’re in a thunderstorm, watch for lightning, which happens because of electricity. And it’s estimated that there are about 40,000 thunderstorms on the planet every single day—so yes, you could say we all live in an enormous electric field.
Spiders use that electricity to rise into the air. Morley and Robert suggest in their paper (Electric Fields Elicit Ballooning in Spiders, Current Biology, 23 July 2018) that the strands of silk pick up a negative charge as they leave the spiders’ bodies. Now the air around the spiders, close to the surface of the earth as they are, is also negatively charged. As you have no doubt guessed, these two negative charges repel each other (as do the charges on individual strands, producing the fan). That’s enough to lift the spiders into the air.
Such a simple idea. So simple that it was actually proposed in the early 1800s—years before Darwin observed them swarming onto his beloved ship—as the mechanism by which arachnids fly. But nobody had shown that spiders could actually detect electrical charges and use them in this astonishing way. Nobody, that is, until Morley and Robert.
How did they do that?
They write: “We presented adult Linyphiid spiders with [electric] fields quantitatively commensurate with atmospheric conditions.” That is, they put these spider subjects into electric fields similar to what they encounter in their real lives. This happened inside a box, so there was no real air movement, meaning that floating behaviour by the spiders—if any occurred—would be triggered only by any electric field inside the box. Which there was: Morley and Robert put two horizontal metal plates (top and bottom) and, between them, a vertical strip of cardboard. They connected the top plate to an electric supply, the bottom one to the ground. Finally, they put some intrepid spiders on the cardboard strip.
When they turned on the apparatus, they immediately created an electrical field inside the box, between the metal plates. The spiders inside the box and subject to the field, they were thrilled to find, reacted like they were born to electricity. (Well, actually they are). Morley writes: “They try to balloon. They perform this tiptoeing behaviour, and try to balloon.”
That is, they raise their abdomens and extrude silk strands—behaviour only ever seen in spiders before they balloon. Some of them even actually rise into the air, if still inside the boxes. I’d have liked to be a fly on the wall—possibly predatory spiders notwithstanding—when this happened, to watch Morley’s and Robert’s reaction to what their experimental subjects were doing.
Then they turned off the electricity. No more charge for ballooning spiders to count on, so they dropped to the ground. (Slowly). Then they turned it on again. The spiders ballooned again.
There’s plenty of work ahead, several intriguing questions to be answered. For now, though, Morley and Roberts have managed to show, with an experiment simple enough for nearly anyone to follow, that spiders detect and react to electric charges around them; and that this is probably the foundation of their ballooning behaviour. Online, their paper includes a video clip that sums it up thus: “One of the best aviators in the animal kingdom does not even possess wings.”
Just remarkable, fascinating science.
Two questions remain, though. One, how did the floating spiders detect the Beagle on the sea below and descend onto it? Two, how did that butterfly get to where I saw it, halfway to Ocracoke?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun
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