Spiders Can Fly Thousands of Miles With Electric Power
by Becky Ferreira  /  Jul 5 2018

“For the first time, scientists have empirically verified electricity’s role in the ballooning abilities of spiders. On Halloween in 1832, the naturalist Charles Darwin was onboard the HMS Beagle. He marveled at spiders that had landed on the ship after floating across huge ocean distances. “I caught some of the Aeronaut spiders which must have come at least 60 miles,” he noted in his diary. “How inexplicable is the cause which induces these small insects, as it now appears in both hemispheres, to undertake their aerial excursions.”

“A water moat surrounds the takeoff site to prevent spiders escaping over ground. The water was electrically floating, not connected to ground or a voltage.”

Small spiders achieve flight by aiming their butts at the sky and releasing tendrils of silk to generate lift. Darwin thought that electricity might be involved when he noticed that spider silk stands seemed to repel each other with electrostatic force, but many scientists assumed that the arachnids, known as “ballooning” spiders, were simply sailing on the wind like a paraglider. The wind power explanation has thus far been unable to account for observations of spiders rapidly launching into the air, even when winds are low, however.

Now, these aerial excursions have been empirically determined to be largely powered by electricity, according to new research published Thursday in Current Biology. Led by Erica Morley, a sensory biophysicist at the University of Bristol, the study settles a longstanding debate about whether wind energy or electrostatic forces are responsible for spider ballooning locomotion. “Dispersal is a crucial part of ecology and ballooning is one mode of dispersal,” Morley told me in an email. “If we can better understand how this works and the mechanisms behind it we can better understand the dispersal patterns of spiders and other ballooning animals.”

“Position on the tribolelectric series of various materials, including bumble bees and rabbit fur. No material caused a bumble bee to become negatively charged after contact, placing the bee at the most positive end of the triboelectric series along with rabbit fur.”

In the last decade, doubts about the wind power explanation for arachnid flight have mounted alongside observations of spiders ballooning when winds are not strong enough to lift them even a few feet, let alone for thousands of miles. The animals can also reach high altitudes—sometimes five kilometers (three miles) above Earth’s surface—which would be difficult to accomplish with thermal air currents alone.

In 2013, Peter Gorham, a physicist at the University of Hawaii, calculated that it was theoretically possible for spiders to use their silk to conduct static electricity as a means to fly, but Morley’s team is the first to confirm this in the laboratory. Morley and her team placed Linyphiid spiders in a lab-controlled electric field with charges similar to Earth’s atmosphere, so the group could directly observe the impact of electricity on arachnid flight. The spiders ascended into the air when electric fields were present, and descended when the team turned them off, demonstrating a clear correlation between electrostatic forces and ballooning ability.

Wind also plays a role in spider voyages, but the new paper establishes that the rapid launches, even in calm weather, can be explained by electrically conductive spider silk. Morley hopes to expand on this finding by studying other ballooning insects, like caterpillars and spider-mites, which can shed light on the incredible journeys and global distribution of these tiny fliers. Almost two centuries after Darwin noted the odd repulsion between the spider strands, his hunch about electrically-powered flying spiders has been proven right.”

“Finite-element modelling and visualisation of bee/flower electrostatic interaction in pollen transfer. Pollen travelling from the bee to the stigma is positively charged while pollen originating on the flower and travelling to the bee negatively charged.”

Spiders go ballooning on electric fields / 5 July 2018

“The aerodynamic capabilities of spiders have intrigued scientists for hundreds of years. Charles Darwin himself mused over how hundreds of the creatures managed to alight on the Beagle on a calm day out at sea and later take-off from the ship with great speeds on windless day. Scientists have attributed the flying behaviour of these wingless arthropods to ‘ballooning’, where spiders can be carried thousands of miles by releasing trails of silk that propel them up and out on the wind. However, the fact that ballooning has been observed when there is no wind to speak of, when skies are overcast and even in rainy conditions, raises the question: how do spiders take off with low levels of aerodynamic drag?

(A) A finite element model of a bumblebee hair under an electric field produced 1cm away.”

Biologists from the University of Bristol believe they have found the answer. “Many spiders balloon using multiple strands of silk that splay out in a fan-like shape, which suggests that there must be a repelling electrostatic force involved,” explains lead researcher Dr Erica Morley, an expert in sensory biophysics. “Current theories fail to predict patterns in spider ballooning using wind alone as the driver. Why is it that some days there are large numbers that take to the air, while other days no spiders will attempt to balloon at all? We wanted to find out whether there were other external forces as well as aerodynamic drag that could trigger ballooning and what sensory system they might use to detect this stimulus.”

“The white circle containing a plus (+) denotes electrode insertion points in the antennae. The white circle containing a cross (x) denotes approximate electrode insertion points for hair recordings.”

The solution to the mystery could lie in the Atmospheric Potential Gradient (APG), a global electric circuit that is always present in the atmosphere. APGs and the electric fields (e-fields) surrounding all matter can be detected by insects. For example, bumblebees can detect e-fields arising between themselves and flowers, and honeybees can use their charge to communicate with the hive.

“Experimental visualisation of floral electric field using electrostatic dusting. Flowers are shown before (left) and after (right) dusting with positively charged coloured powder. Flowers were connected to electrical ground and no APG was experimentally imposed.”

Spider silk has long been known as an effective electric insulator, but until now, it wasn’t known that spiders could detect and respond to e-fields in a similar way to bees. In their study, the findings of which appear today in the journal Current Biology, Bristol’s researchers exposed Linyphiid spiders to lab-controlled e-fields that were quantitatively equivalent to those found in the atmosphere. They noticed that switching the e-field on and off caused the spider to move upwards (on) or downwards (off), proving that spiders can become airborne in the absence of wind when subjected to electric fields.

“Motion of a bumblebee hair (A) and antenna (B) measured by laser Doppler vibrometry in response to an AC voltage sweep (400 V sine wave).”

Dr Morley added: “Previously, drag forces from wind or thermals were thought responsible for this mode of dispersal, but we show that electric fields, at strengths found in the atmosphere, can trigger ballooning and provide lift in the absence of any air movement. This means that electric fields as well as drag could provide the forces needed for spider ballooning dispersal in nature.”

“Visualising electric ecology. Finite-element model of electric interactions between positively charged bumble bees and grounded petunias (Petunia sp.) against the background of the atmospheric potential gradient. Petunias are modelled as slightly electrically conductive bodies that are grounded to earth (resistivity ~10 MΩm).”

The findings have applications beyond the world of arthropods. Aerial dispersal is a crucial biological process for many caterpillars and spider-mites as well. An improved understanding of the mechanisms behind dispersal are important for global ecology as they can lead to better descriptions of population dynamics, species distributions and ecological resilience. There is, however, more work to be done. Dr Morley said: “The next step will involve looking to see whether other animals also detect and use electric fields in ballooning. We also hope to carry out further investigations into the physical properties of ballooning silk and carry out ballooning studies in the field.”

“Background colour shows strength of the electric field as a function of altitude from ground (scale inset). The positive ionospheric charge is included, located between 60 and 100 km above ground (not to scale). Local distortions in the uniform APG, caused by the presence of the trees, are labelled. The approximate space charge density is shown inset. The APG induces negative charges to build up on the surface of the ground and, in higher density, on the upper surfaces of the trees, which, in turn, induce opposite charges in the surrounding air.”

How Do Spiders Fly for Miles?
by Liz Langley / September 27, 2013

“As if spiders aren’t unnerving enough, did you know that some of them use an electrostatic charge to leap into the air and fly for miles? The flight of the gossamer spider was a curiosity even to Charles Darwin, who noted that his boat, the HMS Beagle, was “inundated by ballooning spiders on a relatively calm, clear day,” according to a new paper by Peter Gorham of the University of Hawaii that’s posted on the arXiv website.

Darwin watched two species of spider, one smaller, one larger. The first raised its abdomen, released a thread and launched itself horizontally with “unaccountable” speed. A larger species released several threads more than a yard long, which he described as undulating “like films of silk blown by the wind.” The spider then let go of the post it had been perched on, and flew away —an arachnid paraglider in action.

Darwin thought thermal air currents could be the secret of the spiders’ aerial abilities, but that doesn’t explain things like why the threads fanned out and how even fairly heavy spiders launched so quickly when the air was relatively still. Also, as the Physics arXiv blog notes, these spiders have been found at altitudes as high as 2.5 miles (4 kilometers) and are not likely to have gotten there by hot air alone.

Darwin and others also theorized that “electrostatic repulsion” played a role in the fanning of the threads. Lo these many years later, Gorham says that indeed, electrostatic forces could determine the spiders’ flight. “There are thus a wide and plausible range of processes by which the strands can acquire initial charge,” Gorham writes. One of these is the charging of the strands by the earth’s atmosphere during spinning in a process called “flow electrification.”

From the arXiv blog: “There must be a source for this charge, of course. Gorham thinks a likely origin is the Earth itself, which has a negative charge density of about 6 nanoCoulombs per square metre on average. That’s more than enough to give the silk a healthy boost and spiders may well be able to pick out prominences where the charge density is much higher.” All this explains the spider’s launch power in still air, why large spiders can get such a lift and why the silk strands fan out: “because their negative charges repel.” The arXiv blog notes that Gorham’s theory still needs to be tested by some “enterprising biologist.”

Ballooning Spiders Fly Without Wind
by Breanna Draxler / September 23, 2013

“Gossamer spiders are best known for their bizarre “ballooning” stunts, but it’s only this week that we’ve learned how they pull them off. They disperse by spinning strands of silk into the open air, which allows them to float through the atmosphere miles above the surface of the earth and out to sea far beyond the reach of land. These 8-legged kites can apparently survive 25 days without food during their aeronautical journeys. Even Darwin was baffled by their methods—as he wrote in his diary on October 31, 1832, from his ship off the coast of Argentina,

“In the evening all the ropes were coated & fringed with Gossamer web. I caught some of the aeronaut spiders, which must have come at least 60 miles: How inexplicable is the cause which induces these small insects, as it now appears in both hemispheres, to undertake their aerial excursions.”

“Atmospheric potential gradient (APG) measured for 30 min periods across 3 days”

Darwin conjectured that unnoticeable thermal currents could explain the initial launch, but since the strands repelled each other, he assumed there was electrostatic force at work, too. But since the 1830s, most scientists have accepted wind to be the force of choice for directing spider flight. In 1874 a study reported that a spider “patiently waits for a breath of air to waft it across the vacant space,” and a hundred years after that, scientists still thought “aeronautic behavior is dependent upon wind currents of specific velocity and direction.”

silk flows from a spiny-backed spider

But this week a researcher in Hawaii determined that it wasn’t wind’s thermal currents that gave the spiders lift. Their flight, he determined, is actually electrostatic. Some of that charge comes from the electrostatic field of Earth’s atmosphere. Some charge comes from friction between the silk and dry air. The rest is thought to result from the spinning process and the launch surface itself. As described in the paper published recently in arXiv:

The presence of this charge will lead both to mutual repulsion among the emitted strands, and an additional overall induced electrostatic force on the spider, providing a component of lift that is independent of convection or aerodynamic effects.

“Spiny-backed spider piriform gland spigots (from the anterior spinneret) through which silk is being secreted (Gasteracantha sp.). Magnification: x490, Type:scanning electron micrograph. Photo Dennis Kunkel.”

As reported in The Physics arXiv blog:  “During his famous voyage aboard the HMS Beagle in the 1830s, Charles Darwin witnessed an extraordinary sight while 60 miles off the coast of Argentina. The day was calm and clear but the ship was inundated by clouds of ballooning spiders. These spiders ranged in size from 2 millimetres to about 7 millimetres and launched themselves by releasing several threads of silk into the air which then carried them away. Darwin recorded the event in his journal: I repeatedly observed the same kind of small spider, 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.”

Even now, the question of how many types of small spider achieve their magnificent flying feat has never been fully explained. Conventional thinking is that these spiders use their silk to catch thermal air currents which carry them to considerable height. But this explanation leaves much unanswered. For example, how can these spiders launch themselves with a surprisingly high velocity even when there is little or no wind; how do thermal currents lift heavy adult spiders of up to 100 milligrams; and when spiders release several threads, why do these threads form a fan-shape as if repelling each other? What’s more, biologists have found these spiders at altitudes up to 4 kilometres. That’s hard to explain using thermal currents alone.”





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