Having never evolved wings, many species of spider instead evolved an uncanny ability to take to the skies using nothing more than a few short threads of gossamer dangling from their dainty butts.
Just how this invertebrate answer to paragliding works has never been entirely clear, though historically biologists have assumed it probably has something to do with swirling eddies of warming air close to Earth's surface.
An alternative suggestion is gaining attention, however, as evidence piles up in support of a rather steampunk mechanism. Instead of riding thermals, spiders might instead sail into the sky on tides of electricity.
Studies conducted by researchers from the University of Bristol in 2018 showed electric fields generated by weather activity could sufficiently drag a single electrostatically-charged strand of web and its aeronautical arachnid off the ground.
Now, a new study modeling the mathematics behind the electromagnetic interactions on multiple dangling spider threads has contributed important new details to the discussion.
This isn't to say electric charges are necessarily responsible for the phenomenon scientists refer to as ballooning, either wholly or partially. But it does answer a bunch of questions on the actual physics at work.
The fact spiders can add a slight charge to their webs in order to catch prey (and potentially pick up pollutants) has been a focus of experimental studies for some time now.
Unfortunately, measuring the electrostatic activity of a short drift of thread is a lot harder to do under laboratory conditions.
So researchers kept things simple, by using simple modeling to determine how a single electrostatically charged thread spun from a spider's bum might interact with an atmosphere's own weakly charged field.
In reality, ballooning spiders can spin two, three, or even dozens of fine strands to get them up, up, and away. Just how each thread, coated in negatively-charged material, might interact with other threads is an open question.
To explore that question, physicists Charbel Habchi from Notre Dame University-Louaize in Lebanon, and Mohammad K. Jawed from the University of California, Los Angeles, combined measurements from previous studies with an algorithm commonly used in computerized graphics to trace hair.
Attaching between two and eight virtual hairs to a 2-millimeter-wide sphere that represented a tiny species of spider, they could tweak a range of variables such as the distribution of charge, atmospheric electric fields, and air resistance, and watch it fly.
At first, the threads all remained more or less vertical. But as the simulations unfolded, the negative charges along the threads pushed apart, expanding the collection of strands into an inverted cone-shape.
This in turn slowed their ascent, causing them to drop and the strands to collect together again, making tension between electrostatic repulsion and atmospheric drag an important factor in determining the thread-count of a spider balloon.
"We think that, at least for small spiders, the electric field, without any help from upward air currents, can cause ballooning," Habchi told Rachel Berkowitz at Physics.
As for larger spiders, it's possible a good kick from a rising air current might be necessary, implying the competing hypotheses behind spider-flight might not be so mutually exclusive after all.
Having a sound model is one thing. Backing it up experimentally will be more of a challenge.
On the other hand, if the mathematics works, it could be grounds for a new avenue of spider-inspired flight technology, used to send nanoscale drones out on Earth's air currents or on far distant worlds.
This research was published in Physical Review E.