There are spiders who fly. Flying spiders don’t have wings or even skin stretched between extended fingers like bats, but they have can travel hundreds of miles even when there's no wind.
At least some flying spiders achieve air travel by poising on a launchpad, which can be anything from a treetop to a flower, raising their tush, and releasing multiple threads of silk that look like flimsy fans. Unlike their web, this makeshift balloon stays attached to their bottoms. Then they get wafted off by the wind.
Or not. That had been the assumption until it was noticed that these aviating arthropods can achieve aerial locomotion on windless days.
And fly they do on still days. It probably helps that most of the flying spider species are tiny, which sounds like a good thing until you consider that unless you have sharp eyesight, you may not see them even though Europe is thronged with the little things, which fly chiefly in the autumn.
Anyway, wind alone cannot predict ballooning patterns or observation completely, hinting at the involvement of other factors, co-author Prof. Erica Morley, who researches sensory systems in insects and spiders, told Haaretz.
"Ballooning is widespread and happens frequently, largely going completely unnoticed," she says - explaining that most spiders that fly thusly are very small, a few millimeters in length.
It's the big ones a whole centimeter in length where things get mysterious. Light winds alone cannot explain how these beefier spiders get airborne, Morley says.
Furthermore, flying spiders only balloon in a light breeze featuring wind speeds below 3 meters per second. Any breezier than that and they don't even try. And there's more.
"The fact that ballooning has been observed when there is no wind to speak of, when skies are overcast and even in rainy conditions, begs the question - how do spiders take off with low levels of aerodynamic drag?" asked Morley and Daniel Robert of the University of Bristol, writing in Current Biology.
In short, current theories failed to predict patterns in spider ballooning using wind alone as the driver, Morley explains. Some days vast numbers take to the air and on others, the spiders stay put and don't even try, she says.
The eureka moment lay in the very fan-like shape of spider balloon: it gave the biologists the notion that repelling electrostatic force could be involved. In other words, atmospheric electricity.
The higher one rises from ground level, the greater the electrical potential gradient. According to the great physicist Richard Feynman, on an ordinary day, as one goes rises from the surface of flat ground or water, the electric potential increases by about 100 volts per meter.
Many insects can detect electrical gradients. Bumblebees can detect electricity fields between themselves and flowers, and honeybees can use the electric charge to communicate with the hive, say Morley and Robert.
Flying spiders aren't insects, they're arthropods, closer to shrimp than to fruit flies. So the comparison is a little stretched perhaps.
But the fact is that in experiments exposing Linyphiid flying spiders to electric fields that were quantitatively equivalent to those found in the atmosphere, the spiders flew.
Switching the field on caused spider to move up. Switching the field off caused spider to move down. The scientists thus demonstrated that arachnids can become airborne in the absence of air movement when subjected to naturally occurring electric fields.
A lot of the species with this special quirk are the type that weave so-called "sheet" nets.
Why would spiders disperse so grandly, anyway? Let alone by thousands at a time, even though most spiders are not social beings?
"One motivation for wanting to disperse could be the density of other spiders or hatchlings. This is something that was not examined in our work," Morley says.
What if spider doesn't want to move? They do not know whether the electric field moves the spider whether it wants to or not. "We know that electric fields are sufficient to enable ballooning, but we do not know how this works in their natural environment yet," Morley told Haaretz.
Nor do they know at this point if in the absence of wind, a naturally occurring herd of spiders, or whatever it's called, all zoom off in one direction or get blown haphazardly all over the place, a delightful thought. Their experiments were done in the lab, Morley points out.