Every so often, otherwise Earth-bound spiders take to the air. Ballooning spiders can travel hundreds of miles through the air (and, horrifyingly, rain down on unsuspecting towns). The common explanation for this phenomenon is that the spiders surf the wind on strands of silk, but there may be other forces at work, according to a new study spotted by The Atlantic.

In the research, published in Current Biology, University of Bristol scientists argue that Earth's atmospheric electricity allows spiders to become airborne even on windless days. To test their hypothesis, the researchers exposed spiders in the lab to electric fields similar to those naturally found in the atmosphere.

When the electric field was turned on, the spiders began to exhibit behavior associated with ballooning—they "tiptoed" on the ends of their legs, raised their abdomens, and released silk. Spiders only exhibit this behavior when ballooning. And when they did become airborne, the spiders’ altitude could be controlled by turning the electric field on and off. When the electric field was on, they rose through the air, but when it was off, they drifted downward.

This provides a potential explanation for why spiders take to the skies on certain days but not others, and how they can fly in calm, windless weather— something scientists have puzzled over since the early 19th century. (Even Darwin was flummoxed, calling it "inexplicable," The Atlantic notes.) However, the researchers note that these electric fields might not be totally necessary for ballooning—wind alone might work perfectly fine on some days, too. But understanding more about when and how spiders become airborne could help us predict when there will be large masses of arachnids flying through the skies (and hide).

[h/t The Atlantic]