Last updated July 5, 2018 at 4:38 pm
New research answers a question that once bemused Charles Darwin.
Naturalists, amateur and pro, all know that many species of baby spiders disperse from their birthplaces by means of extending a silk thread and then floating away through the air.
Especially keen-eyed naturalists, however, might have noticed that the little wee things can occasionally be seen drifting off on days when there is not the faintest breath of breeze.
At first blush, this seems impossible. However, research by Erica Morley and Daniel Robert, both from the University of Bristol in the UK, reveals that wind is not at all essential for the phenomenon of spider ballooning, as it’s termed. Indeed, even on gusty days, it might not be involved at all.
Ballooning spiders have been found four kilometres up in the air, and more than 100 kilometres from their starting points. The behaviour has been a focus for research for a couple of centuries. Indeed, Charles Darwin once mused upon it while sitting on the deck of the Beagle, watching hundreds of baby arachnids float by. He came to no conclusions.
In a paper published in the journal Current Biology, Morley and Robert show that the force that stimulates and governs airborne spider dispersal is not wind at all, but electricity.
Young take off on silk strands
To make their findings, the researchers used spiders from the family Linyphiidae, all characterised by being very small.
The spiders were bred inside closed containers, which were by definition free of wind. When the young hatched, Morley and Robert set up electric fields inside the enclosures. When the fields reached particular strengths – corresponding to those generated between the Earth and the ionosphere, and known as the atmospheric potential gradient (APG) – the spiders let out silk strands and took off.
Detection of the electric fields, the researchers established, is made by special sensory hairs called trichobothria. The hairs, once activated, also stimulate the production of silk.
This did not occur at all APG values – potentially explaining why mass spider flights occur on some days but not on others – but within the effective range the electrostatic strength was sufficient to also provide lift and propulsion.
“We don’t yet know whether electric fields are required to allow spider ballooning,” says Morley. “We do, however, know that they are sufficient.”