29 November 2019

Duck! Flying Spiders

Vicki rushed in, calling, “Come with me. You’ve got to see this.” Vicki’s not in the habit of thinking I need to see anything, so l had to ask, “What is it?” “Floating spiders,” she answered.
Spider-spun fibers (white streak above clouds)
Vicki photographed before inviting me to join her.
Welcome back. A handful of Sundays ago, I dropped what I was doing to follow Vicki, though I couldn’t figure why I had to witness spiders swimming in a puddle. 

Spider fibers on trees
and pole (Vicki’s photo).
She drove us a very short distance to where there were long, thick strands of spider-spun fibers on the road, in fields, on bushes and trees. Spider fibers that Vicki had seen floating high in the air had come to ground. If any spiders made it that far, they were gone.

A passerby who had noticed Vicki walking and photographing the sky told her the webs were floating spiders. It didn’t take much of an internet search for me to learn the common labels were ballooning or kiting spiders and that the distances they covered, though usually short, can reach 1,000 miles or more at heights over two miles.

Ballooning spider fibers
on grass (Vicki’s photo).
It also didn’t take much searching for me to wonder why the topic was completely new to me. Ballooning spiders were first documented in the 1600s and described by researchers since at least the 1800s. But the real How did I miss it? was that technical papers and supporting videos appeared in leading journals just over a year ago.

Maybe the topic is new to you, too. I’ll fill in some blanks and hit the highlights of the more recent studies.

Ballooning on the Wind
It's thought that spiders balloon to avoid being eaten at their birth sites or to find food, mates or new areas to colonize. Most ballooning spiders are young or small adults, generally no more than 3 millimeters (mm).

Researchers affiliated with the Technical University of Berlin conducted field and wind tunnel experiments to examine ballooning behavior and associated web fibers of ground crab spiders (Xysticus sp.), 3 to 6 mm and 6 to 25 milligrams (mg).

They found the spiders evaluate the wind with a front leg and await suitable conditions. Before releasing themselves from an anchoring silk line, the larger spiders spun 50 to 60 extremely thin fibers (diameter less than 325 nanometers), averaging about 3.22 meters (10.5 feet) long, that formed a triangular-shaped sheet.

The crab spider spins out fine silk fibers for its aerial dispersal. A triangular sheet of fibers is observed at the moment of the takeoff (image by Moonsung Cho, Technical University of Berlin, from his video - www.sciencemag.org/news/2018/04/watch-ballooning-spider-take-flight).
The researchers hypothesized that spiders use the ascending air current for their aerial dispersal, which concurs with the fact that spiders usually balloon when wind speed is less than 3 meters/second (6.7 miles/hour).

Electrostatic Flight
Aspects of ballooning aren’t well explained by wind and convective turbulence. For example, how is it that spiders can launch at relatively high acceleration when air movement is imperceptible?

In 2013, a University of Hawaii at Manoa researcher made a case for the role of electrostatic forces, specifically, the global atmospheric potential gradient. Above a flat field or the sea on a clear day, the electric potential increases by approximately 120 volts/meter. (Borrowing from Richard Feynman: outdoors the electric potential at the height of your nose is about 200 volts more than the potential at your feet!)

The Hawaii researcher built his case by analyzing Charles Darwin’s observations of ballooning spiders aboard the H.M.S. Beagle.

It wasn’t until 2018 that researchers with the UK’s University of Bristol reported testing the hypothesis that electric fields commensurate with the atmospheric potential gradient are detected by spiders and are sufficient to stimulate ballooning.

The researchers exposed spiders (Linyphiidae) to laboratory-controlled electric fields quantitatively equivalent to those found in the atmosphere. Switching the field on and off caused the spiders to move upward (switched on) or downward (switched off). They also observed that spider leg hairs (trichobothria) were activated by weak electric fields.

In all, the findings showed that spiders can become airborne without wind when subjected to electric fields.

Wrap Up
Whether carried by wind, the global atmospheric potential gradient or both, watching ballooning spiders (which I didn’t), seeing the scattered fibers and wondering where they came from is not a bad way to spend time on Sunday afternoon. Thanks for stopping by. 

Ballooning spiders on trees after escaping floods in Tasmania, 2016 (photo by Ken Puccetti, from www.smh.com.au/environment/conservation/trees-cocooned-in-webs-as-ballooning-spiders-take-refuge-in-floodhit-tasmania-20160608-gpe8er.html).
Wikipedia article on ballooning spiders: en.wikipedia.org/wiki/Ballooning_(spider)
Technical University of Berlin study in PLOS Biology journal: journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2004405
Technical University of Berlin video on ballooning spiders in Science: www.sciencemag.org/news/2018/04/watch-ballooning-spider-take-flight
Article on study on EurekAlert! website: www.eurekalert.org/pub_releases/2018-06/p-fss060518.php
University of Hawaii study of electrostatics and ballooning spiders: arxiv.org/pdf/1309.4731.pdf
University of Bristol study in Current Biology journal: www.cell.com/current-biology/pdf/S0960-9822(18)30693-6.pdf
University of Bristol video on electric fields and ballooning spiders: www.eurekalert.org/multimedia/pub/174402.php
Article on study on Science Daily website: www.sciencedaily.com/releases/2018/07/180705114027.htm
Richard Feynman lecture on atmospheric electricity: www.feynmanlectures.caltech.edu/II_09.html

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