Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics
The Swifts have finished breeding and are getting ready to migrate back down to Africa. Surprisingly, during the breeding season, Swifts mainly feed their chicks not on insects but on spiders. They catch those spiders high in the air. Spiders balloon on long silk filaments, riding the air-currents to arrive at new homes where they at-the-least know the wind can bring prey to them – since it carried them there. The real puzzle has always been how the spiders launch. They are very good at detecting days when thermal updrafts are strong, and on cross-country paragliding flights it is common to look up at the paraglider lines and see spider webs trailing behind them. However, I’ve often seen spiders launching their gossamer threads early in the day before the sun has warmed the ground enough to trigger thermals – it is a good sign when walking up to launch in the morning. Even if the spiders wait until the thermals have started, thermal updrafts are weak close to the ground, particularly in the bottom few centimetres, so how do the spiders get their ballooning threads up into the air? Ballooning gossamer threads are particularly flexible, why don’t they just flop as they leave the spiders spinnerets?
Recent research suggests the spiders exploit electrostatic forces to launch their webs. Work by Fritz Vollrath’s group has shown that spider can spin electrostatically charged silk.
Some time ago Gorham suggested that those electrostatic charges might help by carrying the silk away from the spider and away from whatever the spider was standing on.
Now Morley and Gorman have shown that electric fields alone are sufficient for spider take-off. They put spiders in sealed boxes and applied an electric field across the box between charged plates above and below it. The spiders responded by adopting their characteristic tiptoe posture, and launching a ballooning thread. The thread rose from the spider and coulomb forces were sufficient, in some cases, to lift the spider into the air, in the complete absence of air currents. Electric launch.
The forecast for this week is perfect for spider ballooning.
K. Kronenberger and F. Vollrath, Spiders spinning electrically charged nano-fibres, Biol. Lett. 11, 20140813 (2015).
Gorham, P.W. (2013). Ballooning spiders: the case for electrostatic flight. arXiv, arXiv:1309.4731v,
E. L. Morley and D. Robert, Electric fields elicit ballooning in spiders, Curr. Biol. 28, 2324 (2018).
Greenstone M.H. Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. Oecologia. 1990; 84: 164-168
E. L. Morley and P. W. Gorham . Evidence for nanocoulomb charges on spider ballooning silk. Phys. Rev. E 102, 012403 – Published 9 July 2020