My shot at a quick translation (Google Translate acknowledged for the grunt work). Note that this is not an example of EHD, as there is no working fluid and it is the spider itself that is attracted by the ionospheric "anode". Nevertheless it is, as the author remarks, a form of electrostatic drive.
Soaring Spiders
H. Joachim Schlichting
Spektrum der Wissenschaft, 12 October 2022, pp. 64-5.
spektrum.de/artikel/2057472
Some species of spiders use their threads to soar through the air. They don't just use wind and thermals for this. In addition, the electrostatic field of the earth provides them with the necessary drive even when there is no wind.
If only a magic cloak were mine, and carried me to foreign lands
- Johann Wolfgang von Goethe (1749-1832)
Spiders don't have wings, so they shouldn't be able to fly. Sometimes, however, you can see them floating through the air on long silk threads and are thereby clearly taught a lesson. This technique of gliding has long been known in many species of spiders. Darwin had previously received eight-legged visitors on his research ship "Beagle". He described how a spider that landed on board said goodbye by "spitting out four or five threads. They were more than a metre long and diverged upwards from the gland openings. Suddenly the spider released its grip on the post and was quickly carried out of sight."
According to Darwin, it was hot and windless that day. He attributed tiny thermal convection movements to explain how the spider was able to take off despite the lull. The thesis is indeed able to explain some essential aspects of the phenomenon. In addition, Darwin already suspected that electrostatic forces could also be involved. This was supported by the fan-like spread of the threads shooting out - as if they were repelling each other. However, despite such indications of possible charge effects, the prevailing belief up to the present day is that spider flight depends solely on aerodynamic factors. That was well supported scientifically. In recent years, however, research results have been published that attribute an additional major role to electrostatic processes.
Anomaliies, which appear on closer inspection if one wants to attribute aerodynamic lift alone, have contributed to the most recent developments. It is unclear how the spiders, which can weigh up to 100 milligrammes, catapult themselves up with their thread remarkably quickly, while hardly a breeze is blowing. Added to this is the spreading of the bundles of ejected threads, already noted by Darwin. In addition, flying species of spiders were found at an altitude of four kilometres - quite a challenge in terms of flow dynamics.
Such problems can be eliminated if one takes into account the effects of the so-called atmospheric potential gradient. This is caused by the charge differences between the atmosphere and the ground: the ionosphere, which is more than 70 kilometres high, is predominantly positively charged, while the Earth's surface is negatively charged. The difference affects the entire space in between. The undisturbed atmosphere above the ground is positively charged compared to it, so there is an electrical voltage between a point on the Earth and a point in the air above it. It can be at least 100 volts per metre. However, the strength of the effect varies and depends very much on the weather conditions.
The mutual repulsion of the threads released by the spider shows that the silk fibres are also electrically charged. There is therefore an inevitable interaction between them and the atmospheric field, which causes a movement. But how can the spider know in advance whether the forces are strong enough to carry it and its thread? To do this, it must perceive electric fields and judge them according to their strength. It has been known for several years that at least some insects are able to do this (see "Spectrum" April 2020, p. 60).
In 2018, Erica Morley and Daniel Robert at the University of Bristol studied how spiders react to fields. In laboratory experiments, they exposed canopy spiders (Linyphiidae) to electric fields that corresponded in strength to those in the atmosphere under different weather conditions. The spiders indeed responded with clear preparations for flight. The soaring animals gained or lost height as the fields switched on and off. Thus, not only can aerodynamic lift be responsible for the ascent, but so too must the electrical interaction between the animals and the external fields play a role. Specific movements of certain whiskers on the animal's body surface in response to electric fields suggest that these so-called trichobothria enable the spiders to perceive the fields.
The flying spiders are largely at the mercy of the atmospheric potential gradient and the aerodynamic conditions. Nevertheless, they can probably exert some influence on what is happening. In a 2022 publication, Charbel Habchi of Lebanese Notre-Dame University, Louaize and Mosbeh M. Khalid Jawed from the University of California in Los Angeles presented the results of computer simulations. From these, the two research engineers concluded that the spiders can control both the fluidic and electrostatic effects by varying the number of threads and their length. This gave the little creatures in the air at least certain control options.
So far it has not been clarified how the spinning threads are charged. It is likely that further secrets will be elicited from this fascinating natural implementation of an electric drive for years to come.
[Image caption: Weft. A spider ejects a strand of silk into the air from its raised abdomen.]
References
Darwin, C.R.: Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. John Murray, 1845
Habchi, C., Jawed, M. K.: Ballooning in spiders using multiple silk threads. Physical Review E 105, 2022
Morley, E.L., Robert, D.: Electric fields elicit ballooning in spiders. Current Biology 28, 2018
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