In recent days the plains of the provinces of Reggio Emilia, Modena and Parma in Emilia-Romagna have been adorned with enormous cobwebs enveloping trees and cars. Despite the time of year, the phenomenon is not due to the Halloween holiday but probably to the recent floods that affected the Region. Various conspiracy theories and conspiracy theories have arisen around this curious phenomenon, but it is a natural and quite common phenomenon: the ballooningan ingenious way that spiders and other arthropods have to move through the air. Spiders do this in case of heavy rain to take shelter high up and escape the water, which climate change is making more frequent. A plausible explanation for the sightings of this phenomenon in the Emilian plains in recent days would therefore be a natural response of spiders to torrential rains and floods.
What are the giant spider webs in the fields in Emilia Romagna: the explanation of ballooning
A morning of clear skies from far away 1832sir Charles Robert Darwinyoung British biologist, naturalist, geologist and explorer, overlooking the bridge of Beagles as he sailed the seas of the southern hemisphere, he noticed that everything around him was covered in a thick silky blanket of cobwebs. The creators of that work were thousands of small spiders, clearly distinguishable on the web. The ship, in those days, was about 60 miles from the Argentine coast.
Where did the small arthropods come from? They landed there after hours suspended in the air. More recent observations, in fact, report the presence of airborne spiders up to 4000 m altitude! This is possible thanks to ballooning. Spiders climb to the top of a prominence, such as a plant or mushroom, and from there shoot threads of silk to which they remain attached across the abdomen, being carried by the currents. But how do they do it?
How spiders “fly”: theories and a recent hypothesis
The opinion that has prevailed over all to date is that according to which they would be the wind drag forces light to allow spiders to fly, which is the same principle by which a spider moves hot air balloon. However, according to a recent study, aerodynamic drag alone is not enough. It has been observed, in fact, that i seritteri of spiders (i.e. the glands that produce silk) produce multiple silk threads at a timewhich are distributed like a fan and seem to be attached to a balloon, remaining separated from each other and dragging the arachnid in the light current. This is due to the action of one electrostatic repelling force which does not cause the wires to tangle together.
Fine, but how do spiders take flight? Until now it has been assumed that the driving force was the updrafts hey temperature gradients on days of good weather, but the ballooning phenomenon can also be observed when the sky is covered by clouds or it rains.
THE movements of electric charges between earth and atmosphere (atmospheric electrical circuit) and the consequent atmospheric electric potential gradient (i.e. the rate of variation of the vertical electric field that originates from the movements of free electric charges in the air) would provide an additional explanation to the phenomenon.
The hypothesis from which a study by the University of Bristol started is that spiders detect atmospheric electric fields and that the latter are sufficient to start ballooning, while the information on the potential gradient would indicate the right moment for the ballooning to take off. flight.
A possible scientific explanation of ballooning
An atmospheric potential gradient is always present, regardless of weather conditions. What varies in case of good weather or clouds and rain is his intensity and his polarity.
Let’s take a day with clear skies on a flat meadow: the potential gradient is approximately 120 V/m (i.e. it varies by 120 volts for each meter of altitude). In more unstable weather conditions, electrically charged clouds passing over the meadow bring the potential gradient to different thousands of V/m. In this context, any long-limbed structure protruding vertically from the lawn such as a post, plant or mushroom will cause a substantial increase in local electric fieldscondensing on itself the electric discharges that will reach the ground, as a lightning rod would do.
Not only that: plants and mushrooms, in addition to having the right geometric shape for this to happen, have ahigh percentage of water and electrolytes which allows the earth potential to be equalized, while the intensity of the electric field surrounding leaves and branches can reach several thousand V/m. In slightly unstable weather conditions, for example, the electric field 10 m above the crown of a 35 meter tree can exceed 2000 V/m, but around the tips of leaves, needles and branches it can reach up to ten times superior. This potential difference is often sufficient to trigger the emission of ions via corona discharge. The electrostatic charge of the air above the plant is positively charged, while the spider silk – which is an electrical insulator – acquires a negative charge.
Now, we know that opposite electric charges have an attractive effect and what happens in ballooning is that the threads produced by the spider (electrically negative) are attracted by an air mass (positively charged) just like what happens when we rub a balloon on the head of someone with long hair.
How spiders measure the movements of electrical charges
It has been observed that spiders, at the top of branches, on the umbrella of a mushroom or on any other elevated structure, raise their front legs to detect the surrounding electric field and, having found the ideal conditions of potential gradient, they position themselves on ballooning tips. Once airborne, turning the electromagnetic field on and off would cause the spider to move up (on) or down (off).
But how do spiders detect the movements of electric charges? Through gods mechano-sensory hairs on the surface of their paws, activated mechanically.
The Anglo-Saxon study managed to demonstrate that the atmospheric potential gradient and the electrostatic forces that derive from it are sufficient to stimulate ballooningand that the associated aerodynamic drag of light wind can assist the latter to facilitate this.
A natural phenomenon, but accentuated by climate change
Ballooning has become an impressive phenomenon in recent times. This is due to the fact that spiders are using ballooning as a strategy to escape the waterwhich has invaded their natural environment due to dramatic floods originated mainly from climate changes.
A real one survival techniquetherefore, which is implemented by spiders in emergency situationswhich however are becoming more and more frequent. Once the emergency is over, the arachnids they will return to the ground in the dens.
In all this, some open questions remain: are spiders able to control their flight altitude and navigation direction? But above all, do they know where they will land? Answering these questions could be the challenge for arachnological research in the coming years.
