Countless spider webs fell from the sky, covering California, USA

The net of justice is vast and wide.

October 5 was originally an ordinary Thursday, but a strange scene appeared in California, USA. Mysterious white floccules suddenly floated in the sunny sky, and they fell one after another, sticking to telephone poles, vehicles and the ground, and even falling on pedestrians. When people looked closely, they found that the large clumps of things turned out to be spider webs! Many California residents recorded this large-scale "spider rain" on social media, which can be regarded as a warm-up for the upcoming Halloween.

Image source: pixabay

In fact, the phenomenon of spider webs falling from the sky on a large scale is not uncommon, and some residents in Australia may be more familiar with it. In May 2015, spider webs like snow fell from the sky in the southern highlands of Australia; in March 2012, a small town in southern Australia that was hit by floods was completely covered by spider webs, presenting a scene of silver. In addition, there have been similar reports in Brazil and Pakistan. Some people also affectionately call this silver thread falling from the sky "angel hair".

In fact, "angel hair" is just a tool used by spiders to migrate.

Spider webs cover the ground in an Australian park. Photo credit: Byrce and Leslie Anne Schmidt via BuzzFeed

Flying Spider

In order to avoid competing with their own kind for food, mates, habitats and other resources, spiders often need to migrate. Although they have no wings, they can cleverly use spider silk to travel long distances. In November 1832, the Beagle had sailed about 100 kilometers from the coast. Darwin, who was enjoying the breeze on the deck, suddenly discovered many "balloon pilots". They were a group of dark red spiders, flying on "balloons" made of thin spider silk.

These little guys have superb "driving skills". They go up and down in the air along the spider silk, and can even glide on the water with the help of spider silk "balloons". Some of them will climb to high areas, then raise their abdomens, shoot spider silk, and fly away with a "whoosh". Other spiders will stand on the top of the pillar, secrete four or five spider silks distributed in a fan shape, and then go away with the wind.

Darwin believed that flying skills were an innate instinct of these small spiders, just like water spiders (Argyroneta aquatica) are good at diving. Later, scientists named the spider's balloon-like behavior "ballooning". The small spiders not only fly very fast, they can travel 30 kilometers in 6 hours, but also fly very far, and can reach a distance of 3,200 kilometers (equivalent to Kunming to Shenyang) through continuous flight. The flight altitude is also quite amazing. Some people have even found flying spiders in the stratosphere.

A large number of spiders were filmed flying in Brazil. Image source: Guardian News via YouTube

Many spiders have the ability to fly. Researchers have documented at least 27 families of flying spiders, which are found all over the world, including China. The flying habits of different spiders vary greatly, with some flying all year round and others migrating in late spring or autumn.

So, flying spiders are not rare, so why do we rarely see spiders flying in the air in groups? This is because, generally speaking, spider flying is an individual behavior. But in some special cases, such as floods that cause large-scale destruction of spider habitats, they need to evacuate collectively, or spiders that were originally prepared to fly have not encountered suitable weather, and they will not fly together until the weather becomes suitable.

The Secret of Spider-Man

As Darwin observed, spiders commonly use two methods of flight.

The first method is called "tip-toe" . The spider will first walk to a high surface, then lift one or both front legs and hold this position for 5 to 8 seconds, repeating several times. This is them using the hair on their legs to feel whether the surrounding environment is suitable for flying. After the assessment is completed, if they decide to fly, they will rotate their bodies in the direction of the wind, then lift their abdomen (also called "tiptoeing"), and start spinning silk to make flying props.

First, the spider will use its hind legs to stick the silk spitted out by the spinneret to the plane where it is located. This is the "safety line" it ties for itself. Then, the take-off process officially begins. It uses other glands in its abdomen to spit out multiple 2-4 meter long spider silks, which unfold in the air to form thin triangles and dance wildly in the wind. At the same time, it will use its hind legs to control the safety line in a reasonable position to prevent it from blocking the silk balloon from unfolding in the air.

After a while, the spider lets go of the surface it had been firmly grasping, stretches all its legs outward, and then flies away obliquely. During the flight, the spider does not forget to "hold on" to the safety line. The safety line will not break until it flies 3 to 5 meters away. If the weather conditions suddenly become unsuitable for flying before takeoff, it will immediately cut the spider silk and wait for the right time again.

A spider using the "tiptoot" flying method. Image source: original paper

Another way spiders fly is "dragline" , which is like a spider dropping a raft made of spider silk in the air. When spiders choose to fly from a high pole, the safety line originally fixed to the pole becomes a "bungee rope". They will descend 0.4 to 1.1 meters along the rope, and then swing in the air while weaving a "raft".

They keep waving their front and back legs in the air to keep their abdomens pointed in the direction of the upward wind. A raft of spider silk slowly forms from their abdomens, bending upward under the influence of the wind, and the spiders float upward with it. When the spider silk raft has enough lift, the little spiders cut the safety thread and fly away happily.

A spider flying in a "rafting" style. Red indicates the "safety silk" used for fixation, and blue indicates the spider silk raft involved in flying. Image source: original paper

Why can spiders fly into the air with just a few soft spider silks? Scientists have different opinions on this.

Some scientists believe that this can be explained from an aerodynamic perspective. For example, in the spring and autumn when the sun is bright and breezy, the upward airflow generated by the sun heating the ground provides the spiders with the power to fly, so these are also the days when they fly most often. But in the hot summer, although there are more updrafts, due to the lack of horizontal breezes, spiders cannot fly even if they take off. More extreme, in the cold winter, the strong winds will interfere with the upward airflow, so it is not conducive to the spiders' flight.

However, as the research deepened, some scientists questioned this theory. Why can spiders achieve such a high initial acceleration only under low wind speed conditions? Moreover, spiders cannot actually spray out spider silk like Spider-Man in the movie. External force is needed to pull the spider silk out of the gland (such as using the hind legs to pull the spider silk out and stick it to a surface). How can rising air currents do this?

A spider flying due to updrafts. Image source: original paper

Scientists have observed that spiders produce silk that fans out when they fly, rather than tangling up. So they speculate that charge plays a role in flight. Indeed, spiders and their silk carry a negative charge, and spiders can increase their negative charge by climbing branches, leaves, and grass. The negative charges on the silk repel each other electrostatically, which keeps them separated in the air currents.

On the other hand, the Earth's atmosphere is positively charged, creating a huge electric field with the negatively charged ground. Even on a cloudless day, this atmospheric potential gradient (APG) can reach 100 volts per meter, and in foggy or stormy conditions, it can even increase to tens of thousands of volts per meter. Therefore, the negatively charged spider silk will naturally experience an upward electric force in this environment. Moreover, the raised surfaces that spiders choose when flying also have stronger local electric fields.

Scientists placed spiders in a windless environment and found that once the electric field in the environment was turned on, the spiders would also behave in a "tiptoe" manner. This shows that they are sensing the electric charge in the air. Moreover, many of the spiders successfully took off. By adjusting the strength of the electric field in the closed environment, the researchers were able to control the spiders to fly up and down.

A spider flying up and down in a changing electric field. Image source: original paper

In addition to the environment, airflow and electric field conditions of the habitat, factors such as temperature, humidity, light intensity and pressure may also affect whether a spider decides to fly. If you encounter a spider web falling from the sky one day, don't panic, because most flying spiders are not poisonous, they are just cute little ones moving.

References

[1] https://www.sciencetimes.com/articles/46426/20231009/spiders-falling-skies-california-biologist-explains-bizarre-phenomenon.htm

[2]https://www.tiktok.com/@karensusy3/video/7286222218852355374

[3] https://www.snopes.com/fact-check/park-covered-spider-webs/

[4] https://www.buzzfeed.com/bradesposito/meowth-thats-right

[5]https://www.gutenberg.org/cache/epub/3704/pg3704-images.html

[6]https://www.biodiversity-science.net/article/2021/1005-0094/1005-0094-29-4-517.shtml

[7] https://www.youtube.com/watch?v=RKV34U2vEk8

[8]https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2004405

[9] https://www.livescience.com/7247-fly-spiders-check-weather.html

[10] https://www.newscientist.com/article/2173544-spiders-can-use-electricity-in-the-air-to-balloon-for-kilometres/

[11]https://www.cell.com/current-biology/fulltext/S0960-9822(18)30693-6

Planning and production

Source: Global Science

Written by Huang Yujia

Editor: Yinuo