Ballooning spiders · funigei
On clear autumn days the air carries threads of silk. Many belong to spiders only two or three millimetres long, which climbed something tall, let out a thread and left. Romanians call the floating threads funigei. How does an animal with no wings lift off? Scientists are still not sure.
A drawing, not a photograph. The real spider is two or three millimetres long, about the size of a sesame seed.
Researchers call this flight “ballooning”: the spider leaves hanging from silk. A common launch has four steps.
A grass tip, a fence post, a twig. Darwin saw spiders launch from “some little eminence” and, later, a larger spider launch from a post.
It lifts its abdomen and straightens its legs. Researchers say spiders do this before they launch.
In a wind tunnel, crab spiders of 16–20 milligrams put out 50–60 fibres, 3.2 metres long on average and thinner than a thousandth of a millimetre.
If something in the air holds the thread up, the thread lifts it. Darwin noted that he repeatedly saw the spider “elevate its abdomen, send forth a thread, and then sail away horizontally”.
Moving air can carry a spider hanging on silk. In a laboratory, an electric field has lifted spiders too. Outdoors, nobody has shown how much each one counts. Choose what is in the air and let go of the thread. The drawing follows a few simple rules taken from the papers; it is not a measurement.
Above 3 m/s spiders usually do not take off.
Observed outdoors
The prevailing explanation. Spiders usually take off only when the wind is below 3 m/s.
Cho and colleagues (2018) filmed crab spiders of 6–25 milligrams taking off outdoors. They calculated that an updraft of 0.04–0.2 m/s would be enough to lift them, and measured updrafts of a few tenths of a metre per second on a grass field. The wind where the spiders took off was not measured.
Old idea, mathematical model
At Santa Fe, Darwin wrote of “an ascending current of heated air”.
A 2007 dispersal model links how far a spider travels to the state of the air at take-off: most go a few hundred metres, and hundreds of kilometres are possible. It is a model, not a tracked flight.
Laboratory experiment
Air has a natural electric field: about 120 V/m on a clear day over flat ground. In 2018 a team in Bristol, England, put 36 spiders of the genus Erigone in a box with limited air movement and applied fields of 1,250 and 6,250 V/m, 10 and 52 times stronger.
The spiders lowered themselves on a thread more often in both and went on tiptoe more often in the strong one. In a later analysis of footage from the same chamber, three spiders were seen rising, in strong laboratory fields.
A 2020 calculation says one silk strand in a clear day's field would give an upward force of about 2% of an Erigone spider's weight; with longer and more strands, in the calculation, the spider would float. A model suggests the field at the tip of a plant stem could approach 1,000 V/m.
The authors write that electric forces are sufficient but perhaps not always necessary, and propose that they and light wind can work together. Whether the outdoor field is enough has not been tested.
On 1 November 1832 the Beagle, the ship on which Charles Darwin sailed round the world, was off South America. Darwin writes that within the mouth of the Río de la Plata the ship's rigging was coated with the web of gossamer spiders, and that on the morning of 1 November the air was full of “patches of the flocculent web”. He wrote:
The ship was sixty miles distant from the land, in the direction of a steady though light breeze. Vast numbers of a small spider, about one-tenth of an inch in length, and of a dusky red colour, were attached to the webs. There must have been, I should suppose, some thousands on the ship.
Sixty miles is between 97 and 111 kilometres, depending on the kind of mile; Darwin does not say. “Some thousands” is his own estimate. A tenth of an inch is 2.5 millimetres. It was a day with a light breeze, not no wind.
On another day he watched one leave:
I repeatedly observed the same kind of small spider, either when placed or having crawled on some little eminence, elevate its abdomen, send forth a thread, and then sail away horizontally, but with a rapidity which was quite unaccountable.
Later, at Santa Fe, a city in what is now Argentina, a spider of about 7.6 millimetres (three-tenths of an inch), “quite different from the gossamer”, stood on a post and darted out four or five threads that fanned apart. Darwin looked to the air for the cause:
The day was hot and apparently quite calm; yet under such circumstances, the atmosphere can never be so tranquil as not to affect a vane so delicate as the thread of a spider's web.
He also mentioned an electrical explanation, credited to a Mr. Murray, for the way the threads fan apart:
the divergence of the lines has been attempted to be explained, I believe by Mr. Murray, by their similar electrical condition.
Almost two hundred years later, part of his question is still open.
Between 1926–31 the American researcher P. A. Glick flew over Louisiana in an aircraft fitted with insect-catching screens. He collected 1,461 spiders from the air, from 20 feet above the ground upward, and the highest was caught on an aircraft at 15,000 feet, or 4,572 metres. It was a single spider.
Romania's highest peak, Moldoveanu in the Făgăraș Mountains, stands at 2,544 metres. The spider was caught about two kilometres higher, 1.8 times the height of the peak; Louisiana is a low plain, so altitude above ground and above sea level differ little. Glick does not say whether it had flown alone on a thread. He did note white silk threads floating “even as high as 11,000 feet or more”, so at least 11,000 feet (3,353 metres).
How far do they go? A 2007 model says most travel a few hundred metres, but hundreds of kilometres are possible. Distances from a model are not tracked flights.
Romania's standard explanatory dictionary defines funigei as “the thread of a certain kind of small spider, seen floating in the air on clear autumn days”. Another popular name, listed in dictionaries, is mătasea-morților, “the silk of the dead”.
A dictionary from 1926–1931 notes, from Moldavia and Transylvania, that many funigei floating through the air in autumn were a sign of a long autumn. The word also reached literature: Legenda funigeilor, “the legend of the funigei”, is a dramatic poem by two Romanian poets, Dimitrie Anghel and Ștefan Octavian Iosif.
German
“old women’s summer”: the floating threads were compared to the white hair of old women, as early as the 18th century.
English
possibly from “goose summer”; the word’s origin is disputed.
Romanian
the 2009 explanatory dictionary lists the origin as unknown; older dictionaries propose a Latin derivation, from “funis” (rope) or “fuligo” (soot).
When you next see a thread glinting in autumn air, you can ask something that has no answer yet: what lifted it?
The numbers on the page (fields, spider counts, weights, heights) come from the papers below or are calculated from them: ratios, unit conversions. Drawn: the spider, the threads, the post, the sky and how they move. The drawing does not calculate the physics of flight. It follows three simple rules taken from the papers: no launch in wind of 3 m/s or more; a lift in the strong laboratory field (6,250 V/m); a drop on a thread in the weak one (1,250 V/m). They are not thresholds measured for every spider.