How can spiders fly without wings?
― What lifts their silk may be not just wind, but "electricity in the sky"
Newborn spiders, tiny as they are, spin silk from their abdomens and rise into the air. Some have landed on ships more than 100 km out at sea. For a long time people thought they simply rode the wind. But it now appears that the weak electricity in fair-weather air also gives the silk a lift.
On a clear autumn morning, the edges of rice paddies and meadows sometimes glint faintly silver. Step closer and you see fine threads stretched over everything.
These threads are not webs. They are traces of tiny spiders that took off into the sky, or came down from it.
How can a spider with no wings or feathers fly? And they take off even on days with almost no wind.
There are two reasons
The ground carries negative charge and the upper air carries positive charge. So for every metre you rise above the ground, the electric "height" goes up by about 100 volts.
The fine hairs on a spider's legs move in response to the electric slope. Charged silk feels an upward force in this slope. A spider is so light that this force is enough.
If a human tried the same thing, it would be hopeless. Let's look at where that difference comes from, step by step.
The sky holds an invisible "electric slope"
Even on a calm day with no rain, the air has an electric slope. The ground is negatively charged. The layer high above, which conducts electricity well, is said to sit about 250,000 volts higher than the ground.
Spread over that gap, the slope near the ground comes to about 100 to 150 volts per metre. So there is a difference of over 100 volts between your head and your feet. Your body conducts electricity so well that you feel nothing.
What matters is that the slope becomes steep at the tips of pointed objects. As on the left of Figure 1, electric lines crowd toward the tip of a blade of grass or a twig. When a spider climbs to the top of a blade or a fence before taking off, it may be choosing exactly this spot.
Spiders sense the electricity, take off, and win by being light
In 2018, researchers in Britain ran an experiment. They put spiders in a box with no wind and created an electric slope about the same as in fair-weather air.
When the electricity was on, the spiders stretched their legs, raised their abdomens high and spun silk. This is the set posture before takeoff. Then, though there was no wind, they floated gently up. When the electricity was switched off, they sank back down.
Spider legs carry very fine sensory hairs. These hairs were also seen to stand up and move in the electric slope. Spiders are thought to sense not only the wind but also the state of the electricity in the sky, "with their legs."
The other deciding factor is lightness. Most flying spiders are babies weighing around 1 milligram. At that weight, even the small charge carried by the silk can balance their weight. A 60 kg human would need 60 million times more electricity under the same conditions. That is an impossible amount.
A spider about to take off spins not one thread but many. They have been seen to spread out like a fan without tangling. Charges of the same kind repel each other, so this is taken as a clue that the silk carries charge.
Experiments show that bumblebees sense the weak electric field around flowers with their body hairs, and tell apart flowers that hold nectar. Electricity in the air, which humans cannot feel at all, is information to small creatures.
Summary
The sky always holds an electric slope of about 100 volts per metre. A baby spider senses it with the hairs on its legs, climbs to a grass tip and spins silk. Charged silk feels an upward force, enough to lift a body of about 1 milligram. Using both wind and electricity, spiders are thought to travel hundreds of kilometres.
What carries a spider is not wings, but lightness.
The smaller the body, the more even the sky's weak electricity becomes a "wing."
For another creature that senses electricity, see the story of the shark that finds prey under sand using electricity. How electric charge builds up is covered in our article on winter static electricity. And how long-distance travellers find their way is introduced in the article on migrating birds.
- On a clear, light-wind autumn morning, shortly after sunrise, look across a meadow or paddy edge at a low angle. Sometimes the threads glitter in the sunlight.
- Look closely at the tips of grass and the tops of fence posts. A tiny spider with its abdomen pointing up is in its pre-takeoff posture.
- Rub a plastic sheet with a tissue to build up charge, then bring it near a bundle of thinly torn tissue strips. The thin paper fans out or is pulled toward the sheet, showing that lighter things move more easily under electric force.
Just watch the spiders; don't catch them. The moment of takeoff often lasts only a few seconds, so patience is the key.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" and "Physics"
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (atmospheric electricity, sensory physiology)
- ResearchNot yet taught as settled fact even at university; what researchers are studying now
Middle schoolTerms: this phenomenon has a name
- Ballooning (aerial dispersal): a spider spins silk and is carried through the air to move. Baby spiders do it to scatter from where they were born.
- Atmospheric electric field (potential gradient): the electric slope in fair-weather air. Near the ground it is said to be about 100 to 150 volts per metre.
- Sensory hairs (trichobothria): very fine, light hairs on a spider's leg. They move with air flow or the electric slope and pass this on to the nerves.
Middle schoolHigh schoolChecking with a formula: how much charge does it take to float a spider?
When the upward force on the charge carried by the silk equals the spider's weight, the spider floats. We find the amount of charge needed for that.
| In symbols | q × E ＝ m × g (so q ＝ m × g ÷ E) |
| In words | Charge × strength of the electric slope ＝ mass × gravitational acceleration |
| Where it comes from | The force on a charge in an electric field (high-school physics: "force ＝ charge × field") balanced against gravity |
| q | Charge carried by the silk and spider (unit: coulomb) |
| E | Strength of the electric slope = electric field (unit: volts per metre) |
| m | Mass of the spider (unit: kilogram) |
| g | Gravitational acceleration (about 9.8, unit: metres per second squared) |
| Example mass of a flying baby spider | 1 milligram ＝ 0.000001 kg |
| Electric field near the ground (fair day) | said to be about 100 volts per metre |
| Example field strengthened at a grass tip | 1000 volts per metre (about the strength used in the experiment) |
| Human weight for comparison | 60 kg |
| Spider's weight (N) | 0.000001 × 9.8 ＝ 0.0000098 |
| Charge needed near the ground (C) | 0.0000098 ÷ 100 ＝ 0.000000098 |
| Charge needed at a grass tip (C) | 0.0000098 ÷ 1000 ＝ 0.0000000098 |
| Human's weight (N) | 60 × 9.8 ＝ 588 |
| Charge a human needs at a grass tip (C) | 588 ÷ 1000 ＝ 0.588 |
| How many times more charge a human needs than a spider | 0.588 ÷ 0.0000000098 ＝ 60000000 |
A spider at a grass tip floats with about 0.00000001 coulombs (10 nanocoulombs). The charge on a person's body when you get a zap from a doorknob in winter is said to be some tens of times more than this. So the amount is within a spider's reach. A human would need 60 million times as much, which is out of the question. The charge needed is proportional to body weight, so only small creatures can fly this way.
High schoolHigh school+Electric fields and the effect of sharp tips
High schoolA charge placed in an electric field feels a force equal to the charge times the field. This relation follows from Coulomb's law. The ground is negative and the upper air positive, so the field points downward. A negatively charged thread is pushed the opposite way, upward.
High school+At the sharp tip of a conductor, electric lines concentrate and the field gets stronger. Grass and twigs are connected to the ground and conduct electricity, so the field at their tips is thought to be several times stronger.
UniversityAtmospheric electricity and spider sensory hairs
The fair-weather field is called the fair-weather electrostatic field (atmospheric electric field) and is studied in atmospheric electricity. It is explained by the "global electric circuit": thunderstorms worldwide keep sending charge between the ionosphere and the ground, and it flows slowly back down in fair-weather regions. The trichobothria on a spider's legs are extremely light, and measurements of hair motion using lasers confirmed that they change direction not only with air movement but also with the electric field.
📖 Derivations and further reading: Wikipedia, "Atmospheric electricity" (Japanese edition) ／ Wikipedia, "Coulomb's law" (Japanese edition)
ResearchWhat is still unclear
- Wind or electricity: which leads? Outdoors, wind and rising air also matter a lot, and how much of the lift comes from electric force is still unsettled.
- How does the silk become charged? One idea is that friction as the silk emerges charges it, but there are almost no measurements of the actual charge outdoors.
- How far and how high do they go? Spiders have been caught in nets several thousand metres up, and found over the sea far from land. But the actual route taken by a single spider has not been tracked.
So this article, too, is "an explanation of what is known so far." The role of electricity was only shown by experiment in 2018, and future field observations may change the picture.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 2: static electricity and current | Charges of the same kind repel; silk fans out |
| High school | Physics: electric field and potential | Force ＝ charge × field; balance with weight |
| High school+ | Physics: field at the sharp tip of a conductor | Why the field strengthens at grass tips |
| University | Atmospheric electricity, sensory physiology | Fair-weather field, global electric circuit, trichobothria |
| Research | Behavioural ecology, biophysics | Division of roles between wind and electricity; charge carried by silk |
| ― | Link to daily life | Silk on autumn-morning meadows; how small creatures spread |
- Morley, E. L. & Robert, D. (2018) Electric Fields Elicit Ballooning in Spiders. Current Biology 28
- Wikipedia, "Atmospheric electricity" (大気電気学, Japanese edition)
- Charles Darwin, The Voyage of the Beagle (1839; record of spiders landing on a ship offshore)
- Clarke, D. et al. (2013) Detection and learning of floral electric fields by bumblebees. Science 340
- Gorham, P. W. (2013) Ballooning spiders: the case for electrostatic flight. (preprint)
* This article is a popular-science explanation. The figures given are rough guides for understanding the mechanism. A spider's mass and the strength of the electric field vary greatly with species, place and weather.