Why don't spiderwebs get destroyed by rain or wind?
― Not because they're strong, but because they stretch a lot
Just threads far thinner than a human hair, strung up with gaps everywhere. And yet a spiderweb, buffeted by strong wind and pelted with raindrops, is still there in the morning. The reason isn't that it's "built tough." It's the opposite: the threads stretch a long way and let the force flow through them.
On a rainy morning, a web hung with beads of water sits in the garden hedge. Last night the wind was blowing hard, and rain was falling too.
Press it gently with a finger and the web caves in, soft and floppy. Not stiff at all. And yet it springs back without tearing.
Not stiff, yet not broken. That "caving in" is exactly where the answer begins.
There are really just two reasons
Spider silk is said to stretch to roughly 30 to 40 percent beyond its original length before it snaps. Because it absorbs the impact over a long distance, the force involved becomes smaller.
Wind passes straight through most of the openings. And if one thread does snap, the force scatters to the threads around it, so the damage stops right there.
Both come from the same idea: don't be rigid. Let's look at each in turn.
The longer the stopping distance, the smaller the force
When a flying insect crashes into a web, it's carrying momentum, and the web has to stop it. What matters here isn't how much force does the stopping, but over how much distance the stopping happens.
To arrest the same momentum, stopping it abruptly over a short distance produces a large force. Stopping it gradually over a long distance needs only a small force. It's the same logic as bending your knees when you jump down from somewhere high. Land with straight legs instead, and a huge force slams into your feet.
A thin wire or steel filament is said to snap after stretching just a few percent. In other words, it has almost no give. It would stop an incoming insect over a tiny distance, and the thread would take a huge force. Spider silk stretches dozens of times further than that. Look at the left side of Figure 1. The top line is a thread that doesn't stretch; the bottom is spider silk. The deeper the dip, the gentler the force the thread feels.
What's more, stretched spider silk turns force into heat and lets it dissipate. It springs back slowly, like a squeezed sponge. That's why the insect stays caught in the web instead of bouncing back out. The very mechanism that catches prey is the same one that keeps the web from breaking.
Being full of gaps is itself a source of strength
If a web were a solid sheet like a panel, wind would slam into it head-on. The force it takes grows in proportion to the area facing the wind. But a spiderweb is almost entirely empty space. Most of the wind passes straight through, and only the part that hits a thread turns into force.
Even so, on a windy day the web billows and sways widely. That too is a way of letting force flow through. By staying free to move, the web keeps force from concentrating in one spot. It's like how laundry pinned up taut flaps violently in the wind and tears easily, while loosely hung fabric rides it out fine.
Another key point is what happens after one thread snaps. A single sheet, once a crack starts, rips open from there in one go. A web is different. When one thread breaks, the load it was carrying splits across the many threads around it. The tear doesn't spread — it stops right there. That's also why a spider can simply re-spin the damaged section each day, rather than the whole web.
The same goes for rain. A raindrop hitting a thread tugs it sharply downward. But the thread stretches to absorb that tug, and the water either beads and drops off or stays perched on the thread. A morning web strung with beads of water is proof that the threads are holding the water and still haven't snapped.
The outer frame and the radiating spokes are made of a thread that's strong and resists stretching. The spiral part that traps prey is made of a different thread — stretchy and sticky. Sturdy for the frame, soft where it needs to absorb impact: spiders are known to spin different threads for different jobs.
People often say spider silk is stronger than steel, but that's not a comparison at equal thickness. It's a comparison at equal weight, and one measured by how much energy the thread can absorb before it snaps. For something so thin, it can swallow an extraordinary amount of momentum — that's what's remarkable about it.
Summary
A spiderweb doesn't survive because it's stiff. Its stretchy threads absorb force over a long distance, its gap-filled structure lets wind pass through, and when one thread snaps, the threads around it pick up the slack. Rather than standing rigid against the force, it changes shape and lets the force flow through. That's how those thin threads make it through the night.
Strength isn't about staying unchanged.
It's about changing a great deal, and still not breaking.
The idea of absorbing force over a long distance also shows up in how seatbelts work and in why cats are fine falling from high places. Thin threads or fibers tangling together to provide support connects to why wet paper tears more easily, and remarkable devices built by living things connect to how geckos cling to walls.
- If you find a spiderweb, gently press down on part of the spiral with the tip of a thin blade of grass. Watch how far the thread stretches before springing back. Try it gently, near the edge of the web.
- On the same web, lightly press and compare the frame thread and a spiral thread. You'll find the frame thread is taut and stiffer, while the spiral thread stretches much more.
- On a windy day, watch a web from a short distance away. You'll see the whole web billow out and spring back, again and again, without tearing.
※ The web has an owner living on it. Don't touch it directly with your hands — observe through a tool instead, and only within limits that won't damage it. When you're done observing, step quietly away.
Want to know more? ― Terms, formulas, and textbook connectionsLabeled by level, from middle-school science through university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics"
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university-level specialist content (materials science, polymer science)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has names
- Kinetic energy: the amount of momentum a moving object carries. It grows with speed, and with mass.
- Toughness: a measure of how much energy something can absorb before it breaks. This is different from stiffness or resistance to snapping.
- Tensile strength: the amount of pulling force a thread or rod can withstand before it snaps, compared by dividing by thickness.
- Web-building spiders: spiders that spin a web and wait for prey. Some spin circular orb webs; others spin irregular webs.
MSHSCheck it with a formula: if the stopping distance shrinks to a tenth, how much bigger does the force get?
Consider a small insect flying into a web. All we need is the relationship: "the energy a moving object carries equals the force that stops it, multiplied by the stopping distance." Let's first fix what the symbols and units mean.
| In symbols | F × D ＝ E ＝ ½ × m × v² (so F ＝ E ÷ D) |
| In words | Force the thread applies × distance the web stretches ＝ the insect's kinetic energy ＝ ½ × mass × speed × speed |
| Where it comes from | It comes from "force × distance ＝ work," combined with the relationship between work and energy. We're assuming all of the insect's kinetic energy goes into the work of stretching the web, bringing it to a stop |
| Symbol E | Kinetic energy the insect carries. Unit: joules |
| Symbol F | Average force the thread uses to stop the insect. Unit: newtons |
| Symbol D | Distance the web stretches before stopping. Unit: meters |
| Insect's mass | Take it as 0.0001 kilograms (0.1 grams) |
| Insect's speed | Take it as 2 meters per second |
| Web's stretch distance | Take it as 0.02 meters (2 centimeters) |
| Multiply speed by itself | 2 × 2 ＝ 4 |
| Multiply by mass | 0.0001 × 4 ＝ 0.0004 |
| Halve it to get kinetic energy | 0.0004 ÷ 2 ＝ 0.0002 |
| Average force on the spider's thread | 0.0002 ÷ 0.02 ＝ 0.01 |
| If the thread stretched only a tenth as far | 0.0002 ÷ 0.002 ＝ 0.1 |
| How many times bigger is the force? | 0.1 ÷ 0.01 ＝ 10 |
③ In plain terms: with the same insect at the same speed, if the stretch distance shrinks to a tenth, the force on the thread becomes 10 times bigger. A spider could try to make the thread material 10 times stronger, or make it stretch 10 times further. It chose the latter.
HSHS+"Strong" has more than one meaning
HSIn Basic Physics, you learn about the relationship between work done by a force and energy. When the amount of work needed to stop something is fixed, taking a longer distance means a smaller force. The whole framework of this article is just a rephrasing of that one line.
HS+Material properties are compared by graphing the relationship between pulling force and stretch. The area under that line corresponds to the energy the material can absorb before snapping. A steel wire's line rises sharply and ends quickly; spider silk's line continues gently for a long stretch. Compared at equal weight, the area under spider silk's curve is said to be larger.
Univ.The tear-resistance built into the shape of a web
A single continuous membrane concentrates force at the tip of any cut, so it tears open suddenly from a small nick. A web doesn't allow that kind of concentration. Even if one thread is lost, many other load-bearing paths remain. In materials science, this is treated as the advantage of a structure with multiple redundant paths. In circular webs, there's also a division of labor: the outer frame and radiating spokes are stiff, while the spiral threads are soft. From the materials side, the energy a material can absorb before breaking is called "toughness," represented by the area under the stress–strain curve. Under a large force, the soft parts are reported to stretch first and share out the load. You could also see this as a design that decides in advance where it's allowed to fail.
📖 For the derivation of the formula and further reading: Toughness (Japanese Wikipedia) ／ Stress–strain curve (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why it shrinks when wet. Spider silk is known to shrink and change tension when it gets wet. Whether this automatically adjusts the web's tautness, or is merely a side effect, is still debated.
- The properties of freshly spun silk. Inside the spider's body, the silk is a liquid, and it solidifies the instant it's released. How far that solidification process can be reproduced remains a major challenge for making artificial silk.
- What determines a web's shape. Which species spins which kind of web depends both on innate traits and on the branches and wind conditions on site. How much each factor contributes is still being studied species by species.
In other words, this article too describes things "as currently understood." The overall framework has been confirmed, but the specific numbers vary widely by species and conditions.
Connections to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science ・ How forces work / work and energy | The relationship between stopping distance and force |
| HS | Basic Physics ・ work and kinetic energy | The "check it with a formula" section |
| HS+ | Physics ・ material deformation and the area under the graph | How toughness is compared |
| Univ. | Materials science, polymer science, biophysics | The tear-resistance built into a web's shape |
| Research | Biomimetics, arachnology | Shrinking when wet, and artificial silk |
| ― | Connections to daily life | Landing with bent knees, loosely hung laundry, re-spinning a damaged web |
- Shigeyoshi Osaki, The Mystery of Spider Silk (クモの糸のミステリー), Chuokoron-Shinsha (クモの糸の強さと伸び, 巣のつくり — the strength and stretch of spider silk, and web construction)
- Eiichi Shinkai, Spiders of Japan (日本のクモ), Bun-ichi Sogo Shuppan (網を張るクモの種類と網の形 — species of web-building spiders and web shapes)
- National Astronomical Observatory of Japan (ed.), Chronological Scientific Tables (理科年表), Maruzen Publishing (金属や繊維の引張強さなどの物性値 — physical property values such as tensile strength of metals and fibers)
- Ministry of Education, Culture, Sports, Science and Technology, Commentary on the Course of Study for Upper Secondary Schools: Science (高等学校学習指導要領解説 理科編) (仕事と運動エネルギーの扱い — treatment of work and kinetic energy)
※This article is a general-audience science explainer. The figures given are approximate, meant to help convey the underlying mechanism. Silk strength and stretch vary widely by spider species and conditions. When observing living creatures or their webs, please avoid touching more than necessary and take care not to disturb the surrounding environment.