1. Spiders Do Not Fly, but Their Silk Can Lift Them
Darwin described the scene in a note published in 1873. The ship was 60 miles from land, and a light but steady breeze was blowing. The spiders were about one-tenth of an inch (2.5 mm) long, and there must have been "some thousands" of them in the ship's ropes and the webs hung on them. [1]
The short answer is this. A spider climbs to a high spot and lets out silk. Moving air pulls on the silk, and the silk pulls the spider up with it. Researchers call this behavior ballooning. [2] In one study, the silk that spiders let out for flight turned out to be a bundle of extremely fine fibers. [2] Another study found that spiders can also take off in still air if there is an electric field around them. [3]
The rest of this page looks at three questions. What does the spider do, what is the silk like, and what tells the spider that it is a good moment to go?
2. What a Spider Does Before It Lifts Off
A team in Germany filmed spiders in the field and in a wind tunnel. They studied crab spiders of the genus Xysticus, which are 3 to 6 mm long and weigh only 6 to 25 mg. [2]
The spider climbs to the tip of a plant or onto a platform. It stands on tiptoe, raises its abdomen (the rear part of its body), and lets silk flow out. If the air catches the silk, the spider is lifted off. [2]
A tiny spider is easy to lift, and that matters. The researchers also estimated how much rising air a spider of this size would need. For Xysticus of 10 to 25 mg, their physical model gave a very gentle upward flow of about 0.04 to 0.2 meters per second (about 0.1 to 0.5 mph). [2] That is slower than a slow walk. The next section shows what kind of silk these spiders use to catch even that gentle lift.
3. What the Flight Silk Is Made Of
The silk that the German team measured came as a bundle. Each flight line had 48 to 58 thin fibers and 2 thicker ones. The thin fibers were between 121 and 323 nanometers across, averaging about 212 nm. The thicker pair averaged about 722 nm. A nanometer is a millionth of a millimeter, so these fibers are hundreds of times thinner than a human hair, which is roughly 0.1 mm wide. The lines were long, too: 3.2 meters on average (about 10 feet), and up to 6.2 meters (about 20 feet) in the longest case. [2]
Why a bundle? The authors suggest that many ultra-fine fibers together create enough drag in the air to hold up a small spider. Fibers this thin interact with air in a different way than a thick rope does. The authors themselves present this as a hypothesis that still needs more testing. [2]
4. How Spiders Seem to Choose a Good Moment
A spider does not jump the moment it reaches the top. Before tiptoeing, the spiders in the study often raised their front legs. The authors suggest this is a way of actively sensing the wind. [2] On the test platform, only about 1 in 10 tiptoe attempts (9.8%) ended with a takeoff. [2] The spiders seem to wait.
In the study, ballooning was observed when the wind was weaker than 3 meters per second (about 7 mph), a light breeze. [2] That seems odd. Why would a weak wind be better than a strong one? The team measured the air outdoors, and even when the average wind was only about 2 to 3 m/s, small bursts of rising and sinking air came and went. When the average wind was 1.99 m/s, vertical speeds ranged from about -0.5 to 0.5 m/s. When it was 3.07 m/s, they ranged from -0.6 to 0.7 m/s. [2]
The authors suggest that the spiders may be using rising air created by swirling turbulence near the ground, which is more likely when the horizontal wind is light. They call this idea a hypothesis, and they note that how often and how long these updrafts last is not well known. [2]
5. Can an Electric Field Lift a Spider in Still Air?
Air currents are not the whole story. The air above the ground normally carries a weak vertical electric field, called the atmospheric potential gradient. Over open ground on a clear day it is about 120 volts per meter. [3]
Erica Morley and Daniel Robert at the University of Bristol, UK, tested small money spiders (Erigone, a group in the family Linyphiidae) in a closed box that limited air movement. They applied a vertical electric field between two plates. In the lab, the fields were 1.25 and 6.25 kilovolts per meter, which is roughly 10 to 50 times the natural fair-weather value (1.25 kV/m is 1,250 volts per meter, or about 1,140 volts per yard). [3] With the field on, the spiders showed more climbing, tiptoeing, and silk release. In a video shown with the paper, a spider that was airborne moved upward when the field was switched on and downward when it was switched off. [3][4]
The spiders also have fine sensory hairs on their legs, called trichobothria. The researchers found that these hairs moved when an electric field was applied, which is one possible way a spider could notice the field. [3]
This does not prove that wild spiders rely on electricity. The authors say fields are "sufficient" to trigger ballooning but may not always be necessary. They suggest that air movement and electric forces may work together, and they list field studies as a next step. [3][4]
6. What Scientists Still Do Not Know About Spider Flight
Back to the ship. When Darwin wrote about it, he proposed that the spiders were carried up by "an ascending current of heated air." [1] Today's studies add silk drag, small updrafts, and electric fields to the list of possible forces. Which one matters most, and at which moment of the flight, is still being worked out. The Bristol team also points out that nobody has settled whether spiders in nature need electric fields at all. [3][4]
The Morley and Robert paper mentions reports of spiders as high as 4 km up and of travel over hundreds of kilometers. These are reported observations, not results of the experiments described here. [3] Silk, weather, and possibly electric fields may affect how spiders travel; their relative roles in nature remain uncertain.
Keep this in mind: a single experiment shows what can happen under certain conditions. It does not always show what happens most often in nature. That gap is where new questions come from.
7. How to Watch for Spiders Getting Ready to Balloon
If you happen to see a tiny spider on a fence or on tall grass in a garden or park, watch it from a distance. These conditions do not guarantee that you will see ballooning, but a tiny spider standing on tiptoe with its abdomen raised may be getting ready to balloon. Watch from a distance. Do not touch or pick up the spider, and do not blow on it.
Write down what you see: whether it lifts a front leg, whether silk appears, and how strong the wind feels (for example, whether leaves are moving a little or a lot). If you find no spiders, write that down too, along with the weather. Over several days you may notice a pattern, and it will be your own observation to compare with the papers listed below.