No air currents required: Ballooning spiders rely on electric fields to generate lift

Charles Darwin witnessed hundreds of ballooning spiders landing on theHMS Beagle. Since at least the days of Aristotle, ballooning has been known, but scientists have only recently made progress.

A new mathematical model incorporating all the forces at play as well as the effects of multiple threads was published in the journal Physical Review E. One day, the work could contribute to the design of new types of ballooning sensors.

There are differing theories about how ballooning spiders float off into the air. One proposal suggests that as the air warms with the rising sun, the silk threads the spiders emit to spin their parachutes to catch the rising currents that are caused by thermal gradients. A second hypothesis holds that the threads have a static electric charge that interacts with the weak vertical electric field in the atmosphere.

There are two hypotheses about ballooning spiders. A study in Current Biology found that spiders can detect electric fields. The study suggests that the electric fields provide enough force to generate lift.

Experiments with ballooning crab spiders were reported in a second study. Moonsung Cho, an aerodynamic engineer at the Technical University of Berlin, was inspired after seeing several wild ballooning spiders. He and his colleagues built an outdoor platform, and one by one, they put a dozen crab spiders on the platform, videotaping their ballooning behavior. The platforms emit a fine powder so that the wind can be seen. In the wind tunnel, they put a reel behind the spiders and sprayed silk lines as if they were fishing line.

The spiders seemed to test the conditions before taking off by raising one or two front legs. If the winds were low enough, the spiders would turn around, stand on their toes, and raise their butt to the sky. The silken threads formed a triangular parachute. The spiders would anchor themselves to the ground with a drag line and then detach themselves when the parachute began to carry them off.

The average strand of spider silk was 9 feet long and 200 nanometers across, which is less than the wavelength of visible light. Cho told The Washington Post that the air at those scales is very sticky, like honey, and that the combination of sticky air and narrow threads gives spiders enough lift to soar through the air.

Simulation studies have been done with a single thread. The effect of the repelling force on the shape of the threads as well as the ballooning velocity were taken into account when simulation was done. Jawed and Habchi wanted to correct that gap.

The silk thread is divided into several segments that can bend, stretch, and twist. The spider was modeled as a sphere, measuring 2 millimeters in diameter. The model assumed that the threads were coated with electric charges, as different variations of the simulation had either two, four, or eight threads attached at the top of the spider-sphere.

Jawed and Habchi incorporated several other factors into their model, including gravity, the atmospheric electric field, the electric charge of the threads, and drag. Per APS physics.

In the simulations, the spider starts at rest on the ground and is lifted by the electric field. While the charged, initially straight threads remain attached to the spider, their mutual repulsion causes them to spread apart over a period of time. As the spider accelerates upward, downward drag increases and—combined with the spider’s weight—eventually cancels the lifting force. This competition between upward and downward forces determines a spider’s final (terminal) upward velocity.

The multiple threads did not become entangled when they were charged. The electric charges distributed uniformly across the thread or merely at the tip were equally capable of generating lift, and the vertical velocities were consistent with recent experimental studies.

The Physical Review E was published in 2022. The About DOIs is 0.1103/PhysRevE.105.034401.