Spider webs are literally attracted to some insectsS

Spider webs are well known to be excellent insect — and sometimes bird — catchers due to their mechanical strength, stickiness and numerous other properties. But research now shows that the webs of some spiders have an additional, hidden ability: Essentially, they can reach out and grab free-falling insects that have become electrostatically charged by the environment.

Similar to the way we generate static electricity by, say, rubbing our feet on the carpet, insects and other animals can also attain an electrostatic charge. In their normal environment, they may become charged by walking on some surfaces, such as glass or plastic. "And when they are flying and flapping their wings, insects can acquire a charge by rubbing their wings against the air because the air has charged particles," says Victor Ortega-Jimenez, a biologist at the University of North Carolina at Chapel Hill.

Few studies have actually looked at the consequences of insects' electrostatic charge. Recent work on bees found that positively charged bumblebees can detect the electric field of flowers, helping the insects identify flowers that have previously been visited by other bees (which would alter the flower's charge) and are therefore worth visiting themselves. Other research suggests that electrostatic charges help honeybees communicate with one another.

While playing with his daughter, Ortega-Jimenez came up with the idea to test if the electrostatic charge of insects could attract spider webs. "I was trying to show her how electrostatic charge works," he tells io9. He bought a Fun-Fly-Stick, a toy with a mini Van de Graaff generator that imparts a positive charge on certain small items — the wand can then be used to control the levitation of the item because both objects have the same charge and repulse one another. "I was playing with this toy and charged some insects, and then I approached spider webs with it," Ortega-Jimenez recalls. The wand attracted the spider webs, leading him to wonder if charged insects would do the same.

As they approach the web, positively charged insects and water drops attract spider web threads. Courtesy of Victor Ortega-Jimenez.

To find out, Ortega-Jimenez and his colleague Robert Dudley used the nifty wand to charge various recently killed insects, including honeybees, green bottle flies, fruit flies and aphids, as well as water drops of two different sizes. They also collected webs from cross-spiders (Araneus diadematus), which produce silk threads that are very sticky, thin and flexible, and mounted them horizontally onto a grounded wooden frame. The researchers dropped the insects and water drops (individually) onto the webs from 25 to 35 cm (10 to 14 inches) above, and filmed what happened.

The scientists found that the charged specimens deformed the radial and spiral threads by 1 to 2 mm, which is equal to the mesh spacing of the web. The web threads effectively reached out to the insects and water droplets before the objects hit the web. In contrast, uncharged (neutral) specimens didn't attract the web threads during free fall. In the wild, thread deformation caused by charged insects could enhance the likelihood that the web makes physical contact with the potential prey, increasing the chances of prey capture, Ortega-Jimenez says.

Neutral insects and water drops fall onto the web without attracting it first. Courtesy of Victor Ortega-Jimenez.

The researchers also found that when a water drop hit the web, some of the water remained on the web. If another water drop fell towards the web, it would repel the water on the web, subsequently damaging that area of it. "There are some studies that say that spider webs can attract water drops and act as a reservoir of water," Ortega-Jimenez says. "But if the water is affecting the spider web in this way, it can't be good for the spider because it has to keep rebuilding its web."

Ortega-Jimenez is now interested in seeing how the interaction between web and charged insect plays out in the field. He's also curious about how electrically charged particles, such as pollen and fungal spores, fit into the equation — these particles could accumulate onto a web, potentially degrading it while giving it a highly negative charge that makes it more attracted to positively charged insects. "The important question to answer is if [electrostatic charges] are negative or positive for the spider," Ortega-Jimenez says.

The research was published today in the journal Scientific Reports.

Top image via Victor Ortega-Jimenez.