Spiders webs are beginning to weave an intriguing new story of how they ensnare their prey. Capture of flying and crawling fodder may not just be a lucky accident – for the spider – but the result of an electrostatic interaction between web and insect, according to a recent study conducted by University of California, Berkeley biologists and published in the journal, “Scientific Reports.”
What are electrostatic charges? A simple explanation is that electrostatics encompass the buildup of an electrical charge on the surface of objects in reaction to contact with other surfaces. If you remove a cap from your head and your hair stands straight up, that’s an example of electrostatics at work. The same is true when cellophane adheres to your hand after you unwrap a package. Or when a balloon sticks to a wall after you’ve rubbed it against your body.
New research indicates that spider webs may entrap prey by drawing them in as a result of electrostatic attraction. Some flying insects create an electric charge as they flutter their wings. These charged insects could then be drawn into and trapped by sticky, negatively charged spider web strands as they fly close by.
According to the study’s co-author, Victor Manuel Ortega-Jimenez, a UC Berkeley postdoctoral fellow who typically studies hummingbird flight, and who made the initial observation, “Charged insects can produce a deformation of a spider web. Any insect that is flying very close to the spider web can be trapped by the electrostatic effect.”
Ortega-Jimenez first observed this occurrence when playing with his daughter using an electrostatically charged “magic wand” that induced small objects to rise. He also used the wand to charge up several insects. And when he brought it close to a spider web, the web changed shape in response.
“We were outside of our apartment, and we put the wand close to a giant spider web, and there was a strong attraction between the web and the wand,” he describes. “It kept on getting closer until the web touched the wand.”
The biologist was aware that honeybees’ flapping wings can produce an electrical charge of up to 200 volts, which may assist them when collecting pollen from negatively charged flowers. He knew studies had divulged that in response to prey, webs were capable of radically deforming. This piqued his interest as to whether or not spider webs could ensnare prey through electrostatic magnetism.
To test this possibility, Ortega-Jimenez and colleague, Campus Professor of Integrative Biology, Robert Dudley, collected webs of the common cross spider (also known as a garden spider) from the UC Berkeley campus. Under laboratory conditions, they studied how the spider webs reacted to objects containing an electrical charge.
They discovered that webs and positively charged objects were drawn to each other. Additionally, the spider web’s silk threads moved in an arc toward each other beneath a charged honeybee that was plummeting toward them, increasing the likelihood that the insect would become a victim of the sticky strands. A substantial change, the web’s deformation was approximately half the length of the insects.
“This is quite intriguing,” commented Markus Buehler, a Massachusetts Institute of Technology materials scientist who studies spider silk, but was not involved in this particular study. “The attraction pulls the insect to the web and enhances the likelihood that it is being caught in the web.”
Dudley, who has researched insect flight and was one of the chief researchers of the study, attests that any airborne object – from tiny insects to helicopters – receives a positive charge while flying, produced by friction with the air.
During their lab experiment, Ortega-Jimenez retrieved dead insects such as aphids, fruit flies, green bottle flies and honey bees, which were given a positive charge from an electrostatic generator and dropped onto a horizontally situated, neutrally charged spider web. Utilizing high-speed cameras, researchers witnessed the web bending toward the falling insect before the insect actually made contact with it.
“Using a high-speed camera, you can clearly see the spider web is deforming and touching the insect before it reaches the web,” confirms Ortega-Jimenez. Insects lacking a charge didn’t elicit this reaction. “You would expect that if the web is charged negatively, the attraction would increase.”
Ortega-Jimenez intends to carry out additional tests at Berkeley to ascertain whether this effect takes place in the wild, and to discover whether static charges on webs derive more dirt and pollen, and are therefore a primary reason why spiders rebuild their webs daily. He also theorizes that light, flexible spider silk may have evolved because it deforms much more easily, when faced with electrostatic charges, to assist in capturing prey. “Electrostatic charges are everywhere,” he says, “and we propose that this may have driven the evolution of specialized webs.”
Conversely, when insects devoid of a charge were dropped, the web did not bend toward the insect. According to Dudley, the web’s movement “enhances the likelihood of catching an insect. The web would stretch toward the insect – which is very clever.”
