Cross spider. Credit: Jürgen Howaldt
Cross spider. Credit: Jürgen Howaldt

Spider Webs Reach Out To Flying Insects. Cool, But So What?

Spider webs turn the airways of fields and forests into a gauntlet of traps. Once spun, these silken snares lie in wait for insects to blunder into them. But they’re not entirely passive. Victor Manuel Ortega-Jimenez and Robert Dudley from the University of California, Berkeley have shown that in the moments before a bee or fly careens into a web, the web reaches out to meet its victim.

Bees and other flying insects frequently collide with microscopic mid-air particles like dust and small molecules. These strip electrons from their cuticles—their outer shells—leaving them with a positive electric charge. In this way, a flying bee can build up a voltage of up to 450 volts.

We’ve known this since at least 1929, but a handful of studies published this year have shown how important the electric world of insects can be. Pollen, which is usually negatively charged, can fly over to a bee before it lands on a flower. Daniel Robert showed that bees can even sense the electric fields of flowers and use them to tell which blossoms they’ve recently visited. And Uwe Greggers found that they might be able to move each other and communicate with their own electric fields.

So, if electric charge can influence an insect’s relationships with its peers or its partners in pollination, what about its predators? Ortega-Jimenez started wondering about this while playing with a magic wand. The wand, one of his daughter’s toys, had ability to attract spider webs because it produced a positive charge. If it could do that, why not a bee?

The duo collected webs of the common cross spider (Araneus diadematus) from around the UC-Berkeley campus and mounted them on a horizontal stick frame, so they had a neutral charge. They then dropped dead bees, flies and aphids onto the web and filmed their collisions with a high-speed camera.

If they first imbued the insects with a positive charge using a generator, he saw that the webs’ silken threads would stretch up to meet them around 70 percent of the time. And if the insects weren’t charged before their fall, the web never moved.

The webs in the experiment were grounded and had a neutral charge. Wild ones might have a negative one—no one has ever measured that, but it’s plausible given that plants are typically negatively charged. If that’s the case, webs would be even more strongly drawn to positively charged insects than they were in this study.

Ortega-Jimenez and Dudley write that this attraction “likely increase[s] the risk of capture for free-flying prey”.

The threads only move over 1 or 2 millimetres, but the duo point out that this is similar to the gaps between them. Perhaps by reaching out to incoming insects, they might stop prey from flying between the strands.

Wait a minute, say other scientists who study the electric fields of insects. Ortega-Jimenez and Dudley haven’t made their case. Yes, webs might move towards positively charged insects, and yes, that’s interesting in itself. But so what? Would a thread that moves a millimetre closer to an insect that’s already hurtling towards a web make any difference to a spider’s success?

“The way the web bends was surprising to me, but this is what you when you buy a high-speed camera,” says Greggers. “The harder job is to demonstrate that it is relevant for the animal. Such an experiment is time-consuming but not very difficult to do. We had to demonstrate the relevance for the animal in our paper.”

Robert agrees. The team needs to check if charged insects are more likely to be captured than neutral or negatively charged ones. “As a sensory ecologist, I have to wonder whether spiders themselves can sense the charge of their webs or of insects approaching, or whether they are using their webs to measure that charge,” he says. “By extension, could bees detect the presence of a web using their electro-reception sense? I sense that were are only at the very beginning of discovering electrostatics in the living world, and the way it can be sensed and used by plants and animals.”

Reference: Ortega-Jimenez & Dudley. 2013. Spiderweb deformation induced by electrostatically charged insects. Scientific Reports. http://dx.doi.org/10.1038/srep02108

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