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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

More on insects and electric fields:

9 thoughts on “Spider Webs Reach Out To Flying Insects. Cool, But So What?

  1. But wait, isn’t this likely to just be a fortuitous benefit, not an evolved adaptation? Given the negative charge of the plants and the positive charge of the flying insects, this is just something that happens according ordinary physical principles. So, even if experiments showed that the moving web impacted success, this would still not be sufficient to conclude that it is an evolved adaptation.

    [Agreed – Ed]

  2. More experiments are definitely needed. Flying insects have excellent eyesight, and we know that they can see spider webs. In fact, there is a whole body of research about the interaction between the optics of webs and the eyesight of insects. If an insect is already this close to a web, mightn’t it be likely to hit the web even without this attraction? There are interesting questions about how orb weaving spiders who use catching threads covered with a kind of silk that works by Van der Waals forces evolved to become spiders who cover their (different) catching threads with glue. It would be interesting to think about whether this new finding has any implications there. It’s always great to read a paper that suggests many new questions.

  3. In addition to Leslie’s thoughts, I’d be most interested to see further studies using different webs; it would be interesting to see if this is a real influence on web effectiveness, and if so, how it changes as the water content of the silk changes over the course of the day. How about web construction – does any electrostatic effect vary with web size or shape? How about Bolas Spiders? Then, what about the cribellate orbweavers like Uloborus? Lots of fun questions there. Not that we didn’t already have endless questions and too few people looking at them…

  4. John–Cribellate silk holds insects thanks to Van der Walls forces. Brent Opell has been looking at what happens when bug hits web for a long time. He found back in the 1990s that static electric attraction appears to play no role in cribellate stickiness: http://www.faculty.biol.vt.edu/opell/publicaton_pdfs/1995%20JEZ%20273.186.PDF
    My gut feeling is that the effect described in the new paper isn’t a selected adaptation (but I’m a writer, not a biologist), but, like you, I’m happy to see anyone focused on how webs actually work. Just a few decades ago, even most arachnologists thought webs were just passive sieves.

  5. This certainly helps explain why I get so many spider webs across the face when I running on woodsy trail!

  6. Hmm…I have a garden with lots of spider webs and lots of bees (mostly carpenter bees), yet, I have never seen a bee caught in a spider web. Come to think of it, this is amazing.
    Thanks Ed for an interesting article.

  7. Thanks, Leslie – I missed all that at the time, obviously. I wonder if this might have any effect on pollen capture by the webs of young spiders – I used to see early-instar Argiope webs with a fair bit of pollen on them when I was out in the field a lot. Maybe not; there’s a lot of pollen out there, and probably random encounters are enough. It’s interesting about bees using electrostatic cues on flower visits, too – I assumed it was generally UV-reflectance-based, but who says there has to be a single cue.
    Kathy’s note also raises a nice point – I used to see Honeybees caught in webs quite often, but now that I think about it, I don’t recall ever seeing a Bumblebee caught, even in the big Argiope webs that held Grasshoppers and other large, heavy insects. I’m sure somebody has been looking into web/insect interactions (Soldier Beetles are the only insects I’ve ever seen get out of a web unscathed after being stuck; it’s very cool how they do it, but I can’t imagine Bumblebees getting out the same way), and that sounds like something you’d know more about.

  8. I don’t know about bumblebees specifically, but other bees and flies frequently extricate themselves from webs. What’s more, many of them learn to avoid the same web in future. My co-author, Catherine L. Craig, did a lot of work on this in the ’80s and ’90s and there has been a lot of follow-on work by other researchers since. We talk about this in one of our chapters. It’s all still a bit mysterious, and, given the diversity of spiders, the diversity of their prey, and the diversity of where different species pitch their webs, it seems likely not only that more than one thing is going on in any single web but also that different things are going on in different kinds of webs. Cribellate orb webs and araneoid webs definitely have different optical properties in addition to different sticking mechanisms.

  9. The copyeditor in me got stuck on “The way the web bends was surprising to me, but this is what you (?) when you buy a high-speed camera.”

    The bug-lover in me is fascinated, as always, by new developments like these. Maybe spiders have already evolved to build their webs to better take advantage of this effect?

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