Mystery of spiders’ webs unravelled23rd December 2013
With a beautiful structure, elegantly constructed from silk stronger than steel, spiders’ webs have long been one of nature’s marvels.
But now scientists have discovered yet another remarkable property. According to researchers at Oxford University, Spiders’ webs electrostatically reach out and ‘grab’ airborne particles, droplets and even insects. Electrically conductive glue spread over the threads of the web enables the web to spring out at passing charged particles (including insects), regardless of whether they are positively or negatively charged. This helps to explain how webs are able to efficiently collect airborne particles and why they actively reach out to passing insects.
Not only is this discovery fascinating, but it could also have practical applications as an alternative to expensive industrial sensors used for environmental monitoring. ‘The elegant physics of these webs make them perfect active filters of airborne pollutants including aerosols and pesticides,’ said Professor Fritz Vollrath of Oxford University’s Department of Zoology, who led the study.
‘Electrical attraction drags these particles to the webs, so you could harvest and test webs to monitor pollution levels – for example, to check for pesticides that might be harming bee populations.’
Together with Dr Donald Edmonds of Oxford University’s Department of Physics, Professor Vollrath goes on to show how conductive webs cause a local distortion of the Earth’s background electric field. As many insects such as bees use their antennas to sense electrical disturbances, the researchers hypothesised that the spider’s intended prey might be able to electrostatically ‘see’ the web and avoid flying into it.
‘Pretty much all flying insects should be capable of sensing electrical disturbances,’ said Professor Vollrath. ‘Their antennae act as “e-sensors” when the tips are connected to the body by insulating materials, meaning the charge at the tip will be different from the rest of the insect. As insects approach charged objects, the tips of their antennae will move by a small amount, which they may be able to feel.’
‘Bees already use e-sensors to sense flowers and other bees, so it now remains to be seen whether they might also use them to avoid webs and thus becoming dinner.’
The distortion effect of the web is short ranged and weak, yet many flying insects can detect very small changes in the electric field. Further empirical testing is required to establish whether these insects can avoid webs in this way, the research concludes.
How it works:
1. Spider’s webs are made of sticky silk thread with a glue coating that absorbs water, resulting in evenly distributed aqueous droplets along the length of the thread. The study showed that there is a thin conductive film between these droplets even when the thread becomes stretched; this means that the whole web is electrically connected.
2. Airborne particles (and insects) become highly charged when moving through the air, similarly a water droplet suspended in moving air by an insulator (such as the silk of a spider’s web) becomes charged.
3. The ionic charges in an aqueous solution are highly mobile. As the insect approaches the web, it repels like charges and attracts opposite charges in its vicinity, therefore inducing a charge opposite to its own in the nearest droplet on the web.
4. The web now feels an attractive force towards the insect (and vice versa) so reaches out towards it. As the thread in the web is very soft and stretchy, a small electrostatic force can exert a significant effect.
Photo/Prof. Fritz Vollrath