Spiders sometimes travel great distances by “ballooning”, carried high in the air on silken threads. Biologists have long been puzzled, however, as to how they do this in winds that appear inadequate, particularly for larger species. Electric fields have been suspected, but only now has this theory been proved.
"Current theories fail to predict patterns in spider ballooning using wind alone as the driver,” said the University of Bristol's Dr Erica Morley. “Why is it that some days there are large numbers that take to the air, while other days no spiders will attempt to balloon at all?”
As with so much in biology, Darwin contributed important observations, reporting how hundreds of spiders landed on the Beagle while at sea on a calm day. However, even the great man himself could not explain how they subsequently took off rapidly, despite minimal wind.
Thunderstorms can electrify the atmosphere, but aren't spiders' favorite times to be airborne. It is less widely known that an electric field, the Atmospheric Potential Gradient (APG), averages 120 volts per meter (40 volts per foot) near ground level in flat territory even in fine conditions. This field is even more intense around protruding grounded objects, such as tree branches.
Sadly, you can't generate useful amounts of electric power by tapping into this (although some people have registered patents saying they can). Ballooning spiders, on the other hand, need tiny amounts of energy. Strands of their silk become charged by the field, with repelling charges explaining how the strands fan out, rather than flopping or tangling with each other. Then it appears the APG provides some of the force needed to give the spiders lift.
In Current Biology, Morely demonstrates that spiders know when fields are suited to flight. She exposed Linyphiid spiders to fields of similar strength to those in the near-ground atmosphere. The spiders responded to the fields by straightening their legs and releasing silk, just as one would do if hoping to take off, but stood down when the field was turned off.
It has previously been shown that some bees can detect the electric fields between themselves and flowers, and vertebrates have been using electric fields for more than 400 million years. However, Morely showed that the spiders don't need anything as advanced as the electrosensors in a platypus' bill. Instead, she notes the tiny hairs on the spiders' legs rise like our hair when touching a Van de Graaff generator, giving the spiders a simple field sensor.
Spiders have been found reaching altitudes of 4 kilometers (2.5 miles), but usually prefer to balloon on low-wind days, perhaps to avoid such heights. The combination of electrostatic forces and light winds probably provides the best opportunity for safe floating.
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