The phenomenon of baby spiders gliding through the air on the end of a length of spun-silk has been reported for centuries. Now scientists have uncovered the surprising mechanism by which the feat is achieved.
Charles Darwin once referred to the gossamer spiders he noticed following him on a sea voyage as “little aeronauts”. They accomplish this through a practice called “ballooning”, spinning a tiny silk sail and jumping from a high platform. Until recently, it was believed that wind was needed to enable their flights.
However, ballooning has been observed in windless conditions, which raises the question of exactly how the spiders take flight. Now, researchers from the University of Bristol, UK, believe they have shed light on this mystery.
Their study, reported in the journal Current Biology, shows that naturally occurring electromagnetic fields can not only trigger this process, but also provide lift and velocity, even without a breeze to glide on.
“We don’t yet know whether electric fields are required to allow spider ballooning,” says biologist Erica Morley. “We do, however, know that they are sufficient.”
Morley and her colleague Daniel Robert were presented with the notion of electrostatic ballooning by another researcher in 2013. The theory that electricity could aid spiders in getting airborne has, in fact, been discussed since the early nineteenth century but until now, it had never been taken seriously in spite of little in the way of testing.
The atmospheric potential gradient (APG) – a global electromagnetic circuit between the Earth and the ionosphere – is ever-present around the world, Morley explains. But the strength of the APG can vary greatly; on a calm, clear day, it may reach 100 volts per metre while in stormier conditions, it can increase by two orders of magnitude.
The electric field surrounding all of us can be detected by insects – bumblebees, for instance, use them to find their way to flowers – but only now is it coming to light that spiders are similarly equipped to respond to atmospheric charge.
In their study, Morley and Robert created steady fields of electromagnetic current inside sealed tanks, thus eliminating other stimuli such as air movement. They then introduced baby spiders from the family Linyphiidae.
The researchers noticed that ballooning increased significantly when the fields were switched on. Furthermore, switching the electric field on and off once the spiders were airborne caused them to move upwards or downwards, respectively.
The research also found that the spiders’ trichobothria, tiny sensory hairs located on the surface of arachnid exoskeletons that have previously been shown to respond to sound, also appeared to be stimulated by the electric fields.
There are days when many thousands of spiders take to the air in mass ballooning events and others when none disperse at all. The new findings suggest this could be explained by fluctuations in the strength of the APG.
The findings may also help to predict when such events will occur in future, not only in spiders, but also in other ballooning animals, including caterpillars and spider mites. This could bring about a deeper understanding of population dynamics, species distributions and ecological resilience.
The researchers believe more work is required. “The next step will involve looking to see whether other animals also detect and use electric fields in ballooning. We also hope to carry out further investigations into the physical properties of ballooning silk and carry out ballooning studies in the field,” Morley says.
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