Once, when Charles Darwin noticed the gossamer spiders following him on a sea voyage he referred to as “little aeronauts”. They achieve this by an exercise called “ballooning” that involves spinning a tiny silk sail and jumping from a high platform. Till recently, it was held that wind was needed to enable their flights. But the fact that ballooning has been observed in windless conditions raises the question of how exactly the spiders take flight. Now, researchers from the University of Bristol, UK, have confidence that they have decoded this mystery.
Reported in the journal Current Biology, their study 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.
Biologist Erica Morley says, “We don’t yet know whether electric fields are required to allow spider ballooning. We do, however, know that they are sufficient.”
Another researcher presented the notion of electrostatic ballooning to Morley and her colleague Daniel Robert in 2013. In fact, the theory that electricity could assist spiders in getting airborne has been discussed since the early nineteenth century but had never been taken seriously enough for testing. until now.
Morley explains that the atmospheric potential gradient i.e. APG is a global electromagnetic circuit between the Earth and the ionosphere which is ever-present around the world. But the strength of the APG can vary significantly from a maximum of 100 volts per meter on a calm, clear day to an increase by two orders of magnitude in stormier conditions.
The electric field surrounding us can be detected by insects who use them like the bumblebees to find their way to flowers. Now it has been revealed that spiders are equally equipped to respond to the atmospheric charge.
For their study, Morley and Robert created steady fields of electromagnetic current inside sealed tanks, to eliminate other stimuli like air movement. Then they introduced baby spiders from the family Linyphiidae. The researchers observed that ballooning increased significantly when the fields were switched on. Additionally, switching the electric field on and off after the spiders were airborne caused them to move upwards or downwards, respectively. This research also revealed that the spiders’ trichobothria i.e. the tiny sensory hairs located on the surface of arachnid exoskeletons that were previously shown responsive to sound, also seemed to be stimulated by the electric fields.
On some days, many thousands of spiders take to the air in mass ballooning events while on others none disperse at all. The new findings submit that this could be explained by fluctuations in the strength of the APG. This understanding may also help to predict the future occurrence of such events will occur not only in spiders but also in other ballooning animals such as caterpillars and spider mites. This could lead to a deeper understanding of population dynamics, species distributions, and ecological resilience.
The researchers realize that more work is required.
Morley says, “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”.