At first glance, spiders might not seem like the most likely of aeronauts, but there are in fact many species that take to the skies through the art of ‘ballooning’. By pointing their spinnerets in the air and releasing silken strands in a fan-like fashion, these spiders are able to ascend skywards up to altitudes of over 4000 metres! The mechanics of this ballooning phenomenon were first examined over 200 years ago when the prevailing ideas were that these spiders were relying on either aerodynamic or electrostatic forces in the environment to stay airborne. Even Charles Darwin shared his thoughts on the subject after witnessing scores of such spiders taking off from the Beagle, suggesting that thermal currents may have a role to play. Although the aerodynamic and thermal gradient theories have previously been investigated experimentally, their results do not appear to explain observations of the spiders ballooning in unfavourable weather conditions, suggesting that an important piece of this puzzle has been overlooked.
In a recently published study, Erica Morley and Daniel Robert from the University of Bristol, UK, set out to investigate the previously unexplored relationship between spider ballooning and naturally occurring electric fields (e-fields) known as the atmospheric potential gradient (APG). In order to recreate these e-fields in a controlled environment, the team constructed a chamber with electrically stimulated aluminium plates at the top and bottom to generate either neutral-, low- or high-strength e-fields. This chamber contained ballooning take-off sites for the spider that replicated natural ballooning launch pads such as tree branches and was constructed inside a Faraday cage to block any unwanted electrical interference. Once the tiny money spiders (members of the family Linyphidae) had been introduced to the chamber, the electrically charged plates were switched on to generate the e-fields and the spiders were filmed performing their amazing aerial acrobatics.
Through analysis of the video footage, the team found that ballooning was always initiated by two specific behaviours: first, the spider would stretch its abdomen upwards into a position fabulously referred to as ‘tiptoeing’, and second, the spider would drop a silk ‘dragline’ to keep them anchored before releasing the ballooning silk fibres and taking flight. Once the spiders were airborne, the researchers were able to demonstrate a strong relationship between e-fields and ballooning by repeatedly switching the e-field on and off, which reliably resulted in the spider rising and falling in the air. Interestingly, the spiders initiated significantly more of these tiptoe and dragline drop events when exposed to higher strength e-fields, suggesting that the spiders were somehow able to detect these atmospheric changes and alter their behaviour to capitalize on them.
Not satisfied with simply watching levitating spiders, the researchers then wished to determine how the spiders were detecting these e-field changes. By narrowing their focus to the hair-like ‘trichobothria’ on the spider's legs, known primarily for their detection of minute air vibrations, the researchers were able to examine the physical responses of the trichobothria to e-fields using laser Doppler vibrometry. These micro-measurements revealed that the hairs do indeed become stimulated and move in response to the activation of these fields and are quite likely acting as electro-mechanical receptors that can detect subtle changes in the APG. Finally, the authors discuss the ecological significance of these findings, emphasising the important role of spiders as natural pest controllers throughout the world and how this research may help us to better understand their dispersal mechanisms in the wild.
One last question posed by the researchers is, I think, a crucial one: once the spiders get up there, are they able to steer their balloons or navigate their flight paths at all? Perhaps it’s simply a case of holding on and crossing their trichobothria for good luck.
