http://www.bristol.ac.uk/news/2018/july/spiders-electric-fields-.html

Scientists have attributed the flying behaviour of these wingless
arthropods to ‘ballooning’, where spiders can be carried thousands of miles by
releasing trails of silk that propel them up and out on the wind.

However, the fact that ballooning has been observed when there is no wind to
speak of, when skies are overcast and even in rainy conditions, begs the
question – how do spiders take off with low levels of aerodynamic drag?

“Many spiders balloon using multiple strands of silk that splay out in a
fan-like shape, which suggests that there must be a repelling electrostatic
force involved,” explains lead researcher Dr Erica Morley, an expert in sensory
biophysics.

“Current theories fail to predict patterns in spider ballooning using wind alone
as the driver. 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? We wanted to
find out whether there were other external forces as well as aerodynamic drag
that could trigger ballooning and what sensory system they might use to detect
this stimulus.”

The solution to the mystery could lie in the Atmospheric Potential Gradient (APG),
a global electric circuit that is always present in the atmosphere. APGs and the
electric fields (e-fields) surrounding all matter can be detected by insects.
For example, bumblebees can detect e-fields arising between themselves and
flowers, and honeybees can use their charge to communicate with the hive.

Spider silk has long been known as an effective electric insulator, but until
now, it wasn’t known that spiders could detect and respond to e-fields in a
similar way to bees.

Thanks to Miles Fagerlie.