I came across this little fella (let’s call him Butter-sticky-feets because it’s a Pelopidas mathias oberthueriI, a species of Skipper butterflies, and he can stick to vertical glass surfaces) late one night while waiting for the train. I was surprised Butter-sticky-feets obliged when the phone camera was pointed at him for countless close-up shots – butterflies usually flutter away the moment you cross their physical boundary.
The train arrived and it was time to bid Butter-sticky-feets farewell, “Bye Butter-sticky-feets, you may not be the prettiest but you’re definitely the most qualified butterfly model”.
I looked out one last time and saw Butter-sticky-feets fly onto the outer glass window. I thought it would fly away once the train started – Japanese trains pick up quite a lot of speed so I doubted Butter-sticky-feets would be able to withstand all that drag.
Miraculously, it stayed on and even found a way to minimise the wind resistance. It first flew onto the middle section of the glass and gradually moved towards the corner ledge where it would be exposed to relatively less drag.
From left to right: (1) indicates Butter-sticky-feets’ position when it first flew onto the window; (2) indicates Butter-sticky-feets’ position on the window after one stop.
That’s not what surprised me the most though – I never imagined that their grip was so strong that they can withstand wind resistance while on a speeding train. How did Butter-sticky-feets manage this? Was it some sort of natural adhesive on its’ feet or super strong suction cups?
Turns out it’s neither.
Skipper butterflies generally rely on tiny claws (tarsal claws, SEM of a moth’s foot) on their feet to grip onto pores of seemingly smooth surfaces – although glass appears to be smooth to the human eye, its surface roughness becomes apparent at the microscopic level.
Other insects have a few additional trekking tools under their belts (SEM of a fly’s foot): aroliar or pulvilli (a natural adhesive footpad that is covered with a sticky substance made from sugars and oils), setae (the tiny hairs on the footpads from which the adhesive substances exude) and tiny bristle-like hairs that act in a similar way to the tiny claws of the butterflies.
Perhaps even more interesting are spiders and geckos, which rely on opposing forces, called Van der Waal forces, between its setae (in the millions) and smooth surfaces for dry adhesion. This phenomenon has even inspired scientists to create an adhesive that withstands extreme temperatures.
Wonder if our trekking boots can come equipped with all these funky tools one day.