Spider Man, insects and geckos all share the same effortless superpower: no matter how smooth a surface, they can hang on to it upside down. We have long been mesmerised by this remarkable ability and more recently engineers have been keen to replicate these animals' exotic attachment surfaces to produce artificial reusable adhesives that leave behind no residue. But there is one snag: ‘A dead gecko doesn't stick anymore, so we think it [adhesion] is an active process, not a passive one like a suction cup’, explains Jonas Wolff from the University of Kiel, Germany. According to Wolff, geckos and insects activate their adhesive surfaces by pulling their legs inwards to align the microscopic structures that attach them to smooth surfaces. Which made Ellen Wohlfart, Eduard Arzt and Stanislav Gorb wonder whether spiders use the same active process to secure themselves to walls. Intrigued by the possibility, the team tested how well the American wandering spider (Cupiennius salei) clings on to smooth surfaces after their adhesive pads have been inactivated (p. ).
But spider wrangling is not without its risks. ‘Normally they are peaceful, but if you try to grab them because they have escaped, they can bite you. The poison is not dangerous for humans, but it is not very nice’, recalls Wolff. So, Wohlfart overcame the danger by briefly anaesthetising the bulky arachnids with a puff of carbon dioxide before attaching a human hair tether to it. Having allowed the spider to recover, Wohlfart gently lowered it onto a glass plate and then attached the tether to a force sensor, which was attached to a motor that gently pulled the spider in an attempt to tug it free in order to measure the adhesion force. Having measured the full attachment force, Wohlfart gently applied a dab of wax to the spider's front pair of feet to see how the loss of two sticky pads affected the animal's adhesion force. She then systematically disabled the second and third pairs of feet, eventually leaving the spider with only its hind feet to cling on with.
Repeating the experiments with another spider, only this time disabling the rear feet first, Wohlfart discovered that the spiders cling on with an impressive 97 mN of force when all eight legs were in action: three times more than the average weight of this species. But when Wohlfart measured the attachment force of the spider after its two rear-most legs had been disabled, the attachment force was reduced significantly to just 26.2 mN (27% of the original force), which is 42.7 mN less than expected if the spider was clinging on passively like a piece of sticky tape. And when both rear pairs of legs were out of action, the spider was left with just 9% of its original sticking power, certainly not enough to support its body weight.
The spiders were actively attaching themselves to smooth surfaces by pulling against the diagonally opposite leg to produce the shear force that aligns the sticky foot hairs that allow spiders to hang on. Wolff explains that by inactivating the hind-most pair, the spiders had effectively lost the attachment power of two pairs of legs. He also adds that the front two pairs of legs contribute less to adhesion than the rear, because spiders use their fore legs to manipulate prey, relying more on their hind legs for secure attachment.
