Science Figures Interpreted and Analyzed by RealClearScience
Spiders, as you very well know, are creepy, crawly animals with eight long legs. At the end of these spindly legs are claws, as well as hairy attachment pads -- claw tufts. ("The better to grasp you with, my dear.") The tufts consist of hundreds of tiny bristles, called setae, each with hundreds of thousands of microscopic hairs. It's these miniscule hairs that grant the spider its notorious grip.
But how do these tufts function precisely? They're not thought to be particularly wet or sticky, so they must be working in some other, dry manner.
Previous research determined van der Waals forces to be at work. With all the tiny hairs touching a surface, stickiness on the molecular level is created. This is also how geckos can cling to a glass surface with one toe.
To discern more details on spider attachment, researchers from Kiel University in Germany closely analyzed the friction of spiders' claw tufts on smooth glass. They found it likely that "friction forces play the major role in the attachment of spiders," and these forces depend on the specific arrangement of the claw tufts at the tips of each leg.
The highest friction in each claw tuft is directed away from the center of mass, the researchers said, and due to the radial arrangement of legs, there is always a frictional counterpart.
"Thus, the whole animal works like a holdfast," they concluded.
It appears that a spider's ability to cling to walls or ceilings strongly depends on the friction forces generated by the interaction of the animal's legs on the surface. It is not the contact area alone that boosts attachment forces, like in geckos. In the spider they tested -- the American wandering spider -- the researchers observed that at least two leg pairs were necessary to affix the arachnid to a glass surface.
Coupled with the observation that Spiderman only seems to rely on his tiny hairs for climbing, this would seem to indicate that he is really more of a "Geckoman."
Source: Jonas O. Wolff & Stanislav N. Gorb (2013) Radial arrangement of Janus-like setae permits friction control in spiders. Scientific Reports 3, Article number: 1101 doi:10.1038/srep01101