Pretty simple question, but not an obvious answer at least not to me. I mean you

Question

Pretty simple question, but not an obvious answer at least not to me. I mean you can't just place a dead fly on the wall and expect it to stay there, he will fall off due to gravity. At first I thought it maybe friction, but that would require a normal force (perpendicular to the wall, and then I remembered spiders, geckos etc. which can even walk around on the ceiling. How is it possible? What kind of forces are involved? Would they still be able to do it on a hypothetical wall/roof which was perfectly flat?

Explanation / Answer

See http://www.sciencephoto.com/media/100845/enlarge for an absolutely awesome picture of a fly's foot. It has two claws that can grip any irregularities. For smooth surfaces like glass it has a pad covered in tiny hairs, and each hair is coated in tiny oil drops. The capillary attraction of the oil drops holds the tiny hairs, and therefore the fly, to the surface.

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