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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?

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Related: physics.stackexchange.com/q/12953/2451 and links therein. –  Qmechanic Sep 19 '12 at 16:14
    
I love that Spiders, Flies, and Geckos all utilize different forces to accomplish the same task. –  aslum Sep 21 '12 at 15:30
    
@aslum Actually, that's not true. Spiders and geckos both use VdW forces. –  Glen Wheeler Sep 24 '12 at 18:15

3 Answers 3

up vote 31 down vote accepted

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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To add to John's answer, scientists have an enlarged image of Spider's legs (up to $5\mu m$) whose smallest hairs are named setules. This make the spiders, a level up than flies or geckos... Also, These attractive (adhesive) forces in those setules can make the spiders capable of sticking to an overhead wall, just using a single paw..!

It should be noted that even small amounts of water could wash off this capillary attraction and it would flush away these tiny organisms 'cause these forces are only negligible..!

Reference: http://whyfiles.org/shorties/152sticky_spider/

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Capillary attraction? Don't you mean capillary action (sometimes capillarity, capillary motion, or wicking): the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity? I guess water hogging the grooves would detach the hairs from the wall, but that would mean capillary action negates the stickyness of these tiny feet. –  Cees Timmerman Jan 22 at 15:24
    
@CeesTimmerman: I did mean "capillary action" back there :) –  Waffle's Crazy Peanut Jan 22 at 15:26

Geckos use van der Waals forces: http://www.pnas.org/content/99/19.cover-expansion

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