# Why doesn't a fly fall off the wall?

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 (i.e. perpendicular to the wall), and then I remembered spiders, geckos etc, they like to walk around on the ceiling.

How is it possible? What kind of forces are involved? Would these creatures still be able to do it on a hypothetical surface which was perfectly flat?

• Related: physics.stackexchange.com/q/12953/2451 and links therein. Sep 19, 2012 at 16:14
• I love that Spiders, Flies, and Geckos all utilize different forces to accomplish the same task. Sep 21, 2012 at 15:30
• @aslum Actually, that's not true. Spiders and geckos both use VdW forces. Sep 24, 2012 at 18:15

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..!