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?