How can ants walk on walls and ceilings?
Dear Straight Dope:
Why can ants walk up walls and ceilings but when humans get on hands and knees they can't? Ants don't have suction cups on there feet. Does it have to do with gravitational force and their relatively small size?
You don't need suction cups in the usual sense. Ever see one of those plastic thingies you can slap on a window and they stick, even though they're dry and smooth? Well, lots of insects have little pads, called arolia, between the hooked claws on the tips of their legs. They can stick to smooth surfaces with these pads using the same principle as those pieces of plastic, which use the same basic principle as a suction cup--a vacuum.
When climbing on rough surfaces, such as a wall or ceiling, a vacuum can't be formed, so they use those tiny little claws. If you could see the surface of your walls and ceilings through a microscope, you'd see there are plenty of places for tiny things to put their hooks and climb away.
Ants can generate more than enough force to keep their tiny, air-filled bodies from falling, generally speaking. Sometimes they do fall, though; you just need to watch them often enough, and on the right kind of surface. Most insects have two sets of climbing tools on their feet, one for smooth surfaces and one for rough surfaces.
Some insects have the additional capacity of being able to secrete a thin film of oil on the pads, which gives them even greater sticking power, mostly because it gives an even better vacuum, and also has surface tension effects. Just try taking a square millimeter of thin plastic with a micro-drop of corn oil on it, and see if it won't stick to your ceiling or any other surface.