Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
Related: 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
up vote 38 down vote accepted

See 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.

share|cite|improve this answer

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..!


share|cite|improve this answer
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 '14 at 15:24
@CeesTimmerman: I did mean "capillary action" back there :) – Waffle's Crazy Peanut Jan 22 '14 at 15:26

Geckos use van der Waals forces:

share|cite|improve this answer

protected by Qmechanic May 2 at 22:35

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.