How come geckos stick to the ceiling but centipedes don't?
Dear Straight Dope:
Here in Hawai'i we view geckos as guardian angels and locals allow them to live around the house and eat small insects and make marvelous loud calls to one another from time to time. Now it seems they've been attending post-graduate physics seminars all these years: they walk on ceilings and up and down glass windows through advanced adhering principles at the molecular level! Meanwhile, centipedes — who are viewed here as representatives of Satan and whose venom packs the wallop of ten wasp stings — seem to have been asleep in class: they fall off walls and the ceiling all the time in the houses of those poor accursed souls who are infested with them.
So, what's the scoop? Are geckos really operating at the molecular level (molecules on hairs on their footsies adhering to the molecules of the ceiling), while centipedes are using some failed outmoded mechanical technique (like tiny hooks on their paws)?
SDStaff Doug replies:
You've hit the nail on the proverbial head. Geckos do indeed stick to things because of the close approximation of their toe filaments to the substrate. Think of it as a complete absence of air in between, allowing a combination of suction and Van der Waals forces (that is, direct molecular contact) to create adhesion. Centipedes in contrast only have tiny hooks at the ends of their feet. You, too, have fingers and toes that work like those in geckos, except that your body weight is orders of magnitude more than the adhesive forces you can generate (unless you're Spider-Man). But just press your thumb onto a microscope cover slip (a very thin piece of glass) some time, and you'll see that it sticks to your fingertip. Nothing special, really.