Popular curiosity has always been fascinated by creatures that seem to defy gravity by walking up perpendicular walls or even upside down across ceilings. In recent years, we've finally deciphered how some of the big creatures, such as the lizards that we know as geckos, do it. Now, we've managed to figure it out for one of the wee animals, the common housefly.
We've known for a while that the pads on the ends of fly feet leave
little spots of moisture as the critter walks. Finally, we've discovered more of
what's going on. The attachment pads at foot's end are covered by tiny bristles
called setae. Each bristle ends in a flat plate shaped like a kitchen spatula. A sticky
fluid is secreted from the central portion of the attachment pad and directed to each
bristle's terminal plate through a small channel. The channel acts like a capillary
tube, holding onto the liquid that, in turn, sticks to whatever the fly's walking
on. Now you know how the "fly on the wall" does it! 
Listen to the Audio (mp3 format) as recorded by KTEP, National Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.