How many times have you heard the expression, 鈥渋f only I could be a fly on that wall?鈥� But how do flies walk up walls and across ceilings? At the Marine Science Laboratory in North Wales scientists have spent a lot of time watching flies walk up a wall. No, the scientists are not bored. Quite the opposite. They鈥檙e very interested in finding out how the flies appear to defy gravity and stick to the wall. Why? Because any insight into zygology can have very important practical applications. And what is zygology? It is the science of joining things together. Rivets, nails, screws, welds and threads of course play a large part, but where would we be without adhesives? Planes would fall from the sky, furniture would disintegrate, books and shoes would fall apart and we could forget about capping our teeth. Just imagine life without wallpaper, stamps, scotch tape or Post-It notes!
Any investigation of stickiness has to begin by examining what goes on at the microscopic level. It has long been known that flies鈥� legs have two little claws at the tip which can be used to grip irregular surfaces. And also that their foot pads can deform to create little suction cups. But this still did not seem to explain how do they walk up and down those smooth surfaces like glass with such ease? Was there something else involved, the scientists wondered? Could there be some secretion that played a role? They decided to see if flies walking on glass left a residue. They did! Tiny footsteps appeared when a glass slide on which flies had walked was treated with a stain that detects fatty materials. Could this fat act as a glue?
An ingenious experiment to test this hypothesis was quickly concocted. A thin tether was attached to the back of a fly with superglue. (You see, even fundamental research relies on glue.) Then a gauge was used to measure the force needed to lift the fly off the glass. Next, the fly was made to walk over a filter paper soaked in hexane, a solvent that dissolves fatty materials. Once more the fly was placed on a glass slide and hoisted into the air. This time the force needed was one-tenth of that needed before the hexane bath! It seems that flies really do glue themselves to the wall! The fatty substance they secrete has an attraction to surfaces as well as to the feet. It really is a glue! Undoubtedly some inventive glue manufacturers are talking about capitalizing on this discovery. I wouldn鈥檛 mind being a fly on their wall.
