Have you ever wondered how a fly can walk upside-down on a ceiling without falling off? This question has puzzled biologists and physicists for years,and now a study by Langer and colleagues has brought us one step closer to understanding this remarkable feat. Geckos can cling to walls and ceilings using millions of tiny hairs on their toes that adhere to surfaces via molecular attractive forces called van der Waals interactions. The adhesive pads on fly feet are similarly endowed with tiny hairs (or`setae') that end in flat structures called terminal plates, but the mechanism of insect adhesion is not well-understood. There is some evidence that flies'feet secrete a sticky fluid that might mediate foot adhesion, so Langer and colleagues decided to investigate the adhesive properties of both terminal plates and the fluid secretion.
The team reasoned that if fluid secreted from the terminal plates of individual setae is responsible for adhesion, then the footprints left behind when a fly walks on glass should be stickier than the glass alone. Furthermore, if the stickiness is really caused by the terminal plates'secretion, the footprints' stickiness should be similar to the stickiness of the terminal plates. To measure these tiny adhesion forces, they used an atomic force microscope, which allowed them to measure attractive forces between an extremely sharp triangular probe and individual footprints as well as between the probe and the terminal plates of the flies' feet.
The sensitivity and resolution of the atomic force microscope is such that they could measure forces in the nanoNewton range and map the adhesive properties of the entire surface of a single terminal plate, which is only 2μm long and 1 μm wide. When they measured the adhesive forces between the microscope tip and micro-drops of fluid left by flies on glass, they found that these values were very similar to the forces from the centre of the terminal plate, suggesting that the terminal plates' secretion is responsible for the adhesive properties of the fly footprints. They also found that the adhesive forces between the microscope tip and the footprints left on the glass were considerably more than the attraction between the microscope tip and a clean glass surface. This suggests that the secretion creates capillary forces that probably assist the fly in its sticky endeavours. The team noticed that the adhesive forces between the microscope tip and the footprints decreased considerably over time, and corresponded to a decrease in the volume of the micro-drops due to evaporation. So as the fluid evaporates, its sticky benefits disappear.
If foot fluid secretion is important for adhesion, the team reasoned that it should be possible to make a fly's feet less sticky by simply washing off the secretion. Sure enough, they found that rinsing flies' feet with a buffer capable of dissolving the sticky footprints caused a dramatic ninefold decrease in terminal plate adhesion, suggesting that the sticky substance really does help the insects scale walls and ceilings.
Langer and colleagues conclude that flies don't fall off the ceiling because the terminal plates on their feet secrete tiny fluid droplets that create capillary forces and help the insects stick. We'll have to stay tuned for the answer to the next obvious question – how do they become unstuck when they want to fly or walk away?