How come flies don’t fall off the ceiling?
How come flies don’t fall off the ceiling? asks reader J. Jones.
If we could look closely at our ceilings, we’d see the crisscrossing paths of thousands of tiny footprints, left by flies, ladybugs, and other insects (as well as by spiders). In fact, the problem for flies and other bugs may not be holding onto the ceiling, but breaking free from it. Turns out, a fly strolling across a ceiling is a bit like a person walking across a field of wet mud.
How do flies walk upside down, apparently effortlessly? Being tiny certainly helps. Very-low-mass animals like wall-walking insects and spiders feel less of a pull from gravity. So it’s easier for a fly than a pig to stick to the ceiling (even Spiderpig needed Homer’s help).
On a rough surface, an insect can use its claws, rappelling up or across like a climber on a rock wall. But many insects and spiders also rely on special leg or foot pads, often covered with bristly hairs, when they need to climb up surfaces. Scientists once thought that the rough, bumpy hairs allowed flies to cling to tiny nooks and crannies on even smooth-looking surfaces, including ceilings. A substance secreted by the hairs helped, adding a bit of adhesion.
But in 2006, scientists at the Max Planck Institute in Germany discovered that the substance secreted by the hairs on a fly’s feet is a sticky glue, tailor-made for striding confidently across the ceiling, upside-down.
The glue-y stuff oozing out of a fly’s footpad hairs is a mixture of oils and sugars. Researchers say that all insects may secrete the glue, since all 300 wall-climbing insects studied at the Institute left a trail of tiny, sticky footprints on the wall.
The adhesive is strong enough to keep each foot planted on the ceiling, fly standing still. Walking, however, isn’t trouble-free. Although the journey across the top of a room may look effortless from our perspective, it’s a struggle for the fly. The researchers found that flies use at least four different techniques to get a foot unstuck and moving again.
Watching slow-motion tapes of each foot detachment, scientists found that a fly sometimes pushed a foot away from himself, popping the footpad off the surface like a freed suction cup. Flies also twisted their footpads until they loosened from the wall, or jerked them quickly like a yanked-off band-aid. Flies also used the handy, built-in claws on their feet to pull a footpad off the ceiling, like a person tugging off a boot.
According to the scientists, the techniques that involved peeling the pad off the ceiling or wall work best, because they require less energy.
Using four of six legs as they crawled across the ceiling also helped the flies make their gravity-defying journeys. (On the ground, scientists say, flies often use just three legs at a time to move around: two legs on one side and the middle leg on the other, forming a stable triangle, alternating sides with each step.)
