Dear Carmen,
Right now, there’s a fly walking on the window in my office. I asked my friend Rich Zack how it does that. He’s an insect scientist at Washington State University.
It turns out my window isn’t as smooth as it looks. If we zoomed in on the glass, we’d see bumps and dips. It’s kind of like a rock face or a climbing wall.
“Surfaces look pretty smooth,” Zack said. “But if you get really, really close, you see there are all kinds of little indentations and crevasses. Even a pane of glass is not perfectly smooth. It’s got little nicks and bumps that insects can grab onto.”
There are also electric forces that help stick together the atoms in an insect and the atoms in the surface—like van der Waals forces. Those act like a weak magnet. They pull together any atoms that are near each other. Big animals like us are too heavy to use those forces to climb stuff, but insects are usually small and light.
Most importantly, insects have legs and feet adapted to do whatever that insect usually does—like cling to stuff.
Insect legs are a lot like human legs. Some of the leg parts even have the same names—even though insects don’t have bones. Thighs are femurs. Shins are tibias. At the end of the legs, the feet are tarsi.
“There are specializations when we get down to those tarsi,” Zack said. “Many insects that hang onto things have one of two adaptations or maybe both. They may have little suction cup pads on the bottoms of their feet. We call those pulvilli. Or they may have little hook-like claws on the ends of their feet.”
House flies have both claws and suction cup pads. They use their claws to grab onto things. They push their hairy pads against the surface. The hairs help grip the bumps and dips—with the assistance of the van der Waals forces. Some of the hairs ooze an adhesive to help the foot stick. Then the fly peels up its foot to take a step.
House flies are good examples of insects that use tarsal claws and pulvilli to hang on. But there are all kinds of specialized insect legs and feet out there.
Some insects have sensors on their front feet. That’s how flies and butterflies taste what they touch. Sometimes insects have long, slender cursorial legs. Those help cockroaches and other insects run very fast. Water insects may have paddle-like natatorial legs. That’s how they swim. Big jumpers have strong saltatorial legs. Those muscular back legs propel grasshoppers and fleas into the air when they leap. Some predators have grasping, arm-like front legs. Praying mantises use those raptorial legs to nab prey. Sometimes insects have beefy, shovel-like front legs. Mole crickets use fossorial legs like that to dig tunnels.
It’s easy to tar-see how different insects use their legs and feet to hold onto their very best lives.
Sincerely,
Dr. Universe