Of all the animal powers you could have, being able to walk on walls is a pretty basic one. Sidebar, if you want to walk on walls you might as well fly with the tacked on sidegrade of hovering. Anyway sticking on walls and stuff is pretty effortless when a gecko or that housefly does it. But how do they get it done? Side question, what’s the biggest animal that can stick to a wall?
Further Reading: 10 Weird Animals That Are Basically X-Men
Some Animals Are Hairy
This applies mostly to bugs. If you’ve ever had a big bug zipping around or crawling on your ceiling (preferably not above your bed) you might have gotten a decent look at their legs. They’re a little spiky. Arthropod legs (this includes insects and spiders) are covered in little hairs for lots of reasons. Many are sensory organs inside their leg joints. These sensory hairs are called hair plates and they’re mostly used to tell the little guy when to move.
Sensing doesn’t have much to do with sticking onto walls, though. You’ve probably picked up that the bugs sticking onto your wall aren’t simply sticky. Some animals are sticky, but they’re not particularly interesting. Obviously the sticky thing sticks to walls.
Anyway, all surfaces have imperfections. They can’t always be perceived but even microscopic bumps are enough for hairy bug legs to latch on and get a grip. This may be accompanied by pulvilli or arolia, which basically means “the foot actually is a little sticky–” most insects use some kind of adhesive with their anatomy. The hairs on their feet have some oil on them. There are also tarsal claws, which are like the hairs but bigger.
If this sounds automatic, bugs seem to control whatever sticking-mechanism sits at the end of their leg. It’s not like they all have Velcro on their feet (which it may sound like at first). Long short, we’ve found that houseflies have enough control over their legs to splay out the hairs on their feet to attach and detach from surfaces.
So when you think about it, bugs stick to walls the same way humans stick to rock climbing walls.
No Adhesive Necessary
You’re probably not surprised that animals use sticky substances on their feet to stick to walls. Frogs use a water-based mucus to stick to stuff, and the feathertail glider uses sweat. By the way, the feathertail glider is considered the smallest gliding mammal (12 grams). So it’s no surprise that water-based adhesion is largely considered the most effective means of walking on walls.
But there are animals capable of sticking to walls without any kind of oil or mucus. In fact, the biggest animal that sticks to walls doesn’t use any kind of adhesive substance. We’ll start with a weird one: the disk-winged bat. Its thumbs and feet literally have suction cups on them. There’s also the sucker-footed bat, but this one uses its own sweat, and therefore doesn’t count for dry adhesion.
Spiders have lots of hairs on their legs, but they don’t mix them with any kind of adhesive. These hair tufts are extremely dense, and these individual hairs have their own smaller hairs on them. The width of these smaller hairs is measured in nanometers. That’s small enough for the spider to exploit Van der Waals forces–which is the attraction or repulsion between individual atoms.
The Biggest Sticky Boy
The most important things to an animal sticking to walls are surface area and volume. Specifically, the ratio between the two. To stick to walls with foot pads you’re going to need a lot of surface area in relation to your volume. That’s why spiders are covered in thin hairs–it’s also the same reason your brain is wrinkly.
Through basic geometry, volume increases a lot faster than surface area. So the bigger you are, the more space you take up in relation to your own surface area. So a house spider is mostly surface area instead of volume, but you are mostly volume. A 10 gram spider, then, only dedicates about 1% of its surface area to its foot pads. A 0.00001 gram dust mite only uses 0.02% of its surface area for sticking.
That brings us to what is largely considered the upper limit for animals that can stick to walls. It’s the gecko–but it’s not going to be selling any insurance to you. They hover around 100 grams and use a method of adhesion remarkably similar to spiders. Their feet are covered in tiny ridges that serve the same purpose as the hairs on spider feet. Geckos dedicate about 4% of their surface area to their foot pads, which is 200 times more than the mite. On top of their feet, geckos need to exploit static electricity in the same way your hair sticks to a balloon when you rub one on your head.
If you were wondering what it would take for you to stick to a wall, you’d need to be 40% sticky foot pad. Which would be about the entire front of your body.
Speaking of spiders, see fi you can solve a spider-themed word scramble here, or something.