Have you ever noticed how insects, such as flies, seem to clamber over walls and other vertical surfaces, no smatter how smooth they seem to us?
Whether its smoothened indoor wall surfaces or glass panes – they don’t seem to have problems traipsing up and down.
So what gives?
The Common Notion
If you pose this question to a random person, the chances are that they mention suction pads or adhesives at the bottom of the feet.
The truth is a bit more complex. Adhesion is definitely a factor, but there’s more.
It’s a Complex Ecosystem Down There!
As a matter of fact, there are a number of appendages and surfaces at the bottom of an insect’s foot that allows them to grasp and cling to surfaces. In no particular order, they are:
- Pulvilli (also called Arolia)
- Setae, which work with the Pulvilli
- Tiny, stiff hairs
- Tarsal claws – these are unique, since they actually help the insect to tear its foot away from the wall as it moves forward
- Suction Cups – yes, the arolia can also function as such
We will discuss each of the above features, along with some other fundamental questions (e.g., do insects actually know that they are defying gravity by walking on walls?).
All the Enablers Work Together
The different appendages we mentioned work in tandem, every little system interacting with the others to produce gravity defying feats like walking up walls.
Before we start describing the functions, let’s remember one thing. No matter how smooth they look to us, regular walls or even glass surfaces are not completely flat.
While we are too big and clumsy to perceive it, they actually have rough spots – mountains and valleys – that tiny hairs or foot parts can actually grasp. This is critical to insect locomotion.
Pulvilli or Arolia
Insects, such as flies, have dual footpads (called pulvilli or ariola) covered with a perpetually regenerating adhesive which is manufactured within the foot itself.
The natural glues, made from sugars and oils, cover the pulvilli and is strong enough to be able to support the body weight of the insect (see next section) as it traverses its path.
Interestingly, each insect may in fact produce just enough glue to support it’s own body mass.
Setae
The arolia would not be able to do their job if it were not for the setae – tiny, hollowed out tubes that carry the natural glue from where its generated down to the pads of the foot.
These little tubes in fact open out at the bottom. This is the reason that flies leave tiny footprints (aka oily residues) behind them as they walk!
Tiny, Stiff Hairs
While the adhesive material is a major help, the true grip on the surface is delivered through microscopic, stiff, bristle-like hairs that cover the bottom of the insects’ feet.
These bristles are tiny enough to find the cracks, crevices and smaller irregularities on the surface that the insects walk on, but stiff enough to actually grip to tiny protrusions on the surface.
Without getting carried away with the analogy, think about our hands grabbing at ledges on a rock surface as we climb.
The hairs are strong enough to support their bodies, same as our hand strength can be adequate to balance ourselves for a bit.
Tarsal Claws
Another crucial part of the anatomy of insects are bigger, and strong, tarsal claws. Flies, for example, have two at the ends of their feet. These serve a slightly different function from the arolia, setae and microscopic hairs.
The tarsal claws are used to “dig” into the surface and used as a pivot point to free up and lift the feet to allow the insect to step forward.
In a certain sense, the insect would be glued to the surface (pun intended) without the points of the tarsal claws to rescue them.
They do often peel or twist their feet to get unstuck, but the tarsal claws are a surefire way to succeed – and then proceed on their way.
What About Suction Cups?
So, what about the “suction” you hear about?
It turns out that the arolia and setae can in fact act like suction cups. In fact, for certain species such as hunting spiders, setae can help them function in wet (adhesive) and dry (suction) mode. More about this later.
For an insect, each pulvillus has thousands of microscopic hairs, as shown below:
(source: https://www.sciencesource.com/archive/Image/Stable-Fly-foot–SEM–SS2441094.html)
Each tiny hair end has an expanded, spatulate tip that is pressed against the surface.
When compressed against a surface, intermolecular forces (called Van der Waals forces after the Dutch physicist Johannes Diderik van der Waals) come into action.
While each individual force is weak, they can collectively create enough suction to hold the insect’s body in place for a bit even without the adhesive forces.
It’s the same principle as that behind why Saran Wrap works for us in the kitchen.
Having said that, the natural adhesive and miniscule body weight makes adhesion and grip more important than suction in supporting insects as they scale walls or walk upside down on ceilings.
One More Crucial Factor – Body Weight and Size
One thing to realize about insects, or for that matter lizards or spiders, climbing vertical surfaces or hanging upside down is that they carry substantially less body mass than larger animals.
Also, their foot to body ratio is much lower – the feet pads, setae etc. have much less to do to prevent the insect from falling down.
As a contrast, consider a human being with adhesive gloves, sticky boots or even grapplers, scaling a vertical surface.
Our body mass is so large and the point through which to “stick” to the surface is so concentrated (compared to our overall size), that an enormous effort is required to keep us from falling off.
An ant, on the other hand, has extremely low body mass (it’s mostly air inside!) and its entire body is not big at all, which makes it easier for them to scurry up a wall.
How Do Flies Walk on Ceilings?
Flies walk on ceilings using the same principles as climbing walls, using a mixture of adhesion and suction, along with the usual tactics to either peel their feet off or use their tarsal claws to extract their feet as they move.
The other thing they will also do, of course, is to hang upside down.
Do Insects Realize They Are Defying Gravity? How Do They Right Themselves?
We don’t know, or more to the point can’t tell, if insects do realize that they are defying gravity. But its certainly the case that they will not only walk up and down walls, or upside down on ceilings, they will even rest with their legs pointing up and their wings below them while on the ceiling.
What’s surprising is how fast they right themselves when they fly off from a stationary position. It’s instantaneous with no glitch whatsoever.
A Groundbreaking Study from Leading French Scientists
Scientists from the French National Center for Scientific Research (CNRS) and The Institute of Movement Science (ISM) at Aix-Marseille Université used a high-speed camera to study how the insect flips itself around when taking off from a resting position.
In particular, they were looking for contrasts with cats, who can land on their feet when falling from a height by first reorienting themselves by turning their heads.
To their surprise, the scientists discovered that insects reorient themselves by first turning their bodies – it happens within six wing beats at a speed of 10,000°/s (i.e., approximately 30 revolutions per second).
The whole movement takes approximately 0.05 s with the head turning 0.016 s later than the body.
The researchers reported that “… during take-off flies flip their bodies before their heads due to an inherent stabilization reflex. Small stabilizers near the wings function as a type of gyroscope.”
There were two more interesting observations aligned to this finding:
First, humans have a similar reflex which kicks in when they continue to stare at something despite turning their bodies around.
Second, further modelling and simulations suggested that, during reorientation, the insect stabilizes its visual system before resuming normal flight.
Some of the study details can be found here:
Other Creatures Who Also Climb Walls
There are a number of other creatures that use techniques and appendages similar to insects to scale walls, walk upside down on ceilings or simply hang out.
Among them are lizards, geckos and certain species of spiders – especially the hunting spiders Lycosidae and Salticidae.
The spiders, for example, have scopulae (or scopula pads), which are dense tufts of hair at the end of their legs. These, like insects, are covered in setae that end in setules.
The end result is a large contact area which whatever surface the spider is scaling. This creates suction through Van der Waal’s force, plus they grip using tarsal claws.
The final piece is the adhesive solutions that are generated.
The combination of electrostatic, capillary, viscous, Van der Waals and adhesive forces are sufficient to keep the tiny creatures balanced as they travel on vertical surfaces or go upside down.
The Final Word: It’s All About Nature and Evolution
Ultimately, it all boils down to nature and evolution. Insects such as flies or ants, and others such as geckos or spiders, have evolved in a way that they can scale vertically or hang upside down, then peel off, fly or jump as necessary. This is necessary for the survival of the species.
Many larger animals, and certainly humans, not only possess much larger and unwieldy bodies that are difficult to drag up and down such surfaces, we also do not have the evolutionary need to do so.
We use our larger bodies and brains in a different fashion.
So, while making like Spiderman is cool, it rarely becomes necessary for us. We have devised tools, implements and techniques that allow us to operate above ground in a different fashion.
Alright guys, that’s it for this article, if you want to learn more than check out some of our other hand selected articles that you might find interesting!
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