I’m sure we’ve all stopped to ponder this at least once before. Do bugs sustain injuries falling from trees and such? Will a spider hurt itself jumping from a rooftop? If I were to drop an ant from a plane, is it likely to survive?
And for that matter, how is it that these little monsters can walk across walls and ceilings with such ease? Why can’t you and I do that?
It turns out that size is a simple but unignorable property that governs exactly how living things may go about their survival; injury from falling is one such effect.
Firstly, or in a pinch, here’s a high-level explanation:
Do insects take fall damage? Not really: insects are so small that their weight is negligible in comparison to their air resistance. So, while falling, they never pick up enough speed to do themselves harm upon landing.
Now I expect you might not be completely satisfied with the chain of logic there. How can smaller creatures have so much air resistance? Why does that limit their speed?
And in case you’re wondering… flying insects don’t count!
Size and the Square–Cube Law
You’ll remember from that traumatic experience commonly referred to as your “maths education” that the surface area of an object is measured in square units and its volume in cubic units. So when that object (say, an animal) is ten times the size, its surface area increases by a factor of only 100, but its volume by a factor of 1000. This means that as an animal gets bigger, its volume increases much more quickly than its surface area.
This is the reason why particularly large animals like elephants have so much trouble keeping themselves cool, and why Godzilla’s proportionally puny legs would actually collapse under his sheer weight.
But what does any of this have to do with insects and their air resistance (AKA drag)?
Air Resistance and Terminal Velocity
The greater an object’s surface area, the greater the force of drag that slows its fall. With this fact and the square-cube law added to our mathematical repertoire, we find that the minuscule size of the insect is precisely why it has such a great deal of drag.
As an object falls, it picks up more speed. The faster it’s going, the greater the force of drag that slows it down. These two facts combine so that, while falling, an object eventually reaches its terminal velocity—a sort-of ‘top speed’ at which its weight and drag cancel exactly.
Insects, being so light and having so much area for their volume, have a very low terminal velocity, so they never hit the ground very hard at all. In his influential essay On Being The Right Size, J. B. S. Haldane says this:
An insect, therefore, is not afraid of gravity; it can fall without danger, and can cling to the ceiling with remarkably little trouble. It can go in for elegant and fantastic forms of support like that of the daddy-longlegs.
How Big Do They Have to be to sustain Fall Damage?
Size is a big regulator of every living thing. There are actually eight orders of magnitude for living things, each of which has different physical laws for different sized animals. The seven orders are…
- Bacteria
- Mites
- Ants
- Mice
- Dogs
- Humans
- Elephants
- Blue Whale
Every size has its own unique universe and rules. But regarding falling, an animal would need to have a volume that was around the magnitude of a mouse and higher in order to sustain life-threatening damage. Generally, then, a magnitude of 4 or higher and it should start to be concerned as the ground rushes toward it!
What Do Insects Have to Worry About?
Okay, so insects are at no risk of harm by falling, but what troubles do insects face that we larger species don’t?
Aside from the obvious risks—natural predators, starvation, being swatted or crushed by giants—you may be surprised to learn that many insects have to be incredibly careful around water.
Have you ever found a fly dead in a drink you left on the table? For a creature of such a small size, getting wet is a matter of life and death; if its body is immersed, even slightly, in the surface of a body of water, it’s likely to drown before it can escape.
What’s the explanation for this? Typically you don’t think of water as being ‘sticky’, but it has a very interesting property of surface tension. Surface tension is what causes water to stick to things, and for (sufficiently small) things to stick to water. At the insect scale, water is like a gloopy sort-of glue—a quicksand-like threat that quickly surrounds and traps them. Their minuscule digits are far too weak for them to swim to the surface, so they are very likely to become stuck and drown in the liquid. Even if they did reach the surface, the surface tension of the water would prevent them from leaping out.
Nature, however, has evolved certain protection techniques to get around this. For example, some insects – like ladybugs, have wings made of the same material as our fingernails, and therefore water will largely glide off them. Other insects have a series of short hairs around their bodies that trap air, which enables them to reak the surface barrier.
Conclusion
It’s a hard life being a bug! but at least their one saving grace is that they can leap from the tallest buildings and not see their short lives flash before them. Indeed, you can throw those spiders out the window without the slightest care of hurting it now! …Feel better?
Here’s an interesting video which helps to explain this further and also adds to it.
