Why Size Matters: Falling from the Sky#

Let’s kick things off with a wild thought. Imagine chucking a mouse, a dog, and an elephant off a skyscraper and onto something soft, like a big stack of mattresses.

• The mouse lands, is a bit stunned for a second, then just shakes it off and scurries away, probably pretty annoyed. It’s a rude thing to do, after all.
• The dog? Well, it breaks all its bones and dies. Not very dramatic.
• The elephant? It just explodes into a messy red puddle of bones and insides. No chance for it to be annoyed.

So, why did the mouse survive, but the dog and elephant didn’t?

Size: The Unseen Boss of Life#

The big reason is size. Seriously, size is probably the most overlooked thing that controls living stuff. It pretty much decides everything about how we’re built, how we see and feel the world, how we live, and how we die.

Why? Because the rules of physics act differently depending on an animal’s size.

Life spans a crazy huge range – seven orders of magnitude! We’re talking everything from tiny, invisible bacteria all the way up through mites, ants, mice, dogs, humans, elephants, and finally, giant blue whales. Each size lives in its own special universe, right next to the others, with its own set of rules, good points, and bad points. We’ll check out these different worlds.

Back to Falling: How Size Changes Everything#

Let’s get back to our first question: Why did that mouse make it after falling? It’s all about scaling – how changing size changes everything. This is a principle we’ll see pop up again and again.

See, very small things are practically immune to falling from high up. The smaller you are, the less gravity’s pull bothers you.

Think about a made-up, perfectly round animal, say, the size of a marble. It has three main features:

• Its length (how long it is).
• Its surface area (the skin covering it).
• Its volume (all the stuff inside – organs, muscles, hopes, dreams).

Now, if we made that little marble-sized animal ten times longer, making it the size of a basketball, its other features don’t just grow ten times bigger:

• Its skin (surface area) would grow 100 times.
• Its insides (volume) would grow a whopping 1000 times.

The volume is what determines the weight, or more accurately, the mass, of the animal.

• The more mass you have, the higher your kinetic energy right before you smack the ground, and the harder the impact shock.
• The more surface area you have compared to your volume or mass, the more the impact gets spread out and softened. Plus, more surface area means more air resistance to slow you down as you fall.

An elephant is so huge it has super little surface area compared to its volume. So, a ton of kinetic energy hits a small spot on impact, and the air barely slows it down. That’s why it gets totally wrecked in an impressive explosion of goo when it lands.

On the flip side, insects have a massive amount of surface area for their tiny mass. You could literally toss an ant out of an airplane, and it wouldn’t get seriously hurt.

The Small World’s Big Problems: Surface Tension#

While falling isn’t a big deal in the small world, there are other forces that are harmless to us but super dangerous for tiny creatures. One of these is surface tension, which can turn water into a potential killer for insects.

How does it work? Water loves to stick to itself. Its molecules pull towards each other because of something called cohesion. This creates a tension on the water’s surface, kind of like an invisible skin. For us, this skin is so weak we usually don’t even notice it.

If you get wet, maybe about 800 grams (less than 2 pounds) of water sticks to you. That’s only about 1% of your body weight. A wet mouse gets about 3 grams of water sticking to it, which is more than 10% of its body weight! Imagine having eight full water bottles stuck to you after a shower – that’s closer to the mouse’s situation!

But for an insect, the force of water surface tension is so strong, getting wet is literally life or death. If you shrank yourself down to ant size and touched water, it would feel like reaching into glue. The water would quickly surround you, and its surface tension would be too strong for you to break free. You’d drown.

Tiny Engineering: Beating Surface Tension#

Because of this danger, insects have evolved ways to be water repellent.

1. Wax Layer: Their hard outer shell (exoskeleton) is covered with a thin layer of wax, kind of like waxing a car. This makes their surface at least partly waterproof because water doesn’t stick to it very well.
2. Tiny Hairs: Many insects also have tiny hairs covering them. These hairs act like a barrier. They hugely increase the surface area and stop water droplets from touching the exoskeleton directly, making it easier to shake droplets off.

Evolution actually figured out nanotechnology billions of years before we did, using surface tension!

Some insects have evolved surfaces covered in a short, super dense coat of these water-repelling hairs. We’re talking over a million hairs per square millimeter! When one of these insects dives underwater, air gets trapped inside this furry coat, forming a bubble of air around it. Water can’t get into this air pocket because the hairs are too small to break the water’s surface tension.

But here’s the really cool part: As the insect uses up the oxygen in its air bubble, new oxygen from the water around it can actually move into the bubble (this is called diffusion). At the same time, the carbon dioxide the insect breathes out moves from the bubble into the water. So, the insect basically carries its own “outside lung” around with it and can breathe underwater, all thanks to surface tension!

This is also the same basic idea that lets pond skaters walk on water – those tiny hairs push down on the water’s surface tension without breaking it.

Air as Syrup: The Fairy Fly’s World#

The smaller you get, the weirder your surroundings become. At a certain point, even air starts to feel more and more solid.

Let’s zoom in on one of the smallest known insects: the Fairy Fly. They’re only about half the size of a grain of salt, just 0.15 millimeters long!

They live in a world even stranger than other insects. For them, the air itself feels like thin Jell-O, a syrupy mass always surrounding them. Moving through it isn’t easy. Flying at this level isn’t elegant gliding; they have to kind of grab and push off the air. That’s why their wings look more like big, hairy arms than regular insect wings. They literally swim through the air, like tiny, gross aliens pushing through syrup.

Things just get stranger the more we look at different sizes of life. The physics rules are so different for each size that evolution has had to constantly engineer new solutions as life got bigger over the last billion years.

So, why aren’t there ants as big as horses? Why don’t we see elephants the size of amoebas? Why? We’ll dig into that in the next part.

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