Giant spiders

WHY CAN’T PEOPLE BE TWICE THE SIZE? Why don’t we see giant spiders, like we do in the movies, in real life? It’s staple of science fiction movies, after all. So is it possible?

In a word, no. There’s a good reason why spiders have thin spindly legs, while elephants have great thick chunky ones. It’s all to do with how length, area and volume get bigger at different rates.

If a person was twice the height, they’d also be twice the width – and twice the depth. So they’d be eight times as heavy, not just twice as heavy. Why does this matter?

Imagine a ball hanging from a thread, so that the thread is just strong enough to support it. If the ball were any heavier, the thread would break. Let’s double the size of the ball and the thread.

If the ball’s twice as big, its volume will be eight times as much, so it will weigh eight times as much. And what of the thread? It will be twice as wide, twice as deep and twice as long. Except that being twice as long doesn’t make it any stronger; in fact, the thread will only be four times as thick. So the thread will break.

So it is with people. If we were double the size, we’d be eight times the weight, but our muscles would only be four times as strong. And if a spider were, say, a hundred times the size, it would weigh a million times more (100 × 100 × 100), but its legs could carry only ten thousand times as much (100 × 100). Like super-sized people, it would simply collapse under its own weight.

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