Eli5 how can insects survive a slap from humans, a creature that’s gigantic compared to them

Question

Eli5 how can insects survive a slap from humans, a creature that’s gigantic compared to them

In: 4969

Answer 1

A matter of scale.

Resistance of our bones or of the exoskeletons (the “skin”, if you want) of insects depends on the *area* of their cross section.

The forces applied when moving, falling (or getting slapped), on the other hand, depend on the mass involved (from inertia), that is the *volume*.

But area and volume don’t scale in the same way: area is proportional to the *square* of the size, while for a volume it’s the *cube* (third power). That’s just geometry. Imagine an ant being the size of an elephant. All its dimensions would be roughly multiplied by something like 500, the strength of its legs by 500^2 = 250,000 but its weight by 500^3 = 125,000,000… and the poor beast would break down under its own weight. (So no giant ants outside of science fiction movies.) And the other way round if you decrease the dimensions, an elephant the size of an ant would be ridiculously over-designed.

That’s the reason why elephants need column-like legs to walk around while spiders, for example, can make do with very long and slender ones. And why small kids can fall without hurting themselves seriously while adults can break a bone just by slipping. (There’s also the matter of bones acting as levers, so another size effect, but we’ll neglect that. It works in the same direction.)

Now for swatting specifically, insect bodies are encased in a very tough but somewhat elastic “hide”, a kind of armor. And the size effects I described make that when they hit something, a windowpane or your angry hand, the forces acting on that armor are spread over a much greater area, relatively speaking, than we are used to at our own scale. Being spread out their effects are mild and the insect doesn’t shatter.

(There are other interesting size effects. Water appears more and more viscous at small scale and the unicellular organism one can see under a microscope are swimming through something which behaves like treacle. And that’s why they use cilia to propel themselves and not flippers.)