How Come Flies Don't Get Hurt When They Fly into Hard Surfaces?

How Come Flies Don’t Get Hurt When They Fly into Hard Surfaces?

Flies travel fast. But they don’t seem to bother about flying into hard surfaces like walls, doors or windows. How do they survive such impacts?

The answer to the above question is presented in five different ways below.


Arthropods that fly have very low mass. They also have a lightweight armour made largely of chitin. This exoskeleton protects their nervous system (brain) organs and muscles. It’s like a body helmet. Lastly, they have an open circulatory system that prevents them from inflammation damage, i.e., bruising.

How come flies don't get hurt when they fly into hard surfaces?

An open circulatory system, unlike in mammals and reptiles, has no high pressure blood vessels such as arteries. Instead, there is a mixture of blood and lymph cells, that white-ish goop inside bugs, which sloshes around cavities. The benefit is that bugs do not easily/cannot internally “bleed” like mammals. The downside is that this lack of high pressure blood is one aspect that prevents them from being too large.


Imagine you had a ping pong ball, and you filled it with cool whip. Now shrink that whole thing down to the size of a fly. Now imagine you threw that tiny little shell full of goop at the wall. Even if you threw it as hard as you could, it’s still soooo tiny and soooo tough and bouncy on the outside that it’ll just bounce off.

Flies are super tiny, and have a shell just like that ping pong ball, but with little flexible, foldable wings. And just like a fly, if you use a slingshot instead of your hand (a moving car instead of a window) you might just get it to pop.

How come flies don't get hurt when they fly into hard surfaces?

Flies have an exoskeleton that’s incredibly tough and hard in some spots, and just flexible enough to be springy and bouncy in others. Just like that ping pong ball, they’ve got a shell that’s good at taking a bit of a hit and bouncing off instead of just squishing like a worm (which doesn’t have that tough shell).

The fact that they’re so small helps in a couple different ways as well. For one, we think they’re flying super fast, but it’s really just because they’re tiny. If you look at a massive airplane, it might be moving at 500 miles per hour but still looks like it’s just crawling along across the sky. Houseflies look fast, but I asked Google and they only go about 5 miles per hour. That means a baseball pitcher can throw a fastball 20 times as fast as a housefly flies.

Not only are they actually super slow (if you don’t let the size trick you), they also weigh almost nothing. Like, it would take about 200 flies to add up to the weight of a single ping pong ball, according to some quick Googling/converting.

How come flies don't get hurt when they fly into hard surfaces?

So your ping pong ball full of cool whip is actually super tough, reaaally slow, and unbelievably lightweight, meaning that dumb little fly was designed to fly into the window several thousand times before it finds the opening. Evolution at work.


Tiny amount of mass. There is an old expression that goes something to the effect of ‘Insects float, cats land, humans break, and horses splash’. Basically gravity becomes much more lethal as your mass goes up.


The phrase, “The bigger they are, the harder they fall”. Here is this exact idea formulated in it’s most general form: wiki link
It’s a wiki article I first found after watching a terrible b-movie, and attempting to research why insects haven’t grown to dinosaur sizes and destroyed us all. The short answer to that one was “lungs”. But in a broader sense, it’s because all of a body’s support systems follow the square-cube law. .


How come flies don't get hurt when they fly into hard surfaces?

Yes it’s partly low mass and their exoskeleton, but that isn’t the entire reason.
It’s because of the square-cube law. This principle states that, as a shape (or creature) grows in size, its volume (and weight) grows faster than its surface area. So when the fly hits the wall, there is less weight dispersed over a larger surface area (relatively) than a larger creature.
This is also why children can comfortably sit on their knees, but adults can’t.


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