Ask HN: Why there are no infinite number of flies in a room? | Hacker News Reader

Ask HN: Why there are no infinite number of flies in a room?

Imagine a room on a sunny day, and flies (insects) in that room. You kill all the flies. Next day, new flies are back. Why there are that many flies in the room, not more or less?

If there was a constant inflow of flies into the room, after some time you would have an infinite number of flies. I never heard of this happening!

One could argue that there is a constant number of flies per volume outside the room and when you kill all the files in the room, there is a negative gradient of flies and the insects will "naturally" move to the room. But such a "fly pressure" would require the flies to bounce from each other, like atoms, which does not seem to happen.

Or maybe flies are territorial animals?

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There is also a constant outflow of insects from the room which is less than the influx but not 0, there is a limited supply of flies outdoors to see and then also make it inside the room. Flies are not, however, a constant pressure, their numbers correlate to food sources in the area, as well as cyclic trends which will influence their numbers (predator behavior, etc.).

Regarding territory, sort of: "In urban areas, where people live in close proximity and there is garbage, flies have small territories and may only fly 1,000 meters or so. Rural house flies roam far in search of manure, covering up to 7 miles over time." Via: https://www.thoughtco.com/fascinating-facts-about-house-flie...

> If there was a constant inflow of flies into the room, after some time you would have an infinite number of flies.

With a constant finite inflow, you’d only have infinitely many after infinite time.

As to why they don’t keep amassing: flies need food, water (moisture), warmth and oxygen. Stuff too many in a room, and conditions elsewhere will be better for them.

Flies have senses like vision, that mean they don't have to bounce off each other like gas molecules do.

Molecules of an ideal gas bounce off the sides of the container, not each other.

This reminds me of the other day when my son and I were standing close to several beehives and watching the bees work. I asked him, "How long do you think we have to stand here to get stung?" and he said "a week". A few minutes later one of the bees had gotten stuck in his hair and was working it's way in deeper before stinging him.

Then the gas analogy might be even better: anyone who's been in a room with one or more flies has observed that flies spend a high proportion of their time bouncing off walls and windows.

If you leave your windows open with no screen, they come in and go out... You would probably end up seeing about as many flies as you see outside, unless you have some rotting meat in your kitchen or something similar.

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