[aetherspawn]

I think it would be fine if you tracked only the flies that entered or exited through the small hole in the wall, ie not tracking the whole room just a tiny corridor.

So yes, you know the delta-fly(er) but not the number of flies.

[anyonecancode]

And if there were a small door over the hole, that opened only to allow fast moving flies to go through it from one direction and slow moving flies to go through it in the other...