Huh..
They are way more dense then I expected... not sure how you expect them to flow around, where they are basically touching between themselves.
It's hard to give precise advice (at least, for me), on this, unless I try scene myself.
But I'd try to push them towards that small entrance, and there I'd test them with some surface test operator, to bring them in new event, and in the new event, I'd make path follow, and they would follow a line (I previously created), in the tube.
They are way more dense then I expected... not sure how you expect them to flow around, where they are basically touching between themselves.
It's hard to give precise advice (at least, for me), on this, unless I try scene myself.
But I'd try to push them towards that small entrance, and there I'd test them with some surface test operator, to bring them in new event, and in the new event, I'd make path follow, and they would follow a line (I previously created), in the tube.