We often have flies in our house, although we keep everything quite clean. We have the terrace door always open, if the weather is nice. Every day there are one or two flies flying about. But why aren’t there 10 or 15? If there is food , or if the house smells nice for flies, why don’t we get more? Occasionally I manage to kill them, but soon we have flies again, and never more two. It is as if they count the other flies in the room, and leave if there are too many.