Our house has been invaded by gnats who seem to be efficient breeders. They are little black fruit fly sized bugs but not brown like fruit flies - definitely black. ALso not significantly present in the kitchen - mainly upstairs. We see them most on our staircase which has a very high ceiling. We do not have overripe fruit hanging around the place anyway, but even if we did, it wouldn't be in the stairs!!
Where have they come from? How can we rid ourselves of them??