Pests

I'm sadly dealing with pests again today - not mice this time, but ants.  The inside of my house has not been full on invaded by them just yet, but it looks like they sent an expeditionary force to check out the lay of the land.  I discovered, and mostly killed, about 10 ants this evening that I found downstairs.  I don't know how they got in, but the fact that most of them were in the downstairs bathroom suggests that there must be access to outside my house somewhere in there - maybe underneath or behind the fixtures.  In all the time I've been living here, I thought that this might be the first time I've encountered ants inside.  But I don't think that's true.  I found some bug spray in my cupboard that I don't remember buying - perhaps I did, perhaps the previous tenant left it there - but let's just say it got used quite liberally this evening.  I haven't seen any more ants around since then - but I'm certainly feeling paranoid about their presence, and I can't stop scanning the non-carpeted floors for signs of movement.  I hope I don't wake up tomorrow morning to encounter a load more of them - that would be the nightmare scenario (though at least that might help me pinpoint exactly where they come into my home from).