This ant was covering a swimming pool in the hundreds, in clustered masses with ants climbing on top of other ants, from its nest sites just under the pool lining. Several local pool owners have reported this, at least one with what sounds like Formica fusca, just in the last 2 years. All pools were salt-maintained. I've a small 100% natural pond, with 8 species ID'd from my back yard, for over a decade and have never observed anything like this; neither has my neighbour with a chloramine-maintained pool. Comments on the phenomenon would be appreciated. Also to note: althoough this ant is common in the USA, this is the first time I've observed it in Ottawa. Our most common ant is Formica fusca.