Saint Swithin's Day and flying ants

Yesterday was Saint Swithin's Day. If the traditional rhyme is anything to go by, we are in for a good spell:

St Swithin’s Day, if it does rain
Full forty days, it will remain
St Swithin’s Day, if it be fair
For forty days, t'will rain no more

I also noticed about eight flying ants while I was walking around. I was always told that they come out for one day a year. I think this is, perhaps, a localised thing. So they have chosen today in Newbury but not in Cornwall, according to my mother, who was the source of the theory.

I have often wondered how all the flying ants decide that it is their day to come out. But presumably it has something to do with localised weather conditions.