Why are there so many flying ants ?

Swarms of flying ants have descended upon some parts of the UK over the past few days, only to disappear just as suddenly. Why?
Over the past few days, the phrase "ants in your pants" could be taken literally as the air filled with ants flying en masse, landing on laundry, dropping onto heads and basking on pavements.
Entomologists say the sudden emergence of the black garden ants - known as Lasius Niger - is the insects' annual mating ritual.
Dave Clarke, head keeper at London Zoo's bugs department, describes the annual flight as "the biggest one night stand in the UK".