This butterfly will always be special. First seen by me near my Yorkshire home in the 1970's in my first experience of what in the UK is called a "Clouded Yellow Year". Once I had seen one, it seemed they appeared everywhere, and it felt like they must have been here to stay. Alas not, and although there was a second such year in the 1980's, the only time I seem to see them now is on the Continent. 

I find them really difficult to get close to, and from many attempts, this Greek pair. male left, female right, were my best butterfly photographs to date.