In Europe the Clouded Apollo has a scattered distribution from the Balkan Peninsula to the Pyrenees and north to Fennoscandia, occuring especially in mountainous areas in southern Europe.

In Bulgaria it is widespread but local in mountain areas but occuring at lower altitudes (from sea-level) north of the Stara Planina (central west-east spine also called the Balkan Mountains). 

It is surprisingly easy for the unwary (sometimes even the wary - when not paying attention!) to confuse this species in flight with the generally much more  common and numerous Black-veined White (Aporia crataegi), espeically with females of the latter species as they too have semi-transparent areas of their wings, are a similar size and the way of flyng can also be similar.

I have noted that older (more worn) specimens sometimes have a yellow collar and yellow on their legs and abdomen. I presume this is pollen that has stuck to their bodies from numerous yellow flowers they have visited, probably largely Asteraceae. This can be seen clearly In three of the photos below.

Mated females often have a whitish sphragus under the end of their abdomen, which is deposited by the male after mating. This is done to prevent further mating by other males.

Foodplants: Corydalis spp.