Species account

Apollo (Parnassius apollo)    Linnaeus (1758)
   Svalehaler (Papilionidae) [Sommerfugle]    APOL    25450
Apollo (gb)   Apolo (es)   Apollofjäril (se)  
Subspecies
apollo
Gotland, Sweden.
jotunensis
Opheim (1945).
nevadensis
Oberthür (1891) - Sierra Nevada, Spain.
norvegica
Menthe (1912) - S Norway and W Sweden.
scandinavica
Harcourt-Bath (1896) - E Sweden, Åland Islands, S Finland.
Measurements:
WS: 62-87mm
Generations:
1
Flight period:
JFMAMJJASOND 
SE
Identification:
Unmistakable in the Nordic region with the large size and black and white pattern with red spots on the hindwings.
Much more can be said about details of the countless subspecies and forms that have been suggested/described, but this will have to wait until I grow up.

In the Alps the Alpeapollo (Parnassius phoebus) [Small Apollo] is very similar but shows a varying degree of red spots encircled with black near the costa of the frontwings.
Similar species:
Life stages:
Distribution:
Europe, Middle East, China, Mongolia and Russia east to Yakutsk.
Habitat:
Open rocky terrain in mixed forests, heather forests with poor soil. Also near the ocean.
Behavior:
Not shy when feeding. Erratic flight mixed with sequences of elegant gliding.
Host plants:
Continental Scandinavia:
Sankthansurt (Hylotelephium telephium) [Orpine ~ Orpina ~ Kärleksört]
Gotland:
Hvid Stenurt (Sedum album) [White Stonecrop ~ Uva de Gato ~ Vitt Fetknopp]
Norway (Jotunheim):
Rosenrod (Rhodiola rosea) [Golden Root ~ Rosenrot]
The Alps also:
Bjerg-Stenurt (Petrosedum rupestre) [Reflexed Stonecrop ~ Uñas de Gato]
Pyrenæisk Stenurt (Sedum dasyphyllum) [Thick-leaved Stonecrop ~ Arrocetas ~ Luddfetknopp]
Almindelig Husløg (Sempervivum tectorum) [Common Houseleek ~ Siempreviva Común ~ Taklök]
Other Sempervivum species.
Feeding plants:
Tidsler [Thistles ~ Cardos ~ Tistlar]
Stor Knopurt (Centaurea scabiosa) [Greater Knapweed ~ Centaurea Mayor ~ Väddklint]
Comment:
It is a highly diverse species and it seems like every isolated population has gained status as a subspecies or form, more than 270! It is disputed, however, how many of them are valid or just representing form variation within the species.

In Scandinavia the following four subspecies are recognized:
apollo: synonym linnaei Bryk (1913).
scandinavica: The largest in the region and more long-winged and with large red spots with white center.
jotunensis: The smallest form in Scandinavia.
norvegica: Almost as large as scandinavica.

Max and min wingspans vary greatly depending on which resource one consults. The extremes I've seen being 62mm - 95mm.