Species account

Sortplettet Blåfugl (Phengaris arion)    Linnaeus (1758)
   Blåfugle (Lycaenidae)    LABL    24863
Large Blue (gb)   Hormiguera de Lunares (es)   Svartfläckig Blåvinge (se)      Synonyms: [Maculinea arion]
Measurements:
WS: 32-42mm
Generations:
1
Flight period:
JFMAMJJASOND 
SE
Identification:
The largest blue in the Nordic region. Both sexes show a broad black rim above with black spots on a blue background. The rim is narrower on the male and the spots are tinier. Below they both show characteristic large black spots on both wings encircled by white. The base color is dark gray-beige. Centrally the wings below show a bluish iridescent cast on both sexes.

Is somewhat similar to alcon (Ensianblåfugl) but that male is cleaner blue above with narrow black rim and no black spots. The female only shows small black spots above but very much black in the wing and less blue centrally. Below the base color is similar but the black spots are smaller.
Similar species:
Life stages:
Distribution:
Europe, Turkey, Russia east toward Japan.
Habitat:
Grassed hot and dry south or west turned sandy fields/meadows where the host ant is present.
Behavior:
Ant-symbiotic.
Strong flyer but only lives few days as imago
Status:
Declined heavily during the last century and is now very rare in Denmark, only to be found at Høvblege, Møn. In Sweden restricted to Öland, Gotland and few localities in Skåne and near Stockholm. In Finland only present a few places in the southeast. Reintroduction projects in England have been successful.
Host plants:
Bredbladet Timian (Thymus pulegioides) [Broad-leaved Thyme ~ Stortimjan]
Smalbladet Timian (Thymus serpyllum) [Wild Thyme ~ Serpol ~ Backtimjan]
Merian (Origanum vulgare) [Oregano ~ Orégano ~ Kungsmynta]

Is competing with Timiankøllesværmer (Zygaena purpuralis) [Transparent Burnet ~ Timjanbastardsvärmare] over thyme plants at Høvblege, Møn forcing arion to use other hostplants.
Feeding plants:
Knopurt sp. (Centaurea sp.) [Knapweeds ~ Klint]
Timian sp. (Thymus sp.) [Thyme sp. ~ Serpol sp. ~ Timjan sp.]
Merian (Origanum vulgare) [Oregano ~ Orégano ~ Kungsmynta]
Møn also:
Foder-Esparsette (Onobrychis viciifolia) [Common Sainfoin ~ Esparceta ~Esparsett]
Comment:
Many subspecies listed on Finnish Maculinea.

danske-natur.dk mentions: "På Høvblege findes der han og hun former med utydelige eller manglende sorte pletter på oversiden, ab. punctifera, ab. postero-immaculata , & ab. alconides som udgør op til 20% af bestanden."
My free translation:
"...different forms present at Høvblege where both sexes show very weak or absence of upperside spots. Up to 20% of the population."
I'm not sure if the forms mentioned and their names are officially recognized or only used by the author.
Large Blue 879    (2 photos)
2022-07-03    Høvblege (Møn), Denmark

Imago
Don't think the species is sexable from below.



Large Blue 876    (3 photos)
2022-07-03    Høvblege (Møn), Denmark

Imago Han
Calling it a male because of the small spots above, though the black border above looks very wide (see Comment section under the species account).



Large Blue 878    (2 photos)
2022-07-03    Høvblege (Møn), Denmark

Imago Hun
Dark underside. Very worn.



Large Blue 880    (1 photos)
2022-07-03    Høvblege (Møn), Denmark

Imago Hun
Though a bit blurry I think it's a female with larger spots on the upperwing.