The Chalkhill Blue (Lysandra coridon) is a small butterfly endemic to Europe (however not in Scandinavia and the Netherlands), where it occurs primarily in grasslands rich in chalk.
The sexes of the Chalkhill Blue are strongly dimorphic; the males being pale sky blue, and the females being chocolate brown. 
Both male and female have chequered fringes around their wings.
The Chalkhill Blue has a preference for dry calcareous grasslands.

This species was previously known as Lysandra coridon. 
In 2010 it was reclassified as Polyommatus coridon and in 2017 it was again reclassified back to Lysandra coridon.

1st & 2nd photo: A female. Nature Reserve Urfttal, Schmetterlingspfad, North-Rhine Westphalia, Germany, 8 August 2022
3rd photo: A male. Nature Reserve Urfttal, Schmetterlingspfad, North-Rhine Westphalia, Germany, 8 August 2022
4th photo: A male. Lampertsbachtal, Blankenheim, North-Rhine Westphalia, Germany, 8 August 2022
5th photo: A female. Nature Reserve Gillesbachtal, North-Rhine Westphalia, Germany, 8 August 2022