In Greek mythology, Themisto (/θəˈmɪstoʊ/; Ancient Greek: Θεμιστώ, romanized: Themistṓ, lit. 'belonging to the law or customs'[1]) was the daughter of the river god Inachus and the mother of Arcas by Zeus.[2]
In other accounts, the usual birth mother of Arcas was called Callisto, daughter of Lycaon or else Megisto, daughter of Ceteus,[3] both women were members of the Arcadian royal family.