Commentary Purg III 113

Manfred does not identify himself as his father's son (Frederick II is, after all, condemned to hell for heresy: [Inf X 119]) but by reference to his paternal grandmother, Constance  (1154-98), daughter of Roger II, king of Sicily, and wife of Emperor Henry VI, by whom she became the mother of Frederick.  As Benvenuto uniquely (among the early commentators) points out, Dante has borrowed this tactic from Polynices who, questioned about his lineage by the king of Argos, Adrastus, in Statius's Thebaid (I.676-681), prefers to omit the name of Oedipus in favor of that of his mother, Jocasta.  Porena (DDP Porena.Purg.III.112-113) notes that Dante himself had remembered this scene in Convivio (Conv.IV.xxx.10), laying the avoidance on Polynices' part both to his desire to omit reference to his own harsh actions against his father as well as to keep concealed Oedipus's evil deeds.  Gmelin (Gmel.1955.1), p. 79, offers further discussion.