Commentary Purg XVII 19-21

For Dante's earlier advertence to Ovid's story of Philomel and Procne see [Purg IX 13-15] and C.Purg.IX.13-15.  There Philomel is dealt with as a sympathetic figure; here Procne, her sister, is made exemplary of the sin of Wrath for murdering her own son, Itys, in order to take vengeance upon Tereus for raping her sister.  It may seem odd to today's readers, but Dante thinks of Procne as the nightingale, Philomel as the swallow (for discussion see C.Purg.IX.13-15).  This ecstatic vision (see [Purg XV 85-86]) is sent into Dante's mind, we must assume, by God.