Explain the legend of Philomela.

This myth first appeared in Ovid's Metamorphoses. A warrior named Tereus desired to marry Pandion's daughter Procne; however, there were bad omens even on their wedding night that all would not be well with their marriage. Not long after, Procne delivers a baby boy to Tereus, and after five years, she expresses her desire to see her sister, Philomela. Tereus agrees to go to Athens to fetch Philomela, but when he sees the beautiful maiden, his "burning lust was fir'd." He presents his wife's wishes for a visit and he wins the sister over. When Philomela hugs her father, Tereus wishes he could be that father and enjoy her embraces. They board his ship for Thrace, and he soon rapes the "bloomy girl" who was "Tender, defenceless, and with ease o'erpower'd." Philomela says that she is "devoid of shame" and "Thro' the wide word [Tereus's] actions will proclaim." Angry and guilty, Tereus cuts her tongue out so that she cannot tell. When he arrives home, he counterfeits grief and tells his wife that her sister has died.
The narrator says that "all our wants by wit may be supply'd," and Philomela does find a clever way to "speak" to her sister. She weaves the scene of her rape, paying a slave to convey it to Procne. Procne receives it, understands, finds her sister, and she tells Philomela not to feel grief and shame. Despite the tenderness she feels for him, she kills her son, cutting his throat even as he begs her for mercy. She says that he will be like his father, and why should the boy have the power of speech when her sister has lost hers. She carves up his body and cooks him for dinner. Procne invites Tereus, implying that they will perform some mystic rite, and when he calls for his son to come to the table, Philomela steps forward — all covered with blood and gore — and casts the son's head down onto the table. Tereus gives chase to the two women, and all three of them turn into birds: Philomela becomes a nightingale.
http://classics.mit.edu/Ovid/metam.6.sixth.html

Philomela goes to visit her sister, Procne, in Thrace, under the care of her sister's husband, Tereus. Rather than care for her, Tereus rapes her. He then tells her to keep silent, but she refuses to comply. Therefore, in a rage, he cuts out her tongue and abandons her in a cabin.
Because of her inability to speak, Philomela weaves a tapestry depicting her rape. When Procne sees it, she becomes so enraged at her husband that she kills their son, Itys. She boils him and serves him to his father as a meal. When Tereus discovers what he had eaten, he races after Procne and Philomela, intending to kill them. They flee and pray in distress to the gods to transform them into birds so that they can fly away and escape Tereus. The gods turn Philomela into a nightingale and Procne into a swallow. Because of what happened to Philomela, the nightingale's song is said to carry a note of lament and sadness.