In Hyginus' version of the myth of TEREUS and PROCNE, Laethusa was the wife of Lynceus, the king of Thrace, to whom Tereus had entrusted his sister-in-law Philomela. Laethusa, a friend of Procne, revealed to her the serious crime which her husband had committed and incited her to revenge.
{E2 DICTIONARY OF CLASSICAL MYTHOLOGY}