From Pierre Grimal's Dictionary of Classical Mythology
Twin gods, originating in Sicily. They were sometimes said to have been the sons of Zeus by Thaleia the daughter of Hephaestus, sometimes the sons of Zeus by Aetna. While she was pregnant with the twins, Thaleia, fearing Hera's jealousy hid in the earth and when the time came the twin boys emerged from the ground, which explains their name 'the Returners'.
Their place of worship was close to the Lago di Naftia, not far from Leontini and the site of various volcanic phenomena. For example a fountain of warm water spouted in the lake itself, forming a dome which then fell back into the basin without spilling a drop outside it. A strong smell of sulphur floated over the lake and it was even said that the birds which flew over it died at once and that men who approached it incautiously died within three days.
Such is the site of the Palici, redoubtable deities by whom the Sicilians swore their solemn oaths: when someone wanted to support an assertion with an oath he would write it on a tablet which he threw into the waters of the lake. If the tablet floated, the oath was sincere; if it sank the oath was clearly invalid. It was said that the Palici struck blind all liars who falsely called upon their name.