Apollodorus Library 3.5.4

1. As to the departure of Cadmus and Harmonia to Illyria and their transformation into snakes in that country, where their tomb was shown in later ages, see Ap. Rhod., Argon. iv.516ff. ; Dionysius, Perieg. 390ff. , with the commentary of Eustathius, Comm. on Dionysius Perieg. v.391 ; Strab. 1.2.39 , Strab. 7.7.8 ; Paus. 9.5.3 ; Athenaeus xi.5, p. 462 B ; Stephanus Byzantius, s.v. Δυρράχιον ; Tzetzes, Chiliades iv.393ff. ; Ov. Met. 4.563-603 ; Hyginus, Fab. 6 ; Lactantius Placidus on Statius, Theb. iii.290 ; Scriptores rerum mythicarum Latini, ed. Bode, i. p. 48 (First Vatican Mythographer 150) . Euripides mentions the transformation of the couple into snakes, but without speaking of their banishment to Illyria ( Eur. Ba. 1530ff. ), probably because there is a long lacuna in this part of the text. According to Hyginus, the transformation of the two into serpents was a punishment inflicted by Ares on Cadmus for killing his sacred dragon which guarded the spring at Thebes, which Hyginus absurdly calls the Castalian spring. It is a common belief, especially among the Bantu tribes of South Africa, that human beings at death are turned into serpents, which often visit the old home. There is some reason to think that the ancestors of the Greeks may have shared this widespread superstition, of which the traditional transformation of Cadmus and Harmonia would thus be an isolated survival. See Adonis, Attis, Osiris, 3rd ed. i.82ff.