E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The Thracian Bosphorus, or Bosporus, unites the Sea of Marmra, with the Euxine (2 syl.) or Black Sea. According to Greek fable, Zeus (Jupiter) greatly loved Io, and changed her into a white cow or heifer from fear of Hera or Juno; to flee from whom she swam across the strait, which was thence called bos poros, the passage of the cow. Hera discovered the trick, and sent a gadfly to torment Io, who was made to wander, in a state of phrenzy, from land to land. The wanderings of Io were a favourite subject of story with the ancients. Ultimately, the persecuted Argive princess found rest on the banks of the Nile.
Dionysius of Halicarnassus and Valerius Flaccus give this account, but Accarion says it was a ship, with the prow of an ox, sent by some Thracians through the straits, that gave name to this passage.