This article on A Reader’s Guide to Orthodox Icons begins with a very brief history about the snakes, but it is particularly interesting for its discussion on the symbolism of these serpents in Orthodoxy.
In a tiny Greek village in the south of Kefallonia, a miracle occurs every year after the feast of the Transfiguration (Aug 6). Around the bell-tower of the chapel at Markopoulo, small venomous snakes appear. These snakes crawl around the church, and upon the icons of the Mother of God in an act of apparent veneration. The snakes remain in the confines of the chapel, docile throughout, until the feast of the Dormition (Aug 15), when they disperse and become almost impossible . . .