Prudence, mirror and serpent

Why is Prudence often represented with a mirror and a serpent?

Is she looking at herself in the mirror or using the mirror to look over her shoulder?

What do the mirror and the serpent represent?

http://www.prudential.co.uk/prudentialplc/aboutpru/history/values/prudence/

One of the things that a Serpent symbolizes is Wisdom, and some of the things that a Mirror symbolizes are Truth and Self-Realization. Add them up and you get “prudence”.