Sankes in many cultures are linked to immortality - renewing oneself continuously. Also, they were symbols of fertility, both feminine and masculine. I have also heard that some Native Americans tied snakes to the forces of lightning. I'm currently studying snake dancing, in tribal bellydance traditions and it's tie to ancient snake worship.
Here's something about the Minoan snake goddess that you may find interesting - http://witcombe.sbc.edu/snakegoddess/
A webpage about snake symbology and myth (beware annoying MIDI) - http://www.thelema.net/ramsey/naga.html
Some stuff on Nagas - http://www.khandro.net/mysterious_naga.htm
More snake mythology with more snake links at the bottom - http://www.khandro.net/animal_serpent.htm
Hope that helps!
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1.0 sinaloan milksnake
1.0 western hognose
1.0 colombian boa