This information came from http://www.foundingfathers.info/stories/gadsden.html I especially love the part about the rattles on the snakes tail!
Benjamin Franklin diverts an idle hour
In December 1775, "An American Guesser" anonymously wrote to the Pennsylvania Journal:
"I observed on one of the drums belonging to the marines now raising, there was painted a Rattle-Snake, with this modest motto under it, 'Don't tread on me.' As I know it is the custom to have some device on the arms of every country, I supposed this may have been intended for the arms of America."
This anonymous writer, having "nothing to do with public affairs" and "in order to divert an idle hour," speculated on why a snake might be chosen as a symbol for America.
First, it occurred to him that "the Rattle-Snake is found in no other quarter of the world besides America."
The rattlesnake also has sharp eyes, and "may therefore be esteemed an emblem of vigilance." Furthermore,
"She never begins an attack, nor, when once engaged, ever surrenders: She is therefore an emblem of magnanimity and true courage. ... she never wounds 'till she has generously given notice, even to her enemy, and cautioned him against the danger of treading on her."
"I confess I was wholly at a loss what to make of the rattles, 'till I went back and counted them and found them just thirteen, exactly the number of the Colonies united in America; and I recollected too that this was the only part of the Snake which increased in numbers. ...
"'Tis curious and amazing to observe how distinct and independent of each other the rattles of this animal are, and yet how firmly they are united together, so as never to be separated but by breaking them to pieces. One of those rattles singly, is incapable of producing sound, but the ringing of thirteen together, is sufficient to alarm the boldest man living."
Many scholars now agree that this "American Guesser" was Benjamin Franklin.