Maybe you recognize the flag above. It's known as the Gadsden Flag, and was first mentioned in 1775 when it was painted on the drums of U.S. Marines. There was some sentiment that the rattlesnake should become the national symbol of America instead of the Eagle. Among the reasons given were: because he never starts a fight without provocation, gives clear warning that his anger is aroused, yet never surrenders once the fight begins. If you look carefully at a full-sized flag, you would find that there are thirteen rattles on the snakes tail, one for each of the original states.
The original Gadsden Flag was presented to the first Commodore of the U.S. Navy by Colonel Christopher Gadsden, and was used as Commodore Hopkin's personal standard (flag) on whatever ship he was aboard.
A variation of the flag was adopted by the Navy and every ship flew it in addition to the U.S. flag. It's now known as the First Navy Jack.
Eventually, it became naval custom for the ship with the longest period of active service in the navy to fly the First Navy Jack. In 2009, the aircraft carrier USS Enterprise recieved that honor upon the decommissioning of the carrier USS Kitty Hawk.
Flag images courtesy of www.Gadsden.Info and www.NavyJack.Info, along with much of the information included above. Follow those links for more related history.