This is a folktale, folkbelief, whatever you want to call it that I've heard only in Rowan, Carter, Bath, Lewis, Elliott and Menifee Counties in Kentucky. People of my grandparents' generation believed it sincerely, and so did John I, the second cousin mentioned here previously (and about whom more below). When those wonderful folks still lived, if a smartmouthed grandchild argued "That ain't so" (couple of my cousins were foolish enough to do so), some serious wrath fell down on them. Today people of my parents' generation will sometimes repeat the story. Although they don't seem to believe it, I'd wager a puff adder that got in their way would be a dead one before it was all over with.
I give you the "blow viper:"
The old timers said it was the most poisonous snake in the world. If one got its fangs into a man's leg, that man was dead way before help got to him. There's no antidote nor treatment for the poison, which would kill you before you walked ten steps...
There is however, the fact that whenever a blow viper opens its mouth to strike at a human, those jaws "lock" open, and because the snake can't close them, a human is highly unlikely to ever be actually harmed by this "most poisonous snake in the world."
The old people found the blow viper in their Bibles. They said it was the kind of snake Aaron's staff turned into when he "cast down his rod before Pharaoh", and they would speculate maybe that's why God doesn't let it bite people.
The seriousness with which some old timers took their "blow vipers" can be illustrated with a John I reminiscence. He'd been in the hospital ten or twelve days, but as soon as he was turned loose, that old man insisted on going back to living alone on an isolated farmstead maybe a quarter mile across the Lewis County line. Some in my family took turn about staying with John at night, and one morning when it was my turn, after breakfast we took our coffee onto the back porch, overlooking a small pond. We were having a lazy kind of springtime conversation about whether or not John could more successfully keep groundhogs out of his garden that year than the previous one, when the old man commenced peering hard toward the pond.
"Look on that rock yonder," John said.
"Which rock?" I wanted to know. On those acres, directing someone to "look at that rock" was like pointing at a field and asking, "see that blade of grass?" Finally I found with my own eyes the rock he meant, along with a large, dark snake stretched out across it, probably warming up from the overnight chill.
"That there is a blow viper," John told me. "And you need to kill it."
I fetched a pair of binoculars from the truck and satisfied myself the snake, whatever its species, was neither copperhead nor rattler, the only snakes I willingly kill, but when I told John we could let it alone, the old man told me again all about "the blow viper, the most poisonous snake in the world," and got so worked up I finally took a pistol down to the pond and dispatched it. I felt bad about it, but it came quite clear either I was gonna kill it, or John I was, and he was too sick to wander far from the house.
That's a "blow viper."
Where I live it's our snake: I've talked to people the length and breadth of Appalachia, and have never found its story told elsewhere.
I bet where ever you live there are equally localized folk tales. Maybe you've got an elderly cousin - - maybe an aunt or uncle, parent, grandparent, whatever - - who'll tell what your stories are. Go find out, take them to your children and give those old stories another generation or two of life.