On Google+, John Allder Stephens has a question for D&D Basic/retro-clone referees: How do you handle something like the mundane scorpion?
My first instinct is to handle it as a color scene. “A scorpion falls from the ceiling and clings to your hair. You barely manage to shake it off in time.” No roll required.
“Do you reach into the scorpion infested hole in order to pull that lever?” I’d assume that players would use a rope to pull that lever or flaming oil to burn those scorpions, or any other similarly plausible idea. If so, still no roll required.
If this is a trap, I’d make sure they had seen a bloated scorpion victim before, they see scorpions crawling on the ceiling, and if they don’t tell me how they’re cautious and sneaky or using their shields to cover their heads, then I’d say a scorpion will fall and might sting them with a deadly poison. Save or die!
I would not add a dexterity modifier because in my game saves are not modified by abilities; they are last ditch efforts, strokes of luck, the fruit of experience, an interference of the spirit world. By all rights you should be dead but there is a slight chance that you will not die after all – a saving throw.
I wouldn’t run combat with a mundane scorpion, though. Does plate armor or a shield help against tiny scorpions running around on the floor? Can they even reach you unless you’re helpless or surprised since their legs are about two inches long? Won’t a simple leather boot grant 100% protection? It just feels too weird. There are too many questions; a mundane scorpion might be deadly poisonous, but it isn’t a dangerous opponent in melee.