YES, the bottom one is always the movable one (unless you mistake a crab for a scorpion).
Think about it -- if it varied from scorpion to scorpion, it would be like random people having opposable pinkie fingers instead of opposable thumbs.
If your emperor scorpion stops eating, don't panic.
Be nice -- you were a noob once, too
"Never ask an engineer to explain something, because he will."