The Black Scorpion (1957), directed by Edward Ludwig.
This was one of the favorite creature-features of my youth.
Its good points:
Fantastic stop-motion scorpion effects by the great Willis O'Brien (King Kong (1933)), mentor to Ray Harryhausen.
Scorpions themselves have such a cool design: armored, low-slung, huge menacing claws and the graceful arch of the tail tipped with the sting.
The horrifying scene when the creatures derail a train and feast on the defenseless human passengers.
Descent into dangerous, mysterious caverns is always a good time.
A vision of a fantasy Mexico with volcanos next to an upscale ranch house with its patroness pretty in Spanish riding gear.
Scientist heroes to the rescue!
On the down side:
To my adult ear the dialogue is so poor it sounds like an old boot banging around in a metal bucket.
Irritating, troublesome stowaway kid.
The scorpion face shots are not stop motion and are scary enough, but also spoil the natural effect by giving the arachnids round single eyes like the Zanti Misfits of the original The Outer Limits (1963) series.
Because they ran out of time and money some of the special effects were not finished and we have the blacked-out traveling mattes of the scorpions rather than the stop-motion figures. This doesn't look right, but it does look strange. I think as a kid I thought it was intentional, to show that the scorpions had crossed over into nightmare territory and were so scary that people couldn't see them clearly.
Available on Blu-ray from Warner Archive. This is the first time I have seen it with the correct aspect ratio; the previous DVD was cropped (despite what the case claimed).