Below is a simple setup that might work for a scorpion tail. Light weight and probably could be operated by a micro servo in one direction and have a small rubber/elastic band return to the resting position. Bottom is my latest “time killer”. My nephew’s son has developed an interest in steam things, so I’m working on some DIY pop-pop boat engines. Never too late to add “steam boat captain” to the resume. 
sciencetoymaker.org/robotFinger/assembl.html
youtube.com/watch?v=P-gmVnj_hJ4
youtube.com/watch?v=aLL-V9t78gI