The Scorpion, or Scorpio, was an ancient siege engine probably invented by the Ancient Greeks and later adopted on a larger scale by the Roman legions. It was the Long-Range weapon of the Roman Centurion.
The Scorpion was a primitive Crossbow mounted on a stand. The weapon was operated by a system of springs with torsion (known as tormenta), which made it possible to achieve very great power and a high speed of ejection for the bolts.
A similar yet larger variation of the Scorpion is the Ballista
The Scorpion was most likely inspired by the Macedonian Oxybelles, which was inspired by the Gastrophetes.
The Scorpion was used for two functions. In tended shooting, the Scorpion was used to pick off individual enemies up to 100 yards away. In parabolic shooting, bolts were lobbed at opposing armies up to 400 yards away; the Scorpion also had a higher rate of fire (three to four shots a minute) but was less precise.
Its relatively small size, accuracy, and ability to be used by a single man made the Scorpion more of a sniper weapon than a siege engine. However, due to its complexity of construction and adjustment, and its sensitivity to changes in temperature and moisture, the Scorpion's use on the battlefield was limited.