A ballista's arms are wood, each supported by being spun into the middle of a large rope. The ropes (one per arm) are the springs of the ballista. The ropes are made of human hair or animal sinew. When the bow-arms are pulled back, they twist the ropes. Winches pull the bowstring back.
The Ballista was a very accurate weapon, but it had to compromise its accuracy for range. The bolts (arrows) were not nearly as heavy as a 200-300 pound stones. The lightweight bolts could not gain the high momentum of the stones used in onagers, trebuchets, or catapults.
The precise design of ballistae was a triumph of ancient engineering and mathematics. The surviving manuals provide exact measurements and calculations for building ballistae of any size.
It was traditional in ancient Greek and Roman cities for women to grow long hair as a patriotic gesture in case the city suddenly required new ballistae.
Its wider development was in the Middle Ages. The catapult was an evolution of the Ballista.