The evening primrose is a siege engine. It is a kind of giant catapult that was used by the Romans.
Its name comes from the analogy of its movement with that of the kick of an onager, a kind of wild donkey.
According to the Dictionnaire raisonné of French architecture from the 11th to the 16th centuries (volume 5), Roman historians all agree to rank the onager, like the catapult and the scorpion, among the offensive jet engines, but their descriptions are either succinct or contradictory:we find the term onager as a synonym of scorpion in Marcellin (6th century) or onager as a machine throwing stones (as opposed to javelins) in Végèce, or onager as a vulgar synonym of catapult at John the Lydian.
Some describe it as a small catapult capable of sending small projectiles 30 m away or 40 m high, others as a giant crossbow.
Johann Silberschlag in his Dissertation on the three main war machines of the ancients, namely the catapult, the ballista and the onager (Berlin Academy of Sciences 1760) analyzes with precision the texts of Vitruvius and Marcellin. He then distinguishes the machines intended to throw lines (catapults) and those intended to throw stones (ballistae). He then places the onager rather in the family of triggerfish from which it differs on a few points (p 432). He describes it as a wooden lever arm at the end of which is a spoon. This lever arm is stretched, and when the tension is released, the lever arm draws an arc of a circle and strikes against a stopper while the contents of the spoon fly into the air. He confirms that the evening primrose was used a lot, with or without a wheel, during the wars of Julius Caesar. He supposes he could throw hailstones, flaming projectiles and even corpses