The Semispatha and the Saex

[Renatus]

If the original semispatha was simply a shorter version of the spatha and not a gladius by another name then why did it later become a different weapon?

We habitually call this weapon semispatha (I have myself almost certainly been guilty of this in the past) but that is not the word that Vegetius uses. He refers to it twice (Veg. 2.15.4 - semispathia; 2.16.1 - semispathiis), in both cases using words in the plural. Thus, the actual name of the weapon is semispathium. This is second declension neuter, as opposed to spatha which is first declension feminine. The distinction is preserved in the Carolingian text (spata; semispatum). This opens the distinct possibility that the semispathium is not simply a smaller spatha but another type of sword altogether.