Slingers and their origins in the Trajanic period

[Frank]

It is my understanding that there is no mention of formed units of slingers in the armies of the principate. I know of them in the late Roman period.

Actually I never heard about a slinger unit during the principate, too. Vegetius wrote later, that every roman soldier had a sling and had to practise it. Of course we never know, about which timeframe Vegetius is talking exactly. I doubt, most of them became very efficient slingers. But there are always soldiers which are more talented with the sling than others, and all the guys, who already learned it as child (not just Balearians, also shepherds and others).

So via integrating slingers into regular units, the romans could always build a slinger unit during a campaign, if needed. Similar like legionaries used as velites if needed and were called antesignani in this case. In Arrians battle against the Alans, the first 4 lines of his phalanx used spears, while the last 4 lines used javelins. Also the equites of the numerous cohortes equitatae were attached to the alae during a battle. Or think about the ballistiarii. They had a special optio for training and such, but were not consolidated into a special artillery unit during the principate.

It might be possible, that the romans decided for more flexibility and less specialisation for their standard units during the principate. This also explains the rise of the cohors equitata. If some of the soldiers are able to use ranged weapons, you got a kind of independent mini-exercitus.

What we see on Trajan's column are just slingers. We don't know, where they come from, or to what unit they belong to.