Hellenistic Chiliarchoi

[Paullus Scipio]

Paul wrote:-

But would those be special cases, or is there evidence that, for regular cavalry units, the traditional ilarchia either dissolved or was absorbed into the larger formations used, presumably, by the infantry: the pentakosiarchia and then chiliarchia.

As Paralus remarked, the only units which we know for sure were organised into 'thousands' were the Hypaspists/Argyraspides - the original 2000, organised into 4 pentakosiarchies were re-organised and expanded into 3 chiliarchies by 329 B.C. The Pezetairoi seem to have consistently been organised into Taxeis 2,000 strong ( or more exactly the 2,048 of the Hellenistic manuals) - the names alter somewhat for the various sub-units of the 16,384 full phalanx in Hellenistic manuals and 'taxis' becomes a 128 man sub-unit, while the former Taxis of 2,048 becomes Keras, Telos or Merarchia, but the 1,024 man sub-unit remains chiliarchia, presumably commanded by a Chiliarch.
Cavalry nomenclature is more confusing ...for example Arrian giving an 'Ile' as a unit of 64 men in his tactical manual, yet using 'Ile' in his history of Alexander to describe a squadron/hipparchia! (probably an Ile before Alexander's re-organisation was 256 troopers, and the later Hipparchia was composed of two former'Iles', and later still with the Companions organised into Epihipparchia 1,024 strong ( the equivalent of the Infantry 'Chiliarchia')......but I digress, the significant point is that neither in Alexander's day or afterward do cavalry appear organised into, or referred to as 'Chiliarchia', nor in the Tactical manuals of Aelian, Arrian or Asklepiodotus ( in which an Ile is 64 men, and the old 256 man unit is a 'tarentinarchia', Hipparchia is the 512 man unit ( again) and an Epihipparchia is the 1,024 man unit with a 'Telos' ( c.f. infantry use) of 2,048. Aelian goes one stage further, calling a 4,096 man unit an 'Epitagma' ( the others don't go beyond 'Telos').

So to answer your question, cavalry units 1,024 strong existed following Alexander's re-organisations and later, but do not ever seem to have been called 'chiliarchia', which seems to be an exclusively infantry term.....