Pausanias on Achaean armament, ca. 200 BCE

[Paullus Scipio]

Inaki wrote:-

In fact, Macedonian and Seleucid Peltasts in Polybius are named in a number of occasions together with phalanx, clearly implying they were not part of the phalanx.

...you have tried to put this hypothesis forward before, notably in your Magnesia article in AW2, and in your letter in AW4, but the evidence for these Hellenistic Peltasts being pike armed ( at least in pitched battles) close order armoured infantry is very strong indeed, one may say almost incontrovertible. It is perhaps notable that the only times Polybius mentions 'peltasts' in the Seleucid army they are on one occasion described as leading an assault through a breach in a wall (10.31.11), and on the other as being 10,000 strong (10.49.1).(almost certainly comprising the elite, regular, half of the phalanx) Elsewhere Polybius tells us that the elite portion of the Seleucid phalanx was 10,000 strong and that most of them were called Argyraspides.(Silver Shields, successors to Alexander’s Hypaspists – the Guard Infantry.) It seems likely that these men were 'Peltasts' in exactly the same manner Antogonid pikemen were 'Peltasts'. Macedonian armies elite bodyguards were not called ‘Silvershields’, because of the grudge between the original ‘Silvershields’ and the Antigonids.
Evidence for the Macedonian ‘Peltasts’ being phalangites is strong. In the Aetolian campaign of 218 BC, Philip V leaves them behind with the ‘heavier’ part of the Army, while the mercenaries are sent into ‘difficult’ terrain. We hear (in Polybius II.65.2) of 3,000 such men at Sellasia, and elsewhere, 2,000 in the Peloponnese – who may not be the full strength (Polybius X.42.2). 'Peltasts' are protected as part of the phalanx (Polybius V.7.11). At Polyb,IV.75. the Peltasts are distinguished from light troops ( euzanoi), at Polyb. V. 4.9. they are used as elite shock troops to storm Kephalonia.
You have referred to Livy mentioning the 'Peltasts' and phalanx separately as evidence that they must have been separately armed, ( which does not logically follow) but again this is selective use of the evidence and fails to take into account the whole. When Livy uses the word caetrati in reference to these troops it is a direct translation of ‘Peltast’ ( that caetrati are peltasts is clear from Livy XXXI.36.1). Further these Peltasts contain the Agema, or Royal Bodyguard (Polyb V.25.1 c.f. Livy XLII.51.4 – 2,000 of them form the Agema). It is clear they are ‘heavy’ ( e.g. Polyb V.23.4, where Lycurgus Spartans, having got the better of Philip’s mercenaries, give way and run in the face of the 'Peltasts', who are ‘heavier troops’).
At Pydna, Livy specifically refers to the fight between the 'Peltasts' (drawn up on the right flank of the phalanx) and the Paeligni (Roman Allies) as an example of the risks of fighting sarissa armed troops head-on, and Plutarch also describes a victory for the sarissa/pike, referring to this fight. Plutarch also refers to a force of ‘picked Macedonians’ who are almost certainly the Agema (elite bodyguard) of the 'Peltasts', because the word he uses is derived from Agema.
Furthermore, there has been a consensus on this for well over a century among scholars, that the 'Peltasts' were an elite unit, sarissa/pike armed heavy infantry ( with the possibility that like Alexander’s sarissaphoroi, they carried javelins/longche when not in pitched battle) – see e.g. Griffith, ‘Hellenistic Mercenaries’, Kromayer and De Sanctis.
As I said before, in Hellenistic times, ‘Peltasts’ in Macedonian, Seleucid or Achaean armies generally means elite pike-armed ‘heavy’ close-order Infantry.