Argyraspides are a tough unit of elite veteran pikemen, better armed and armoured than lesser pikemen. They are usually used to hold the center of a Makedonian or Seleukid battle line.
Can Form Phalanx
The veteran Pezhetairoi are often called Argyraspides ("Silver Shields") in Makedonia, while in Arche Seleukeia they are drawn from the sons of mainly Makedonian military-settlers, primarily from the Katoikiai, or military settlements, of Syria. These are tough men often better armed than their fellow phalanx soldiers, who usually are used to hold the center of a Makedonian or Seleukid battle line. They are older and more disciplined than other Phalangitai and are thus able to hold their own against good odds. They are armed and armored much the same as regular Pezhetairoi, but with slightly better equipment and weapons, including scale reinforcement on their Linothorax. As with most elite units, they are best utilized as troops that take the brunt of an assault and can be counted upon to hold their ground.
Historically, the Silver Shields were an elite force within the armies of Alexandros, made up of the veterans often in their mid-40s and upwards. After the death of Alexandros, the term "Argyraspides" came to describe the veterans among the Pezhetairoi, the soldiers aged 45-60. How this happened is anyone’s guess, but it is what came to pass. In Makedonia the term was used to describe the veteran Pezhetairoi who more often than not weren’t a distinct corps, while in Arche Seleukeia they became the royal Guard, and carrying a silver shield was a mark of the elite. The Seleukid Argyraspides seem to have been kept at a constant strength of around 10.000 men. How they were recruited is disputed, but one theory says they were sons of military settlers, who served in the royal Guard while their fathers tilled their land allotments, which were hereditary. This also had the added bonus of making every Argyraspis know who the true king was and where his loyalties were to be. The Makedonian Argyraspides eventually “died out” and was replaced by the reformed Pezhetairoi, while the Seleukid Argyraspides evolved into an assault infantry corps, the Thorakitai Agematos Basilikou, although one source suggests that the 10.000 man Silver Shields corps was divided into two halves, where 5000 men continued to fight as Phalangitai and the other 5000 were armed in the new “Thorakitai” style.