Quote Originally Posted by abou View Post
My theory is that the Hypaspistai were phalangites and that the Agema of the Hypaspistai were hoplites. That seems to coordinate better with their development into the arygraspides and what not.
It could also be something like this: Nobles or richer landowners who wanted to fight on foot fought as peltastai makedonoi when younger and more agile, later as hypaspists when they were more grizzled and finally as argiraspidai towards the end of their military career.

It's all assumptions of course but it would sound logical to me.