The Makedonian phalanx -- why such depth?

[Paullus Scipio]

Aaahhh! Found it....a reference in Justin to a passage by Didymus "He (Philip) received his third wound during the foray against the Triballi, when one of his pursuers thrust a sarissa into his right thigh and made him lame" Justin adds "... Philip was wounded in his thigh in such a way that his horse was slain through his own body"

What is not clear is whether this Triballian is a cavalryman, or peltast/infantryman, but what is all but certain ( though just possible) is that this 'sarissa' is not the two-handed Macedonian pike, but rather a single handed spear longer than a 'Dory'.....the word itself may be Thracian, and simply mean 'long spear'......