How Effective were Spears Against Cavalry?

[Macedon]

The action of topoi, perhaps? Or a suggestion that these kind of tactics were frequently used to defeat cavalry?

;-)

I would here add Plutarch's account of the battle of Tigranocerta where Lucullus attacked the cataphracts of Tigranes, who stood on a certain hill. According to him,the cataphracts did not withstand the attack nor did they countercharge but fled through their own infantry, putting it in disorder and surrendering the victory to the Romans. (Plut. Lucullus, 28.3-4)

Other infantry "charges" examples against any type of cavalry :

Polyaenus also talks of a Persian cavalry attack against 10,000 Greek mercenaries of Orontes who moved a short distance towards the advancing cavalry (Polyaenus mentions only 3 steps), whereupon the cavalry of Autophradates immediately took to flight, thinking that the Greeks were charging them. (Polyaen. 7.14.3.)

and let's not forget the attack of Caesar's 4th line reserve at Pharsalus.

Of course, actual melee would be rare, but if no tactical provisions had been taken for such a case, it could lead to cavalry actually leaving the field.