They also seem to be more organized in combat than we think. Take the house of winter strike for example. Key processors are the hydras, and minotaurs are usually playing the role as communicators, especially in larger firefights. In the cave, where fallen and vex are fighting, theres a rather small force of vex fighting, with one hydra and some basic units. The main "grunts" are probably giving the hydra scenarios and equations, which it rapidly computes and sends back to the units. They probably dont want these hydras destroyed. In larger engagements, such as outside the first big ship, theres minotaurs fighting captains, grunts too. Probably a more dangerous zone for a hydra seeing as if its destroyed the main data processor for them there is out, so the minotaurs probably relay info to the hydras keeping them safe. Last part of the strike, there are 2 hydras, surrounded by grunts probably to protect them incase of attack. As we see in the next area over where the prison cell is, there are vex fighting a rather large force of fallen, and they were relaying info back to the 2 hydras just before the area. Why would they put the data processors at high risk right? The heads of vex look like satellites or antennae probably wirelessly transporting data back and forth. When you destroy the head, seems like the connection to the vex network is severed, they no longer receive orders from superiors and probably enter a state of emergency in which in their dying breath they will try to take you down with them, assuming their fate is sealed. Which in large numbers as shown by fanatics in the raid, can be a handful to deal with while the units still receiving intelligent orders can fight and have more time to fight while enemies are busy fighting vex whose new command is rush the enemy. The templar is obviously capable of massive data processing, and summons forth many many soldiers and can command them all without a sweat while hydras seem to fight in smaller divisions. For all we know, the rotating shield around hydras could be the satellite for data receiving.