Heart of the Swarm- Evolution Guide: Variety Pack

Frenzy: Hydralisks gain the Frenzy ability, which improves their attack speed for 15 seconds and has a 30 second cooldown.

If you’re good at micromanaging and consider hydralisks otherwise well-balanced ranged attackers on their own, this is the option for you.  It may mean frequently re-selecting them to trigger Frenzy over and over again, but the upswing in overall damage is very noticeable and enables them to win fights they otherwise would stand little chance in.  This is a purely offensive bonus, and really means nothing when you’re using hydralisks as a static defense while you manage something else, so don’t bother unless you’re a micromanager.

Ancillary Carapace: Hydralisks gain +25% Life (+20)

This is a considerable upswing in hydralisk durability, and makes it a lot easier to operate them as your main ground force, particularly if you’d rather not bring a screen of roaches along to protect the frail ‘lisks.  This is particularly noteworthy when you’re using hydralisks as your main anti-air attackers and defenders, since many air units that can attack ground will shred through the frail things unless you protect them somehow.  This will, in turn, ease the burden on your resources of drawing up anti-air forces or protective queens to keep your hydralisks active, so it’s pretty much a good choice for stability.  While this isn’t the most overall effective improvement for hydralisks, it’s very efficient in that it doesn’t in any way alter the actual method you use with them, only their longevity in that position.

Grooved Spines: Hydralisks gain +1 range on their attack (from 5 to 6).

This is the greatest offensive boon for hydralisks, giving them a notably longer attack range than roaches and therefore letting you possibly dispense with zergling screens entirely in favor of roach screens.  It also increases their range to match or exceed that of most air attackers you will have to deal with, rendering them invaluable anti-air units, assuming you can keep them alive.  If you’re okay at micromanagement, you can usually take drastic advantage of this, drawing enemy forces closer or chewing away their health before they can actually reach your units.  It also makes them effective as a temporary air defense, supposing you intend to move them out after the enemy base supplying the attackers with aerial units.  This is possibly the least efficient of the hydralisk improvements, as the one point of extra range does not in and of itself even guarantee a single extra attack on an enemy, but the effects on large groups of units is very noticeable- especially when they stop dancing around each other to get in range of Terran basic units.