Heart of the Swarm- Evolution Guide: They’re Everywhere!

Carapace: Your zerglings gain +10 Life.

In any other unit, a +10 life difference would be anything from a bit small to pointlessly immeasurable.  Zerglings, however, come in large numbers and don’t have much life to begin with.  This evolution can make a huge difference in how well your zerglings withstand area attacks, especially from colossi, siege tanks, flamethrower units, and effects like Irradiate.  It may seem a bit counterintuitive to make what is usually considered a fairly disposable unit into something more durable, but it can mean the difference between zerglings doing nothing and zerglings mutilating a defender, so it’s worth consideration.  This works especially well with raptors, who combine the durability with their pounce capability to ensure they survive into melee; but it still grants a large boost to swarmlings provided your enemy doesn’t really overload on area of effect attacks.

Adrenal Overload: Your zerglings attack 50%  faster.

This is essentially the mirror image of Carapace- you will like this if your zerglings manage to survive into melee range already.  The attack speed boost means huge damage gains, especially against units with high armor or special shielding, like thors and immortals.  While it’s useless if your zerglings cannot reach their targets, it makes swarm tactics incredibly deadly against enemies who cannot sweep the zerglings out fast enough.  This combines well with either raptors or swarmlings, although raptors are generally more likely to reach their targets to begin with.

Metabolic Boost: Your zerglings move 60% faster.

As with Carapace, Metabolic Boost helps to ensure that your zerglings will actually reach their target.  The drawback here is that they can’t move at any lower speed, so they immediately become more difficult to manage in tandem with your other units.  This shows especially much with raptors, who will often find themselves leaping in and mostly getting cleaned out of the area before your other Zerg can actually arrive to take advantage.  If you can manage the altered speed well, however, it becomes a boon.  More importnantly, this goes excellently with swarmlings, who will often clutter the field so badly that they can’t reach anything around each other.  The boosted movement rate will help this and will also mean a much easier time taking advantage of the superior numbers they provide in a more obvious fashion, letting you strike with them more successfully.  Again, this can be an issue when you try to use them in tandem with other units, but if you’re good enough at the micromanagement, you can handle that easily.