Zerg

The Zerg are the second creation of the Xel'Naga, after the Protoss.

History
The planet Zerus housed a small, worm-like creature, which was perfect, as the Xel'Naga were looking to give the lowest life-form "Purity of Essence", and finding the Zerg was, to them, a godsend. They changed the Zerg from an earthworm-like creature into a parasitic "worm", which supercharged the hosts' evolution. Wishing to avoid an earlier failure (specifically, the Protoss), they imposed a hive mind on their new creation. This "Overmind" soon turned against them, and almost completely eradicated or consumed them. With the consumption of Xel'Naga, the Zerg were able to learn of the Protoss, and decided to assimilate them, to become truly perfect. However, they needed a weapon. They settled on the Terrans, who were evolving into a close second to the Protoss.

The Zerg possess the ability to terraform any planet or space platform they land on through a thick mat of gray fungus called "Creep", which spreads out from Hatcheries, Lairs, Hives and Creep Colonies, the latter of which can be mutated into defense strains. Each structure in a Zerg Hive Cluster acts as an organ in a huge super-organism, some of which have spread out to cover entire planets or platforms.

Larvae
The Larvae is spawned from the Hatcheries, Lairs, and Hives. Each larvae carries the genetic code for every Zerg strain. At first, the larvae can only mutate into Drones and Overlords, although each new organic building adds more genetic information to their repertoire thanks to the hive mind and super-organism structuring of a Zerg Hive Cluster.

Egg
"Egg" is a misnomer: it is in fact a pupae. Larvae will enter this stage once they have received a command to mutate into one of the many strains of Zerg.

Adulthood
The mutated Larvae is now combat ready. Certain strains are for melee, ranged combat, or air support while others are solely meant to produce horrible poisons. Only the Hatchery/Lair/Hive and Queen carry out any reproduction. Very few Zerg have ever lived out their natural lifespans, as they are either killed in battle, or potentially even immortal. The few exceptions are simply meant as distractions.

Varieties
There are seventeen combative sub-species of Zerg.
 * Drone, responsible for collecting Crystal and Vespene, and construction.


 * Overlord, radiates Psychic energy to control the other Zerg with. It can be evolved into a transport class unit.


 * Zergling, evolved from small lizards, now the primary warrior strain. Spawned two at a time due to their simplistic genetic structure.
 * Baneling, mutated from the Zergling. It is a small, spherical suicide unit. (StarCraft 2 only)


 * Hydralisk, large snake-like beast, produces spines that are hurled with enough velocity to puncture battlecruiser grade armor.
 * Lurker, mutated from the Hydralisk. It burrows and sends a row of vertical spikes into a target.


 * Mutalisk, primary air combat unit. Each shot is a disease, which spreads to two other targets.
 * Guardian, a bomber creature mutated from a Mutalisk. Can only attack ground targets.
 * Devourer, mutated from the Mutalisk. Can only attack air targets.


 * Scourge, a small bat-like suicidal aerial unit, spawned two at a time due to their simplistic genetic structure.


 * Queen, a large beast that originally resembled a sort of flying octopus, although they now move about along the ground for better protection at the cost of mobility. It is used to sow the hyper evolutionary virus, which in turn creates the Infested Terrans.  Its only combat ability is spawning a pair of Broodlings on a ground unit.
 * Broodling, a small, insectoid being that is fired from a Queen during its egg stage. It uses the targeted organism's DNA to fertilize it, and it is quickly born thereafter in an explosion of gore.  They are unstable, however, and cannot live for very long.


 * Defiler, a rove beetle-shaped creature (specifically, a devil's coach horse beetle. It carries the gene codes of every Zerg, which it uses to produce carcinogenic poisons.


 * Ultralisks, a large elephantine beast. It is classed as a siege unit, although it is a melee attacker, due to its enormously destructive potential.


 * Infested Terrans, a "zombified" suicide bomber. It produces chemicals that are not only horribly toxic, but explosive as well.

Post Brood War Experimental Strains
These Zerg may or may not show up in SCII, and will show up in its campaign editor
 * Nydus Worm, a long worm-like organism that erupts from the ground and drops off other units.
 * Infester, a frog-like Zerg that can infest Terran and Protoss buildings.
 * Infested Marine, the evolution of Infested Terrans. They have a poisonous attack instead of its predecessor's destructive suicide attack.  However, the alteration forces them to gain a limited lifespan.
 * Lair Watcher, an evolved Queen that can tunnel to another point on the battlefield.
 * Hive Matriarch, an evolved Lair Watcher that can drop base defenses and poisonous Creep to make it dangerous to traverse.
 * Corrupter, a flying squid-like organism that mind-controls other air units.
 * Overseer, an evolved Overlord with detection capabilities.
 * Roach, a melee attacker with a supercharged regeneration ability.
 * Swarm Guardian, a Guardian bomber that spawns Broodlings every time it attacks.

Hive Mind
The Xel'Naga were concerned that the individuality of each Zerg would cause a war, and therefore they chose to impose a hive mind on their creation, which became known as the Overmind. Later, the Zerg were taken control of by the infested remains of the former human, Sarah Kerrigan, and the Overmind was obliterated.

Hierarchy

 * Overmind/ Infested Kerrigan
 * Cerebrate
 * Overlord
 * Minions