The larva is the base zerg unit from which all other units and, ultimately, buildings are created.
Larvae are the closest creatures to the original zerg insectoids. Although their size and toughness were greatly boosted by the xel'naga during their experiments, they still possess the two traits that originally intrigued the ancient masters—genetic versatility and psychic sensitivity.
Larvae were the first zerg strain ever encountered by the terrans of the Koprulu Sector. Around 2487, larvae were discovered on a number of fringe worlds. These worlds were depopulated by "cholera" and the specimens shipped to a Confederate science facility on Vyctor 5, only to be discovered by the Sons of Korhal nearly five years later.
Maggot-like, larvae often make a sucking sound from their tail section, akin to someone's boot stuck in mud. Physically, they have a segmented, multilegged body, trailing a mucous-like substance behind them and a complex array of internal organs. Every segment of the larva's body contains all the required organs to ensure its survival. Such a trait, combined with near instant wound clotting, allows larvae to function and morph normally even if cut in half. Its small twin antennae probe the air around it. Diminutive larvae exist within the bloodstream of the swarm host, where they feed on damaged tissue, sealing wounds, and destroying foreign matter. They can also head for the creature's incubator to be evolved into locusts.
The need to survive has played a role in the evolution of the larva's carapace. Its plates have turned into a hard, dense material that is not easily scratched, let alone punctured, and is immune to small arms fire. Dominion scientists suspect that the carapace served as a template for enhancements to the ultralisk.
Similarities exist between a larva's pluripotent cellular structures and terran embryonic stem cells. Project Blackstone researchers attempted to find a way to both arrest the obligatory asymmetric replication (the astounding healing factor) of the zerg, and adapt it (in some measure) for Dominion soldiers.
Within a larva's DNA are countless sequences, carrying billions of patterns and nearly infinite possible genetic combinations, and they contain within them the genetic code for all other zerg breeds. When a new breed of zerg is needed, an individual larva will enter a pupal state and begin replicating the DNA of one specific zerg breed at an astounding rate. After a short gestation period, the new mature zerg minion hatches. A young hive will only have the genetic code for the most basic of zerg breeds, such as the drone, but as it grows and develops new structures, its library of genetic strains is expanded. With a command from the Overmind, cerebrate, or overlord, the larva will enter a pupal state and begin the metamorphosis into whichever breed is required by the hive. The inner layer of the creature's skin is populated with receptor cells, and once a morph command is issued, the larva's organs are ejected, forming a hard egg-like cocoon, and its tissues are instantly reconfigured into the desired strain through a brief pupation period. Because of the larva's metamorphic abilities, the Dominion stresses that marines be obsessively thorough when carrying out zerg eradication operations. Just one surviving larva can mutate into a drone, which in turn can mutate into a hatchery, and thus give birth to a reborn zerg colony.
Concept art for larvae appears for StarCraft: Ghost. As seen, the larva appears larger and more adept at battle, carrying a likeness to a Hydralisk, albeit missing arms.