The swarmlings are able to spawn in groups of three and morphs almost instantly (two seconds). This allows the player to amass a huge army of swarmlings quickly.
The swarmling still take 0.5 control per swarmling, or 1.5 per cocoon.
Swarmlings can be very handy if the player wishes to amass a large number of banelings in a short amount of time, and at slightly less cost per zergling. Their short hatching time is useful for times when the player's base is attacked while the main army is away: defenders can be made in seconds.
During Kerrigan's resurgence, Abathur discovered the swarmling strain in a feral zerg brood on the planet Astrid III. The brood's hatcheries developed the means to reduce the complexity in Zz'gashdune runner core thus allowing the production of the swarmling. The simple sequences allow three swarmlings to hatch from a single egg with an almost nonexistent gestation period. However the dorsal projection of the dune runner has resurfaced in the swarmlings.