Locusts are short-lived, ferocious, parasitic symbiotes. Powerful for their size, they are spawned from the millions of larvae that inhabit the swarm host's bloodstream. When the swarm host is threatened or engaged in combat, subtle chemical signals instruct the larvae to head for its incubator, where they are evolved into locusts.
Originally, locusts attacked any nearby enemy with volleys of acidic saliva. Since evolving pressurized air glands, locusts can attack aerial targets, and have developed the ability to fly themselves.
Swarm hosts enslaved by the Cerberus Program were enhanced with microfactories that equipped spawned locusts with remote controlled headgear.
The swarm host creates 2 locusts with a cooldown of 60 seconds. It is not autocast, but can be used whether the swarm host is burrowed or not.  Locusts are also able to fly, and must be ordered to land at a targeted area using "Swoop" in order to attack.
Upgrades and Abilities
Allows locusts spawned by swarm hosts to fly. Flying locusts can use Swoop to land and attack units.
Locust production inefficient. Will greatly benefit from redundancy removal. Sequences are complex, wasteful. Byproduct of wild, uncontrolled evolution. Simplification of sequences will allow swarm hosts to incubate full-grown locusts at rapid speeds.
Swarm host defense reliant on locust organism. Locust secrets solvent fluids, projectiles towards enemy. Can mutate glands, increase pressure. Will allow projectile to reach incredible height. Attack both ground and aerial targets.
Locusts are available to Abathur in Co-op Missions. They can be spawned via his swarm hosts, and can be upgraded at the infestation pit. In addition, Abathur unlocks an upgrade that gives him and his ally the random chance to spawn locusts from the bodies of dead enemies at Level 13.
In early development builds, locusts were named "swarmlings."
Locusts were originally melee units. They had 110 hp, dealt 14 damage, and could be spawned every 24 seconds. By 2012, their stats were altered and they gained a ranged attack which could also target air units.
Balance Patch #1 of the Heart of the Swarm beta decreased their damage from 16 to 14. Balance Patch #2 decreased their attack range from 3 to 2, while Balance Patch #3 reverted this and further decreased their damage from 14 to 12. Patch 2.1.9 increased attack speed from .8 to .6 and added the Flying Locusts upgrade. Locust duration also increased to 25 seconds and the Enduring Locusts upgrade was removed.
Flying Locusts had to be researched at the infestation pit up until the beta version of Legacy of the Void, where locust health was also reduced from 65 to 50. Patch 3.8 increased the range of Swoop from 4 to 6, and reduced weapon damage once more from 12 to 10.
Increases the lifespan of locusts by 10 seconds (to 25 seconds). This upgrade was removed from HotS in the April 2015 update, as the locust's base lifespan was increased to 25 seconds.