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.
The swarm host creates 2 locusts manually, with a cooldown of 60 seconds. It is not autocast, but can be used whether the swarm host is burrowed or not. 
Orders flying locust to land at target location, so it may attack enemies.
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.
In Legacy of the Void, locusts can be spawned every 60 seconds. Spawning locusts is no longer auto-cast, and no longer requires the swarm host to be burrowed. Locusts are also able to fly, and must be ordered to land at a targeted area using "Swoop" in order to attack.
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.
Enemy units have a chance to spawn friendly Locusts upon death.