From Liquipedia Starcraft 2 Wiki
|Description||Fast melee creature. Can morph into a Baneling.|
|Cost||25 0 24 0.5|
|Defense||35 0 (+1)|
|Speed Multiplier on Creep||1.3|
|Speed on Creep||3.83903 (+2.2698)|
The Zergling is a small and fast melee attacker and the backbone of the Zerg army. An individual Zergling is relatively weak, but as part of a group they can rapidly surround and damage enemy ground forces. In addition, their use in large numbers helps to soak up a considerable amount of incoming damage, and they provide a shield for more expensive units.
As an inexpensive "tier 1" unit requiring only a Spawning Pool, Zerglings can be spawned very early in the match. This means that the patented Zergling rush is a threat for which opponents must prepare.
Zergling have unique spawning attributes: two Zerglings hatch from a single Cocoon, with each Zergling taking only half of one supply point. Once a Baneling Nest has been constructed, Zerglings can be morphed into Banelings at any point, even while out on the battlefield.
The Metabolic Boost run speed upgrade gives Zerglings visible wings and makes them incredibly fast, especially while on creep. The Adrenal Glands upgrade, boosts Zergling attack speed by 18.6%. Zerglings with Metabolic Boost are often referred to as Speedlings, and Zerglings with the Adrenal Glands upgrade are often referred to as Cracklings.
 Competitive Usage
Zerglings are produced in every Zerg match-up. If not used as part of an early rush or defense against early pressure, Zerglings are sent out across the map as scouts and early harassment. In the later stages of the game, Zerglings are often used as a flock to force engagements by surrounding enemy units in a battle, they are also very good for counter-attacking when an opponent attacks a Zerg, they can be rushed into an opponent's base very quickly and deal heavy damage before the enemy gets time to react and create units at home.
The Metabolic Boost upgrade is considered essential in nearly all strategies, and it is often researched as early as possible. The Adrenal Glands upgrade, on the other hand, has seen relatively less competitive usage and is generally only used when Zerglings are a central component of a composition.
 Vs. Protoss
Zerglings don't scale well against their Protoss melee counterparts, the Zealot, because as soon as zealots get the +1 attack upgrade they can kill zerglings in two hits, which is why Zerglings are seldom created in mass amounts early. However, against a Protoss rushing Immortals, or going stalker-centric build Zerglings are the perfect counter unit as their fast attacks aren't affected by Hardened Shield and they can easily surround both immortals and stalkers to deal heavy damage. In the later stages of the game, Zerglings are often used to do run-bys since Protoss units are quite slow, which can result in entire Protoss bases being razed thereby damaging their economy and tech. Zerglings also deal extreme amounts of damage in the late game with upgrades since they require very little supply, which is combined with how fast and cheap they are to spawn.
 Vs. Terran
Zerglings are very effective in ZvT due to the improved AI that allows them to easily surround their target. In the early game, Zerglings match up well against Marines and Marauders. When combined with Banelings, Zerglings provide good cover by being able to outrun the Marines and Marauders and surround them causing them to be immobilized allowing the Banelings to get up close to the Terran force and decimate them. Zerglings continue to be effective throughout the entirety of the match-up, but if the Terran player incorporates Hellions into his army, heavy use of Zerglings should be avoided because they melt extremely quickly to the Hellion's area-of-effect flame attack.
 Vs. Zerg
Zerglings are useful against Zerg for very early aggression, and therefore they are often used to punish an early-expanding Zerg. With the Metabolic Boost upgrade they can easily run past Roaches and kill off Drones or snipe isolated Queens. As more and more Roaches come into play towards the midgame, however, Zerglings lose their offensive effectiveness. This is because roaches gain a lot of armor with upgrades and at +1 attack they can 2 shot Zerglings. In addition, as Roach numbers grow the surface area of the army decreases, which causes a swarm of lings to become less and less efficient when surrounding a ball of roaches. However, because of the Zergling's high speed, they offer faster map control and scouting compared to Roaches.
- Zergling's sounds (video and sound) - YouTube video.
Both in the Wings of Liberty and Heart of the Swarm Campaign you can hear that Zerglings have a lemon juice allergy.
In the WoL Campaign this is mentioned in a news report while in the HotS Campaign this is said while going through the Zergling upgrades in the Evolution Pit.