From SC2Pod
The Scourge serve as kamikaze-attack flying units, capable of causing 110 damage to aerial targets. Like Zerglings, Scourge spawn in pairs from a single Larva and can be produced in mass numbers in a very short period of time.
Lore
In stark contrast to the lumbering Guardian is the tiny Scourge. These blind terrors seek out enemy starships and dive into them. Catalytic agents found within the body of the Scourge cause it to explode like a living plasma bomb when it smashes itself against the hull of a larger vessel. Large numbers of these creatures can cripple a squad of fighters or even a Battlecruiser. Like the Zergling, the simplistic genetic makeup of the Scourge allows two of them to hatch from a single egg.
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StarCraft 2
The Scourge's fate was unclear for a while, but they have been scrapped from StarCraft 2. The Corruptor is now the Zerg's primary air to air unit.