The following section contains information from a previous version of StarCraft II which is no longer valid.
You may be looking for:
Swarms were also created when zerg structures were destroyed. The number of swarms produced depended on the importance of the structure. A lair would produce over 15 swarms, a baneling nest would produce fewer, and a spine crawler or extractor would produce none at all.
Swarms had a very short life span (measured in seconds).