From Warhammer 40k - Lexicanum
|Previous Edition Warning: This article discusses a topic from a previous edition of Warhammer 40,000.|
Its canonicity is in question but was once considered canon.
In 4th Edition, Spore Cysts are found on larger Tyranids and are excretion pits for the creation of Spore Mines. The symbiotic relationship between creature and spore cyst is such that they cannot be separated once formed.