Pirahna mites were an invention of Omnius he devised during the siege of Corrin. He sent missiles containing "dumb" programming (not depended on gelcircuitry) so they would pass through the scrambler field satellite network. The missiles landed on Rossak and Salusa Secundus. In Zimia, Abulurd Harkonnen was the first to notice that the mites had purely optical target recognition, as well as that the mites assimilate metallic objects for their factories, never organic.