These creatures were created by the Geonosian queen Katrina was a way of taking about their planet by reviving the dead under her control. After reviving the dead Geonosians in her catacombs she ordered them to infect more Geonosians and kill the Jedi and Clones that remained on the planet. After successfully taking back the planet a, dispatchment of the Grand Army of The Republic was sent to back the planet which they later did. When Jedi Knight Aayla Secura found out and destroyed the catacombs and thinking she stopped the brain worms and the queen. A brain worm made it out and infected a clone trooper who later started the Worm Wars.