Colonization
In the Films; The Arachnid Quarantine Zone consists of many star systems colonized by the Arachnids. At a turning point of their evolution, plasma bugs developed the ability to fire spores into orbit. Utilizing a bioship star drive, the spores can travel to other systems and impact with planets, thus spreading Arachnid eggs and creating a new generation of bugs.
In the first film, Klendathu, Tango Urilla, and Planet P are the only worlds explicitly shown to be colonized by Arachnids, though an unspecified number of planets are under their control. It is also shown that they slaughtered a colony of "Mormon extremists" on a planet called Dantana. In the TV show, multiple celestial bodies, including Pluto and fictional planets, are inhabited by the Bugs.
Read more about this topic: Bug (Starship Troopers)