Arachnid

The Arachnids, also known as the Bugs, the Pseudo-Arachnids, and the Archie, are a race of insectoid organisms indigenous to the planet of Klendathu, which is the hub of their Arachnid Empire. Despite their primitive appearance, they do in fact possess technological prowess, likely in the form of bio-organics.

Differences between the film and the novel
The Bugs in the film are portrayed as generally mindless insectoid beings, ruled and organized by an extremely intelligent overmind. However, at the beginning of the film, when Rico and Carmen dissect Arkellian Sand Beetles, the biology teacher states that the Bugs have millions of years of evolution behind them and are, in the case of survival capability, the perfect species. They have the ability to colonize planets "by hurling their spore into space" and possess a social structure which fits their mental capabilities. They can also communicate with the Behemocoatyl, or the God Bug who led and guided them.

In the novel, it is established that the Bugs have spacecraft, beam weapons and other advanced technology, far from the mindless insects of the movie. The book also describes them as looking like "a madman's conception of a giant, intelligent spider." Interestingly, the book also reveals that the Bugs see infrared: though pitch dark to human eyes, the underground corridors of a Bug colony are well lit when viewed by the infrared "snoopers" used by the Mobile Infantry.

Bug society is based on a caste system in both the films and the books.

Colonization
The Arachnid Quarantine Zone consists of many star systems colonized by the Arachnids. At the turning point of their evolution, Plasma Bugs developed the ability to fire spores into orbit. Utilizing an organic 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 planet Dantana. In the TV show, Zegema Beach is on a planet known as Tesca Nemorosa.