Krana (Earth-4001)

Each Bohrok contained the Krana which acts as its brain and gives it its power. The Krana, besides controlling the Bohrok, provided the intelligence to the swarms.

History
The history of the Krana is the same as their mainstream counterparts.

Biology
The Krana were created and telepathically guided by Cahdok and Gahdok, the twin Bahrag queens of the Bohrok swarms. If the queens were either not awake or had their connection servered, the Krana, and therefore the Bohrok extensively, would not be able to operate to their fullest performance.

The Krana, while the Bohrok's power, was also its weakness: even the strongest Bohrok, without its Krana, became little more than an empty shell, which could then be programmed or trained to repair the damage it inflicted. One of the main functions of the Bohrok Va scouts was to replace a Bohrok's missing Krana. If one felt threatened, it could eject from its Bohrok host; this left the host powerless, but if the Krana attached onto another being's face, it took control of that being's mind and assimilated it into the swarm.

Types
The eight types, which follow, represent a rank in the swarm:
 * Xa: Swarm Commander. They lead their Bohrok on their mission. Bohrok Xa command a single swarm.
 * Za: Squad Leader. They have the ability to communicate telepathically with other Krana of the swarm and lead squads comprised of the remaining 6 ranks below. There are several Bohrok of this rank so that the swarm can be divided into smaller groups of 6 other ranks led by a Bohrok Za, thus providing the right Krana for any situation in all areas subjected to their search protocols. This Krana controlled Lewa and Turaga Matau for a period of time.
 * Vu: Surveyor. Bohrok of this rank can fly short distances ahead of the swarm to confirm that the path is clear.
 * Ca Clearance. They have strong defense shielding; these Krana spearhead the swarm's mission.
 * Yo: Mole. They use their Bohrok to tunnel through anything on the island.
 * Ja: Scout. They have radar-like senses that detect distant obstacles in the path of the swarm and can help make a swarm aware of possible danger or a hold up.
 * Su: Worker. They have colossal energy and inhabit the strongest of Bohrok. They are always around whenever brute force is needed.
 * Bo: Sentinel. They can see in the dark and serve as night-time lookouts and lead their Bohrok into the darkest places on Mata Nui.

Krana-Kal
Krana-Kal were specialized Krana, mutated by the Bahrag's energy and a mutagenic substance. Like regular Krana that were literally the brains of the Bohrok, the Bohrok-Kal were controlled by the Krana-Kal. The Krana-Kal were unlike the other Krana in that they could act independently, without depending on the Bahrag, and were able to communicate with other beings using telepathy. Krana-Kal also had a unique property that allowed them to turn from a different metallic/opalescent color to a high polish silver color and generate an impenetrable shield around their hosts when at the final stages of their mission.
 * Xa-Kal: Liberator. The most important of the Krana-Kal, one of these needed to be brought into physical contact with the Bahrag to complete the Bohrok-Kal's mission.
 * Za-Kal: Overseer. They could communicate telepathically with other Krana-Kal and had the limited ability to read minds and sense strong emotions.
 * Vu-Kal: Transporter. They can move its Bohrok-Kal host over the surface or through the air at great speed.
 * Ca-Kal: Seeker. They are linked to the Bahrag and could sense their presence within a limited range.
 * Yo-Kal: Excavator. They can sense stress points in the earth and detect underground movement.
 * Ja-Kal: Tracker. They can detect distant obstacles and enhance all of the senses of the Bohrok-Kal, especially hearing and smell.
 * Su-Kal: Demolisher. They can provide tremendous strength, endurance and resistance to heat and cold.
 * Bo-Kal: Visionary. They can see in the dark and tunnel through any substance except for the floors and walls of the Mata Nui underground.

Color
Each breed of Bohrok wore a different colored Krana. The Bohrok's Krana color corresponded with their colored eyes:
 * Blue: Tahnok
 * Orange: Gahlok
 * Red: Lehvak
 * Light Blue: Kohrak
 * Green: Pahrak
 * Yellow-Green: Nuhvok

Dormant
The colors of the Krana, while dormant and carried by a Bohrok Va, were different:
 * Yellow: Tahnok Va
 * Purple: Gahlok Va
 * Dark Grey: Lehvak Va
 * White: Kohrak Va
 * Tan: Pahrak Va
 * Black: Nuhvok Va

Trivia

 * Coincidentally, the Ca breed of Krana was similar in appearance to the Kanohi Hau, which also had the power of shielding.
 * The Krana would work just as well as a Kanohi on a Matoran's face to keep them awake and alive; however, it also means that the Krana may be able to easily assimilate the Matoran if the Bahrag were awake.
 * In the Metru Nui Archives, Toa Metru Whenua nearly knocked over an exhibit of Krana.
 * The Krana each have a three letter code, written in Matoran on the back. These codes have no story line importance and do not spell any real words.
 * The Krana controlling Matau and several Le-Matoran were colored like a Gahlok's, despite the Lehvak corresponding with his village, and the Nuhvok being seen attacking it.