Constructor Sentinel

Constructors are small machines that repair Forerunner structures that appear to be damaged. The Sentinels will come to their aid if they are attacked, as they have no weapon systems of their own to defend themselves. Since the Construction Beam is fired from the top half of the Constructor, the bottom half is most likely where the power supply is held, but the constructors on The Ark fire their beams from the bottom meaning the top is now their power supply. Sentinel Constructors also play another role as a security grid. When a Constructor is attacked, an unknown signal causes the Sentinel Launchers to begin deploying Sentinels. Constructors form an excellent security perimeter.

Role
Constructors are devices similar to Sentinels in design, that mainly repair damage to any Forerunner structure they can find, using weak energy beams similar to Sentinel Beams. The beams are small and blue, and are harmful if they come into contact with an organism. This is unknown whether they will harm the Constructor itself, though.

The Sentinels will come to their aid if they are attacked, as they are ill-suited for combat, having no weapon systems of their own; however, their beams will damage you if you get caught in the path (on Legendary difficulty their beams can kill in seconds). They also use their beams to lower the shields on Pistons, allowing them to move throughout the Sentinel Wall quickly.

The Constructors first appeared in the beginning of The Library although only one or two can be seen floating around the corridors and into the air shafts. They also appeared in the opening moments of "Sacred Icon", where a swarm of them flies about trying to fix the platform you start on, then moves away further into the Sentinel Wall. They don't really affect anything in game, they show red on your reticule and you may shoot them if you like, although they won't bother you. If you jump in front of one in the beginning of "Sacred Icon", you can get hurt, though very little, by their thin beams depending on the difficulty level you are playing on.

Appearances
In Halo 2, in the Quarantine Zone level, they appear at the end of the level when the player fights Flood-controlled Scorpion and Wraith tanks along with many Flood using Covenant Ghosts and a Spectre, and two Sentinel Enforcers fight against the Flood near a downed Human Pelican dropship. During all this chaos, if the player watches one of the large and deep trenches, a large cloud of Constructors fly off into the sky. Constructors can also be seen at the beginning of the level Sacred Icon.

In Halo 3, Constructors appear in some levels based on the Ark. In campaign, they appear in the level The Ark, where they are seen inside Forerunner structures including inside the wall to the Cartographer and the map room itself. In multiplayer maps Isolation and Construct, small groups of Constructors can be seen flying around in the distance. Also, in Halo 3, a small Easter egg can be witnessed if a player watches one of the groups of constructors on the multiplayer map Construct. They will fly normally for a while, but when they near a wall, they will hop like bunnies, before passing out of the map, and to a destination unknown, possibly re-spawning on the other side. You can destroy them and the Sentinels in the level.

Trivia

 * Some Sentinels have been seen using the Construction Beam in the level The Arbiter.
 * They look similar in appearance to the Covenant Plasma Pistol, though whether or not their appearance was the inspiration for the Plasma Pistol is another matter entirely.
 * In the Halo 2 level Sacred Icon, Constructors will open the Pistons for you if they are near them.
 * On the level Sacred Icon, where you find the Grunt and the Jackal, if you look off the side into the chasm you will see an endless stream of constructors coming out of a bright hole in the wall.
 * On Isolation and Construct the Constructors can be sniped, though their small size and constant movement make this difficult.
 * In Halo 2 and Halo 3, even though Constructors do not attack you, your weapon reticule will turn red when passed over them.
 * A player can get up close and even touch a Constructor on Construct. First he/she must overload the map with trip mines. When the map is overloaded the player must go through the hole where the purple lift beams are projected. A player then can easy be touched by a Constructor but cannot be moved. They can be destroyed, mainly with a Spartan Laser or a Flamethrower. Note that even when one is destroyed another immediately replaced it in the same place. So in other words, you will see the debris from a destroyed Constructor fall from a full and complete one.
 * In Halo 2, the Constructor's beam light is on the top part of its structure, whereas in Halo 3 the light is on the bottom half.
 * Constructors appear to be able to compact into a smaller form, as their top and bottom halves appear to latch together. However, this has yet to be seen.
 * On the level Sacred Icon, Constructors will scurry away from you if you approach them. If you attack them though, Sentinels will attack you.