Constructor

Constructors are small devices that repair damage to all Forerunner structures they find that are 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 in Halo 3 they now fire their beams from the bottom meaning the top is now their power Supply. Sentinel Constructors also play another role, a security grid. When a Constructor is attacked, a signal causes the Sentinel Launchers to begin deploying Sentinels. They 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 harmful if they come into contact with an organism. This is unknown in the case of the Constructor, 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. Constructors are encountered only during the levels Sacred Icon and Quarantine Zone and are distantly visible on the Halo 3 multiplayer maps, Isolation and Construct, but they can be seen on The Ark (where the light bridge must be activated) and The Oracle.

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.

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.

Appearances
When they appear 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 drop ship. 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. You can also see Constructors at the beginning of the level Sacred Icon, And on Halo 3's Construct, if you look below the bottom floor, there is a small group flying around. They also appear high in the sky of the Halo 3 multiplayer map 'Isolation', above the immense walls.

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 there 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.