From Halopedia, the Halo wiki
Crawlers are capable of both ranged and close combat, with the standard one being armed with a Boltshot, an exotic directed energy pistol that functions as a close range precision weapon and a particle dilator. Crawlers also travel in large packs and work together to destroy enemies using skirmishing tactics.
Crawlers appear as one of three types: the standard one, the Alpha Crawler, and the long-range Crawler Snipe. The standard one is armed with a Boltshot, which it always fires in two-round bursts. The upgraded Alpha Crawler leads lesser Crawlers and fires bursts of Suppressor rounds at its enemies. Finally, the long-range Crawler Snipe is armed with the deadly Binary Rifle, which can instantly kill players on Legendary difficulty, regardless of where it hits.
Crawlers are capable of scaling walls, ceilings, and other steep surfaces and leaping great heights to get a tactical advantage against their enemies. This behavior is commonly observed in all campaign and Spartan Ops levels that feature Crawlers, although it is most prominently featured in the campaign mission Forerunner as part of introducing Crawlers to the player. Watchers can also summon new Crawlers from raw materials on the ground. When Crawlers are damaged, parts of their carapaces start falling off.
At close range, Crawlers can pounce onto a player. The damage done to the player is usually small to medium, but can be increased further to the point of draining the player's shields if the player moves towards the Crawler while it pounces at them, even on Easy difficulty. However, Crawlers also tend to be cowardly, sometimes running away from the player. This leaves them open to being assassinated.
Crawlers are fast-moving enemies that are otherwise easy to kill. They die to a single headshot from any precision weapon, including the Boltshot, so players fighting Crawlers from a distance should snipe them with a precision weapon to minimize the trouble they can bring upon them. In addition, Crawlers tend to die from one or two melee attacks, regardless of the type. If the player is surrounded by Crawlers, the player should have a fully automatic weapon ready, like the SAW, Suppressor, or Assault Rifle.
Alpha Crawlers are slightly harder to kill due to their Suppressors, which they fire in medium-length bursts more accurate than when used by players, Knights, or Elites. However, the Suppressors they drop usually come with a lot of ammunition ready, meaning that Suppressor ammunition should not be an issue if the player feels the need to use it.
Crawler Snipes are the most dangerous of the Crawlers. As previously stated above, Crawler Snipes can instantly kill the player with their built-in Binary Rifles on Legendary difficulty, regardless of where the weapon hits. The most efficient and effective way to destroy Crawler Snipes is to use a long-range precision weapon (e.g. LightRifle, Covenant Carbine, Binary Rifle, etc.) and fire from afar without the Crawler Snipes discovering the player's location. Sidearms with mounted scopes like the M6H magnum are also effective at killing them, provided that the player aims at the head. Crawler Snipes are, however, the rarest type of Crawler; in the campaign, they only appear in Infinity and Midnight.
Halo 5: Guardians
Changes from Halo 4 to Halo 5: Guardians
Crawlers are now more powerful to compensate for Halo 5: Guardians's increased mobility, as standard Crawlers' Boltshots now exhibit tracking behavior except on Easy difficulty and Alpha Crawlers now fire their Suppressors much more slowly and in small bursts for better accuracy. Despite this, they are still just as easy to kill, as they can no longer climb walls or steep surfaces. In addition, Watchers can no longer summon new Crawlers. Fully-shielded players moving towards a Crawler as it pounces at them can no longer lose their shields even on Easy.
In the campaign, Crawlers appear on all missions featuring Promethean enemies except Enemy Lines.
Warzone and Warzone Firefight introduce Legendary and Mythic variants of Alpha Crawlers. They all have the same armament but are much more resilient to enemy fire and are much more powerful. Legendary Crawlers are colored orange with turquoise highlights while Mythic Crawlers are colored black and maroon with red highlights.