From Halopedia, the Halo wiki
Being more powerful than the other common type of Promethean Crawlers, Alpha Crawlers lead packs of lesser Crawlers. They are distinguished from other Crawler subtypes by the array of glowing hard light spikes, which indicate status, on their carapaces, and are armed with a built-in Z-130 Suppressor. While they are designed to lead detachments of varying sizes, they can be found in excess of three or four per pack. They nevertheless serve the same purpose, to lead, augment, and fortify localized Crawler forces. They are easily the most recognizable of their kind. By the end of 2558, Alpha Crawlers had taken on a white body color with bright red highlights while serving in the Created, led by Cortana and the Warden Eternal.
Alpha Crawlers always attempt to maintain a safe distance away from threats, while using their attached Suppressor to unleash a barrage of firepower upon their opponents.
Dealing with Alpha Crawlers in combat is very dangerous, due to the fact that they will often continue to fire on enemies from a distance to overload shields and will follow up with close-in melee attacks to keep enemies' shields from recharging.
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