From Halopedia, the Halo wiki
Due to the difference in mentality of Yanme'e compared to other species, it is possible that Yanme'e become a Major once they have reached a certain age by which point they will have a certain amount of experience, meaning less experienced Yanme'e can follow them, rather than due to distinguished service or bravery as seen in other major creatures such as Unggoy and Kig-Yar. This is similar to how older insects in sociable colonies take on different roles, such as from scout to guard.
Majors are distinguished by their red exoskeleton. Rarely, Majors are equipped with energy shielding. Majors are equipped with anti-gravity technology that is attached to their wings that allow them to fly freely. They are usually armed with Type-33 Needlers in combat. Majors are also armed with Type-25 plasma pistols and, rarely, Type-25 Brute plasma rifles.
Yanme'e Majors are more skilled in combat than Yanme'e Minors and often wield Needlers or Brute Plasma Rifles. On Easy and Normal difficulties, they do not use the Needler's super-combine ability. However, on Heroic and Legendary difficulties, it is recommended the player takes cover frequently to regain lost stamina as Yanme'e Majors do use the Needler's super-combine ability.
Yanme'e Majors normally command a group of six to ten Yanme'e Minors. Some strategies are to stick one of the Yanme'e in the group with a Plasma Grenade, bombard them with Needler or dual-wielded Plasma Rifle fire. As with all enemies who move in groups, super-combining Needler rounds will produce an explosion that will most likely kill multiple Yanme'e in the group. One can also use ballistic fire, but some Yanme'e Majors have shields, especially in the presence of Engineers, hence making ballistic fire ineffective to a certain extent. Using Noob Combo is a good tactic. Yanme'e Majors also land on the ground much less often than their Minor brethren, making them less vulnerable to melee attacks.
In Halo: Reach, however, they are represented differently, with a more unique appearance. This may suggest that the Major is actually a caste that one must be born into in order to be able to hold the rank, or it might still be representative of biological growth that allows them to be promoted. The appearance of the Yanme'e Ultra is even more drastically unique.
List of appearances