Telepathic worms.Their bodies are comprised of multiple worm-like creatures that can form together and work as one. I believe individually, they are called Lekgolo, and when in the form of a hunter they become Mgalekgolo. The individual worms are orange, hence the orange parts under the Hunter's armor, and don the armor of a hunter to gain access to it powerful offensive and defensive capabilities. It seems that they are not "members" of the covenant as much as they are pets, or artillery.Hunters, a.k.a. Mgalekgolo, are a gestalt of smaller serpent-like creatures known as Lekgolo. By uniting into a group, they greatly increase their intelligence, strength, and maneuverability. They are nearly always seen in pairs.
Also, Lekgolo are the main components of Scarabs, so sometimes in Halo 3 you can see them inside the Scarab