Stingers are a variant of the parasite infected marines that appear in N.O.V.A 3, and are equivalent to the "Red Fury" Troopers from N.O.V.A 2 in that they aren't heavily armored and damage to their jetpacks will quickly result in a fatal explosion.
Stingers are in essence like their ground-bound parasite infected marine counterparts. Their tentacles form a wing shape to assist their flying. They dual wield plasma pistols which they fire while airborne, much like "Red Fury" Troopers.
Stingers were humans that got infected with the Xeno parasite. It is possible that they were Red Fury Troopers before they got infected which would explain where the jetpack and plasma pistols came from.
Since Stingers are so weak, almost one shot from anything can kill them. A common tactic is to One-Shot them with the Handgun.