Imago were SwarmDrones that recently matured from Juvies from inside a nest. Emerging pale and naked, they scavenged whatever weapons were at hand and charged at their enemy, lacking the sense to remain behind cover for long.
Imago possess all of the capabilities of your standard Drone, but have slightly less health and fight more aggressively, eschewing cover for rapid aggression. You don't need to prioritize them over other Drones, but you shouldn't ignore them either.
If you seal a nest with an explosive while Imago are still emerging from it, you'll kill them instantly, regardless of health. Frag grenades are your best option for this, due to their availability; always keep a few on hand for this purpose.