The Lurker is a caste of Xenomorph very similar to the Drone. Lurkers are easily identified by their usually smooth head carapaces. Unlike the far more common Warrior caste which tend to employ direct assaults and swarm attacks, using sheer weight of numbers to overwhelm defences and subdue their prey, Lurkers prefer to keep their distance and lurk in the shadows, only attacking when the time is right.
It also (if *anonymous* is correct) is undetected by motion trackers. and crawls on both two and four legs equally