From: Star Trek: The Next Generation

Caption: Devidians

Star Trek: The Next Generation

Episode: TNG 226 - Time's Arrow, Part I

Planet: Devidia II. A shape-shifting race who exist in another "phase" but use time travel in the visible world to collect human life energy from the past as their nourishment. Visible only to those viewers or mechanical scanners that have matched their synchronic distortion of +0.004%, they are silver-gray in color and range from 2 to 3 meters high, with four limbs, no noticeable eyes or ears, and a single orifice -- located where a humanoid's forehead would be otherwise -- where energy fragments are ingested for nourishment.