At the shallower end of the Vampire Squid's vertical range, the view from below is like the sky at twilight: The highly sensitive eyes of deepwater dwellers are able to distinguish the silhouettes of other animals moving overhead. To combat this, the vampire squid generates its own bluish light (bioluminescence) in a strategy called counterillumination: The light diffuses the animal's silhouette, effectively "cloaking" its presence from the watchful eyes below. Its own large eyes detect even the faintest of gleams. A pair of photoreceptors are located on top of its head, perhaps alerting the animal to movements above.