The unnatural world of monsters is fairly rife with subterranean humanoids, from goblins, ghouls and wendel to more recent C.H.U.D.s, Voormis and Morlocks. There's just something endlessly fascinating about our abhuman cousins -- a dark reflection of all we've attained in the surface world.
One of the more singular specimens of this order is a rare species of subterranean humanoids native to the dark caves of the Appalachian Mountains: the crawlers.
As explored in 2005's "The Descent," these blind creatures navigate their environment via echolocation. This is essentially a form a biological sonar. The creature emits a long shriek and the sound waves bounce back from fixed and moving objects in the vicinity. This echo tell the creature where it is in relation to the rest of its environment.
Bats provide the best example of echolocation in the natural world, but humans also learn to navigate by sound when deprived of sight. According to the BBC's Emma Tracey, the blind use both passive echolocation (dependent on naturally-occurring echos) and, rarely, active echolocation (dependent on tongue clicks emitted specifically to aid sensory awareness). The subterranean crawler simply makes more evolved use of this feature.
Descent into Troglofauna
Scientists refer to cave organisms as troglofauna, and we divide these up into three subgroups:
Trogloxenes: Creatures that use caves for shelter but don't complete their life cycle there. Bats are the best example.
Troglophiles: Cave-dwelling creatures that complete their life cycles in a cave, but they can also survive above ground.
Troglobites: These are obligate cave dwellers, so adapted to darkness that they can't survive outside their subterranean home.
Based on what we know about crawler biology, they seem more troglophilic than anything else. Granted, such morphological distinctions as blindness and pigmentation loss might suggest permanent cave denizens, but the creatures clearly exit the cave to hunt surface game.
Like the bat, they presumably hunt at night when their echolocation provides them maximum benefit. And their innate crawling ability would aid them well in the pursuit or ambush of prey amid a rocky, Appalachian landscape.
Scientists believe that most troglobitic organisms descend from troglophilic ones -- which is to say a species already well-adapted to cave life simply descends deeper into the Earth and adapts to permanent life there.
There seems little threat of crawlers following this trend, however, given the limited resources available in deep cave environments -- especially for such a large, mammalian creature. The troglobitic world is one of small bodies, low metabolisms and long, strange lives. The subworlds lack the resources to sustain such a creature, unless they contain rich, bountiful environments currently unknown to us.
And if they do, then there's no telling what other deadly things await us down there...
Monster of the Week is a - you guessed it - regular look at the denizens is of our monster-haunted world. Sometimes we'll focus on the cultural aspects, but mostly we'll look at the possible science behind a creature of myth, movie or legend. Be sure to explore the Monster Gallery as well as the Monster Science video series.