What it is
A brood hangs as grimed, ribbed capsules with no light of their own. A living body close by, or a flame in clear view such as a torch, warms them. They shiver, split, and wake their neighbours. A hatchling then scuttles at the nearest warm body and leaps at the face.
On your face
It gnaws and sucks, and the sound of the world goes muffled. Close your hand on it and pull it away from your head and it rips off. Torn off, it crawls up the rock, hides dark on a wall or ceiling, and creeps back to try again.
Dealing with it
Any blow knocks it off, and a blade held across its leap stops it. A hatchling will not leap through a held flame, but a torch is also what wakes the capsules, so give a brood room.