They itch. They dig in and they itch.
These are the mites that cause scabies, the tiny tunnelers, burrowing into the skin and digesting as they go. If your German’s good, you can read more about them in Brockhaus’ Konversations-Lexikon yourself.
Or, you can rely on a more modern source.
Either way, brrr.
The smaller fellow on the right is a male, I think, and the larger, a female, capable of laying hundreds of eggs.
They even LOOK itchy.