denim

I've seen those before too. If you try to capture them, say in a jar, but don't quite get the whole critter in, any legs trapped by whatever you're using will come off the insect. I don't know what they are either, but they don't seem to harm anything.

Could they be some other kind of silverfish?

I just sent the URL for your image to an entomologist. Let's see what they have to say about it.