I once saw a spider amputate its own leg. It was severely crippled, only able to move in small circles. Then it suspended itself from a small shelf and started

I once saw a spider amputate its own leg. It was severely crippled, only able to move in small circles. Then it suspended itself from a small shelf and started biting off the one leg that was immobile. After it was done it lowered itself on the ground and walked away totally fine. Since the behaviour seemed very innate I figured that it must be something that happens frequently and is a known phenomenon. Or maybe ant scientist just don't read spider papers :)