Spider mites are closest to ticks, the disease carrying blood sucking insects you pick up in the woods or in your yard if you have deer like I do.
The lay waiting for a suitable host to come by and launch themselves on it.
Spider mite nymphs need to mature on a host, because they need nutrients only a warm blooded host can provide... They suck not blood, but the juices that come out of a wound they make on your body. They have a mouthpiece, and pierce the body, then the fresh around the wound becomes hardened, very itchy, and they feed on that wound for days, I think.
I have to control my gag reflex as I am writing this.
Anyway, when they are good and ready, they drop off and molt into a spider mite with eight legs.
The problem is this: if they do this in your house, then there is an unlimited supply of spider mites and wounds and itching, unless you find a way to eliminate the dropped off mites as well.