fun fact: there are living intermediaries between spiders and scorpions, such as the sun spider. If you exam one, you'll see that one set of its legs has shrunk and become vestigial while a front set is developing into something like claws but is not yet usable.
Also fun fact: if you catch one and throw it into a cup of rubbing alcohol so you can exam it's body, the experience will traumatize you for life, more so than waking up to find these things chilling on your shoulder. (seriously, between these, snakes, and the occasional coyote, there were way too many pests in my house as a kid...at least these would eat the lizards I guess...)