Why do spiders have 8 legs?

There appears to be no best variety of legs. People have two, canines have 4, bugs have six and millipedes can have over 1,000. So what made spiders accept eight legs? 

“I believe the most effective reply and the only reply is that spiders have eight legs as a result of their mother and father did,” Thomas Hegna, an assistant professor of invertebrate paleontology on the State College of New York at Fredonia, informed Reside Science. “However then that will get into kind of a regress, and someplace this all needed to begin.”