Not in the family

 When I was young, I was taught that insects had six legs or more than 8 legs (Centipedge or millipedes). Only spiders and scorpions had 8 legs.  

Except that explanation had a flaw in it. I happily dumped daddy long legs into the family of spiders even though they did not spin any web. And before anyone suggests that these are proto-spiders, they are not closely related to spiders. They are not spiders that have yet to evolve webbing. 

This is why education is such a problematic thing. You never know when you need to attend a new class. 

Edit a year later: 

That is why education is never ending. There are arachnids daddy long legs. Daddy long legs are also used to call crane flys.