I think the "jury" is still out on the full genetic story of spiders. Kara (KLG) mentioned that some dominants may have a "super" form that isn't visible, but didn't mention spiders in her example...and I think NERD knows more about spiders than just about anyone. I may have missed it, but I haven't yet read of anyone producing a for-sure "Super-Spider", visible or not.
(And I would LOVE it if Kara or Randy Remington stepped in here to fill us in on the real spidey story!)