If your all wondering why they have more arms and legs than Peter, its because the spider died and they used a artificial form of its venom thus resulting in extra limbs and changing there DNA to more spider than human while Peter's DNA is a more or less balance. Also, like marvel zombies, if they get bitten they will change and seek to infect more instead of eating them, and only peter is immune to the effects for obvious reasons. If your wondering how I know this its because I have a friend who works in Marvel comics and she sends me copies of any comic I like although I still have to pay her.
Apparently everyone goes spider-mutant in that storyline, cover characters included or not. I wonder though, why do all these spider-mutants including old Peter himself never develop 3 pairs of legs or 2 pairs of arms and legs? It's always the convenient Goro-type 6-arms kind of style. Makes you think, doesn't it.