^Did they though? I may be wrong but I thought the fore-wings had a second smaller wing. Again though, it's the eyes that bother me more than the limbs since that's the kind of thing that's established early on in evolutionary terms. The eyes themselves may change but it'd take one hell of a selective mutation to loose two without a trace and have it still be a positive survival trait. The closest terrestrial case I can think of are creatures that became subterranean or deep cave dwellers and went blind over millions of years...and even then they still have their eyes, they just don't work any more.
Anyway, yeah, as I've said several times I know it's just a production thing, but I find it interesting to speculate if something more can be inferred in light of Cameron apparently dead set on doing more films.
Say what you will about Cameron, but to date he's always done something interesting with his sequels...admittedly he's only done two and only one of those was to his own film, but nonetheless they're both considered among the best films of all time, sequel or otherwise.