There tends to be a lot of variation within a certain set of initial conditions. E.g. all insects have 6 legs & three body segments, whereas spiders have 8 legs

There tends to be a lot of variation within a certain set of initial conditions. E.g. all insects have 6 legs & three body segments, whereas spiders have 8 legs and two body segments. This isn't only true in the pedantic sense in relation to your question, i.e. if you did have something evolving from insects that say added an extra pair of legs it would be a new class of arthropods, so it wouldn't be an insect anymore, but nothing major like that has branched off either insects or spiders. Insects evolving 4 sets of legs would be a much smaller change than spiders evolving wings, since insects already have all the genetic material to grow legs, but spiders have none of the genetic material to grow wings and would have to independently evolve them. As an aside you're misusing the term "begs the question", it doesn't mean "prompts a question", it's a reference to a specific logical fallacy that doesn't apply here.