Well, there's no reason to delete this topic that I can see
. But you have to remember is what separates Fantasy or Fairy Tales from Science Fiction. Science Fiction usually has to follow rules, even if they are made up, Fantasy doesn't (though many Fantasy writers still create rules for their magical systems anyway).
This is one of the reasons why literary charges of Deus Ex Machina and Shaggy Dog (literary devices to make a story work when there are plot gaps) don't work against Fantasy writers. Anything can happen. Giant Sand Slugs, Giant Spiders, little kids living on planets barely bigger than a beach ball (The Little Prince), Old Witches who melt when water is thrown on them, whole cities of people living on a speck of dust resting on a clover being held by an elephant sitting on top of a little tree, flame breathing dragons etc. etc. Granted some of those Fantasies are geared towards children, but Fantasy, aka Fairy Story, has always been considered a children's genre by the unimaginative anyway, much to the disgruntlement of Tolkien (and myself
But the main point is, in Fantasy anything can happen. And adults like us, who enjoy Fantasy, have the imagination to make these sorts of things work. Marvel comics always had a nifty way of fixing plot holes--asking the readers to invent a solution and make up a reason why it could work. This is all we have to do to make Giant Spiders work.
For example: most insects, arachnids and crustaceans have an exoskeleton. When they are tiny, they are, as you suggest, cartilagetinous. However, as larger crustaceans such as Crabs and Lobsters have Hard Exoskeletons, we can imagine that a Giant Spider might have an exoskeleton as hard as any of our bones. So it could work