... 2.5 pages. 2. Point 5. Pages. And you ruined it.
They call me El Doúblé Yêo, rerailer of worlds.
I didn't think it was to do with metabolism, I was under the impression that large insects and arachnids simply can't breathe the insufficient oxygen content that we have here on earth at present.
Spiders don't breathe, got no lungs! They absorb oxygen through their body, much like amphibians, and eeyup, not enough oxygen content to maintain a big size.
So caverns are technically jars of oxygen, just waiting to be exploited by hairy midgets...
You're technically right, but overly general:
http://en.wikipedia.org/wiki/Book_lungAlmost any organism human sized is going to need a pretty impressive O
2 collector. So rather than trying to create a complex explaination for how so much Oxygen is in the caves and not elsewhere, I'd theorize that they have adequate organs necessary for survival. It seems to me like the caves would have LESS oxygen anyways for a number of reasons:
1) Most creatures consume it
2) plant life is more abundant topside
3) the absence of sunlight (an no other obvious energy souce except magma in some places) would make photosynthesis difficult for the plants that do live underground.
4) many caves have a tendency to consume Oxygen and fill with CO
2 and other not so useful gases. (due to things like respiration, but also reactions with minerals such as sulfur)
5) Oxygen is not terribly heavy and if there's not enough wind then it would likely seep out of the caves for "higher ground"
there are other things to consider that would make this thread a year long research investigation. Anyways, I can't really get comfortable with the idea of High 0
2 caverns.