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Message: <blockquote data-quote="Dannyalcatraz" data-source="post: 4710026" data-attributes="member: 19675">I know that millions of years ago, Earth's atmosphere had a higher Oxygen content that allowed terrestrial arthropods to grow quite large.Nowadays, the largest terrestrial arthropod is the Coconut Crab (Birgus latro)- at (supposedly up to) 40lbs and about 3' across, its no lightweight, to be sure.<a href="http://en.wikipedia.org/wiki/Coconut_crab" target="_blank">Coconut crab - Wikipedia, the free encyclopedia</a>There are other arthropods out there that get quite large, notorious among them the tarantula family of spiders.  Yet none of the tarantulas known to science are even close to the Coconut Crab in size.  There are stories of spiders big enough to attack dogs and cats from certain areas of the world- creditable enough for some cryptozoologists (legit ones, not the ones who claim to have "Bigfoot" samples) to go looking for them, but as yet, none have been found.My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?</blockquote>