Spiders and insects just don’t have a large enough volume of blood in them, and considering they have open blood circulation (blood isn’t just enclosed in the vessels), this doesn’t cause problems.
Some animals (like geckos) just adapted to this, just like woodpeckers adapted to their head being constantly smashed at a deadly rate.
I think it could be due to a few reasons-
– open circulatory system so blood flow is different
– they weigh very less which might help in upside down position or various different positions
– they are very adept at holding on to various surfaces and not “fall over”
– their balancing (vestibular system) may not be as “ground-centric” in the sense that a human’s is meaning that for us the ground is below and other things are in reference to the surface we stand on
which makes sense as we humans are larger (relatively) and gravity affects us more and might have had more impact in the evolution of our balancing system and correction. But this might not be the case for them, so even if they are upside down, according to them they are in a normal position because there is no up or down for them, any surface is any surface
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