Any member of the class Arachnida; a creature that is a spider, scorpion, mite, tick, or harvestman.
From 'Arachnida' + '-an' (suffix for members of a class). Parallel formation to 'mammal' becoming 'mammalian,' used as both noun and adjective.
Every arachnidan has eight legs, making them instantly different from insects which have six—it's the easiest way to spot spiders versus bugs!
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