The largest living arthropod is a?

The largest non-extinct arthropod in size was a Japanese spider crab caught in 1921 measuring about 12 feet across the claws. The heaviest on record was a lobster weighing about 45 pounds. On land, the largest is the coconut crab at about 3 feet across. Some anecdotal evidence exists for even larger crustaceans, based on recovered exoskeletons/exuviae.