Stygotantulus

Stygotantulus is a genus of crustacean with the sole species Stygotantulus stocki, that lives as an ectoparasite on harpacticoid copepods of the families Tisbidae and Canuellidae. It is the smallest arthropod in the world, at a length of less than . The specific name "stocki" commemorates Jan Hendrik Stock, a Dutch carcinologist. 
Another contender for the world's smallest arthropod is "Tantulacus dieteri", with a total body length of only .