Arachnothryx calycosa is a plant species in the Rubiaceae family. It was originally classified as Rondeletia calycosa by John Donnell Smith in 1913. In 1982, Attila Borhidi reclassified it under the genus Arachnothryx, as part of a broader reorganization within the Rubiaceae family.
Distribution Arachnothryx calycosa is native to Costa Rica, where it typically grows in forested areas. The species is found at an elevation of approximately .
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