Total population counts have rarely been undertaken for species of spider, but among those that have been, a leading contender for the unenviable title of the world's rarest spider is unquestionably the Kauai cave wolf spider Adelocosa anops. Categorized as Endangered by the IUCN, this large 20-mm-long spider with distinctive reddish-brown prosoma (conjoined head and thorax in spiders) and large pale opisthosoma (abdominal section in spiders) is known only from the single Hawaiian island of Kauai, where it inhabits a few caves with a lava flow covering an area of 10.5 square km. Counts taken of this species have never documented more than 30 spiders.
This species is also known by the odd-sounding name of the no-eyed big-eyed wolf spider, because although it belongs to a family of wolf spiders renowned for their big eyes, this particular species is exclusively cave-dwelling, living in absolute darkness, so it has no need of eyes and during evolution has lost them entirely. Interestingly, although it was only formally described by science as recently as 1973, this spelaean species has long been known to locals on Kauai, who call it the pe'e pe'e maka'ole.
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