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,...
