An inventory of the invertebrate occupants of Bat Cave, a cavern on the north wall of the main gorge of the Colorado River within the boundaries of Grand Canyon National Park (GCNP) in Arizona, revealed the presence of a population of an undescribed troglophilic species belonging to the genus Eschatomoxys. This is not surprising inasmuch as among the “triorphine” genera there are several cave-frequenting species. A comprehensive survey of Tenebrionid beetles inhabiting the islands in the Gulf of California resulted in the collection of a series of specimens that represent two insular species, both previously unknown to science, and assignable to the genus Eschatomoxys. The habitats of Eschatomoxys in caves, islands, and extreme deserts, suggests that the species are relictual in distribution and their populations fragmented. Ecological studies of the cave species reported herein sheds light on why these beetles are rare in collections.