Abstract:
Until recently, the distinctive phallostethid genus Phallostethus was known from a single collection made in 1902 from Johor on the Malay Peninsula, described in 1913 by Regan as Phallostethus dunckeri. In 1991 and 1993, specimens of Phallostethus were collected from disjunct coastal localities in sarawak and Brunei, northwestern Borneo. These Borneo specimens differ from Phallostethus dunckeri in morphology of the priapium (a complex, bilaterally asymmetric, subcephalic copulatory organ), pigmentation, and paradentary bone dentition. They are described herein as Phallostethus lehi, bringing the number of recognized phallostethid species to 20. Phenacostethus smithi Myers, 1928, previously unknown from Borneo has been collected from Sarawak, Brunei, and Kalimantan, bringing the total number of Bornean phallostethid species recognized currently to six. The phallostethids of Borneo do not form a monophyletic group but, instead, have complex, repeated sets of relationships with species on the Malay Peninsula and in the Philippines. Components of their biogeographic relationships are shared with other members of the stream biota. Composite biotic relationships are consistent with a composite geological origin of modern Borneo.
