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Show Dan A. Polhemus Dept. of Entomology, MRC 105 Smithsonian Institution Washington, DC 20560 BASINS AND RANGES: THE BIOGEOGRAPHY OF AQUATIC TRUE BUGS ( INSECTA: HEMIPTERA) IN THE GREAT BASIN The endorheic drainage catchments of the Great Basin form a rather distinctive biogeographic subregion for the aquatic true bugs ( Hemiptera) of North America. The Basin as a whole, however, does not have a homogenous regional fauna, due to its broad north to south extent, and the consequent variation of both current day climate zones and past limnological regimes. The northern basins, including the Bonneville, Lahontin, and those of eastern Oregon, have depauperate water bug faunas characterized by the absence or near- absence of many families, such as Veliidae, found at similar latitudes outside the Great Basin. It is hypothesized that this pattern is due to the former presence of cold water lakes in these northern basins at various times during the Pliocene and Pleistocene, which represented generally hostile habitats for many aquatic Hemiptera, and did not allow the possibility of southerly escape along river corridors in response to periods of increased climatic stress. Even among cold hardy families such as the Gerridae, Saldidae, and Corixidae, the present day fauna of these basins is composed primarily of widespread generalist species, and shows little interbasin endemism, indicating that the former lake systems presented relatively homogenous habitats and were sufficiently interconnected to preclude allopatric speciation. The southern basins, particularly those lying in southern Nevada and southeastern California, have had a much different faunal history. One of the most remarkable concentrations of disjunct and endemic aquatic Hemiptera in North America is found in the thermal spring refugia of three of these endorheic drainages: Railroad Valley, the White River, and the Amargosa River. These drainages, which apparently represent former tributaries to the precursors of the modern Rio Grande and Colorado river systems, appear to have been continuous and perhaps integrated watercourses in the Pliocene and Pleistocene, but have been subsequently dessicated and reduced to sets of discontinous spring fed oases. The thermal springs of these three systems contain a large number of endemic or highly disjunct aquatic Hemiptera species. As with other similarly relictual aquatic elements, such as pupfishes, the nearest sister species or conspecific populations of many of these isolates are frequently found in central Texas or in the Sierra Madre Oriental of eastern Mexico, rather than in the more geographically proximal drainages of the modern Colorado River basin and the Sierra Madre Occidental of western Mexico. In addition to these thermally associated disjuncts, the southern basins also contain remarkably isolated populations of certain species, such as creeping water bugs, adapted to cold water regimes. These isolates have their nearest associated populations in sections of the upper Virgin and Colorado river systems lying immediately to the east. It is suggested that this pattern is of much more recent origin, and has resulted from dispersal through post- Pleistocene cold water drainages that existed in the region, followed once again by subsequent dessication and fragmentation. In the current paper we review the evidence for the above patterns supplied by various families of aquatic Hemiptera, and then consider in detail the biogeographic histories of the Railroad Valley, White River, and Amargosa drainage basins. |