ROOSTING, PERCHING, AND HABITAT SELECTION IN ARGIA VIVIDAHAGEN AND AMPHIAGRION ABBREVIATUM(SELYS) (ODONATA: COENAGRIONIDAE), TWO DAMSELFLIES INHABITING GEOTHERMAL SPRINGS

AbstractAlthough Amphiagrion abbreviatum(Selys) and Argia vividaHagen often occur at the same geothermally heated springs in western Canada and the United States, they differ markedly in their abundance at any particular site. There is no relationship between crude data on water temperature, conductivity, or aquatic vegetation and the relative abundance of the two species, but there is a striking correlation with presence or absence of trees. The absence of A. abbreviatumfrom heavily treed areas is associated with the paucity of suitable daytime perching sites, and there may be competitive pressure exerted by A. vividafor the perching sites that are available. Argia vividadoes not live at open sites because it requires trees for night-time roosts. Argia vividaroosted higher than A. abbreviatumin cages and held the body at a greater angle from the cage wall. The roosting posture of A. vividais probably related to interception of solar radiation in the morning, and the body positions of both species possibly provide defence against predation.