Thermoregulation and hovering behavior of salt marsh horse flies (Diptera: Tabanidae)

Males of the horse flies Tabanus nigrovittatus Macquart and Tabanus conterminus (Walker) engage in daily periods of hovering and mating activity; the time period when hovering takes place is species-specific and strongly temperature-dependent. T. nigrovittatus males exhibit two distinct daily hovering periods, which occur over different temperature ranges. Thoracic temperatures of hovering males were elevated 10-12 degree C and were independent of air temperature; temperatures of early-hovering and late-hovering T. nigrovittatus males were significantly different. We found no evidence of endothermy in perching males; preflight warmup apparently was strictly ectothermic. Female T. nigrovittatus in forward flight did not thermoregulate as strongly as males. Males of the two groups of T. nigrovittatus did not exhibit significant differences in wing area, wing length, wing load, body mass, rate of convective cooling, or wingbeat frequency that might explain their different flight temperatures. Differences in the internal temperature associated with hovering may represent a prezygotic isolating mechanism to reduce or prevent interaction between T. nigrovittatus and T. conterminus. Alternatively, the bimodal pattern of mating activity may be a consequence of physiological and biochemical limitations on hovering flight.