ASSEMBLY, MATING, AND ENERGETICS OF HYBOMITRA-ARPADI (DIPTERA, TABANIDAE) AT CHURCHILL, MANITOBA

The mating system of Hybomitra arpadi was studied at a subarctic, tree line site near Churchill, Manitoba. Males aggregated in linear clearin gs on a hilltop in spruce-larch forest, hovering at a mean height of 1 3 +/- 1.3 (SE) cm. Hovering occurred only on sunny days (greater-than- or-equal-to 12-degrees-C) for periods of up to 11 h. Males oriented pa rallel to the longitudinal axis of the aggregation site, independent o f wind direction or solar azimuth. Few copulations were seen but there was a high frequency of aborted copulations involving nulliparous and parous, con- and heterospecific females. Males that intercepted femal es were morphometrically distinguishable from the remainder of the pop ulation on the basis of allometric relationships described by principa l components. Aggregating males had highly variable quantities of carb ohydrate in the crop, with early-hovering males tending to have the la rgest quantities; carbohydrate concentration was much less variable. P otential hovering durations estimated from crop energy varied from a f ew minutes to nearly 5 h, but no individual male had sufficient energy reserves to hover for the entire aggregation period on continuously s unny days. Hovering males maintained a mean thoracic temperature of 40 .0-degrees-C, as much as 23.5-degrees above ambient. Thoracic temperat ures were almost invariant across males and over a wide range of ambie nt temperatures, increasing only slightly with increasing ambient temp erature.