THERMAL BIOLOGY AND FORAGING RESPONSES OF INSECT POLLINATORS TO THE FOREST FLOOR IRRADIANCE MOSAIC

This paper examines the responses of the insect pollinators of the sum mer-flowering, understorey shrub, Lavandula latifolia (Labiatae) to th e irradiance mosaic of the forest floor in a southeastern Spanish loca lity, and assesses the relationship between these responses and inters pecific variation in thermal biology. Pollinators differed significant ly in the mean irradiance at capture points (MICP), and a large percen tage of species (59%) preferentially selected plants in either shaded or sunlit conditions. Taxonomic affiliation (at the order level) and b ody size explained 78% of interspecific variance in MICP. Dipterans te nded to be restricted to situations of relatively row irradiance, wher eas hymenopterans foraged over the entire irradiance gradient. MICP wa s inversely related to body size. Pollinators differed Significantly i n mean thoracic temperature (T-th), thoracic temperature excess (T-exc = T-th - T-a [air temperature]), and slope of the T-th/T-a regression (which may be taken as a rough index of thermoregulatory ability). Hy menopterans had higher T-th and T-exc, and smaller T-th/T-a slopes, th an dipterans. After accounting for this taxonomic effect, T-th and T-e xc, but not T-th/T-a slope, increased with body size. MICP depended si gnificantly on T-exc and, to a lesser degree, on T-th, and species cha racterised by high T-exc tended to select low-irradiance conditions. T he relationship between MICP and body size was thus mediated by the co rrelations between the latter variable and thermal biology parameters. In the hot Mediterranean summer, as in other thermally stressing envi ronments, thermal constraints of the kind reported in this study may g enerate within- and between-habitat variation in the composition and s ize structure of pollinator assemblages, which may be consequential fo r plant reproduction.