Geographic Variation in Flower Size and Flower-Visitor Composition of Two Bumblebee-Pollinated, Spring-Flowering Herbs, <i>Lamium album</i> L. var. <i>barbatum</i> (Lamiaceae) and <i>Meehania urticifolia</i> (Lamiaceae)

Geographic variation of flower size is potentially affected by various biotic and abiotic factors such as pollinator size, abundance of nectar robbers, and temperature. To elucidate the effect of geographic variation of flower-visitor fauna on flower size, we investigated two spring-flowering, bumblebee-pollinated herbs, Lamium album L. var. barbatum (seven populations) and Meehania urticifolia (five populations), in central Japan. Flower-size variations were not correlated with altitude (an index of abiotic environmental change) in either species, but in both species they were related to the local species composition of flower visitors. In L. album, flowers were bigger in populations visited by long-tongued Bombus consobrinus and B. diversus queens, whereas they were smaller in populations visited by short-tongued bees such as Apis mellifera. Flower size did not vary much among populations of M. urticifolia visited by long-tongued bumblebees, but it was uniquely small in a population whose flowers were predominantly visited by the nectar-robbing bee Megachile japonica. In conclusion, both the size and nectar-robbing behavior of bees affect local flower-size adaptation in these two long-tubed, spring-flowering herbs.