2.3 Flower visitor assemblages and size variation
To investigate the flower visitors of A. buergeriana var.buergeriana , we walked through each population and captured insects that were visiting flowers. This survey was conducted during 7:00–14:00 local time, when flower visitors are active. Each population was observed a total of 60–180 min over 1–3 days at the peak of the flowering season. Captured insects were measured from the tip of the proboscis to the end of the abdomen with a digital caliper (precision, 0.01 mm) to determine visitor size (Figure 1c). Observations were made of visitors to both yellow and red flowers to confirm that there were no differences in visitor assemblage or visiting frequency between differently colored flowers.
As the average visitor size for each plant population, the weighted arithmetic average was calculated from the relative abundance of each visitor species and the size of that species:
\begin{equation} Average\ visitor\ size=\ \sum_{i=1}^{n}{Pi(Ni/Nt)}\nonumber \\ \end{equation}
where n = the total number of insect species visiting a A. buergeriana var. buergeriana population, Pi = the average size of the i th insect species, Ni = the number of flowers in the population that the i th insect species visited, andNt = the number of flowers in the population that any of the insect species visited. Thus, Ni /Nt is the relative abundance of the i th insect species visiting the population. Observations of flower visitor frequency showed that large bumblebees (Bombus spp.) were the main visitors, although small bees (Ceratina spp.) also visited occasionally. Bumblebees, which extend their proboscis to the nectar source at the tip of the spur to suck nectar, visited both male and female phases and both yellow and red flowers (Toji personal observation). Smaller bees could not reach the spur tip to forage for nectar. Furthermore, although they sometimes collected pollen from the flowers, they did not contribute to pollination because they rarely moved between plant individuals and did not visit female-phase flowers. Because the pollen visitation patterns of the bumblebees and small bee species were very different, we calculated average visitor size for all visitors (i.e., bumblebees plus small bees) and for bumblebees only.