Makino, Takashi T, and James D Thomson. “Innate or Learned Preference for Upward-Facing flowers?: Implications for the Costs of Pendent Flowers from Experiments on Captive Bumble Bees”. Journal of Pollination Ecology, vol. 9, Sept. 2012, pp. 79-84, doi:10.26786/1920-7603(2012)11.