HARDER, L. D. Measurement and estimation of functional proboscis length in bumblebees (Hymenoptera : Apidae). Canadian Journal of Zoology. 1982, 60, 1073-1079
HIEI, K. A study on pollination systems and reproductive biology of an annual herb, Melampyrum roseum var. japonicum (Scrophulariaceae). PhD thesis, Tokyo University. 2004