Pollination of the long-spurred African terrestrial orchid Bonatea steudneri by long-tongued hawkmoths, notably Xanthopan morganii

Charles Darwin correctly predicted the existence of an extraordinarily long-tongued hawkmoth in Madagascar, based on the length of the floral spur of an Angraecum orchid. This hawkmoth, Xanthopan morganii, is also the longest-tongued hawkmoth on the African mainland, but its ecological role as a pollinator has been virtually unknown outside of Madagascar. We investigated whether X. morganii pollinates Bonatea steudneri, an orchid with floral spurs that vary from 10 to 21 cm in length among populations. At a forest locality in central Kenya, we observed X. morganii individuals with tongues ca. 16 cm in length pollinating B. steudneri plants with spurs ca. 15 cm in length. At other localities in Kenya, we observed visits by the hawkmoths Agrius convolvuli (tongue c. 12 cm) and Coelonia fulvinotata (tongue ca. 10 cm). Pollinaria of B. steudneri are attached to the eyes of hawkmoth visitors. Flowers of B. steudneri show several adaptations for hawkmoth pollination, including anthesis at dusk and nocturnal emission of scent dominated by the volatile compound linalool. This study highlights the potential for predicting certain highly specialized interactions among species by matching of their respective morphological dimensions.