Complex floral morphology and scent chemistry mediate hawkmoth pollination of an African orchid in the diverse Habenaria clade

Abstract

Abstract One explanation for the evolution of complex floral morphology in orchids is that it can function as a component of ‘lock-and-key’ mechanisms that ensure pollen transfer by specific flower visitors. We investigated the roles of floral morphology and scent in the pollination of Bonatea antennifera, an African orchid that is phylogenetically embedded in the hyper-diverse Habenaria clade. Using camera traps and direct observations, we established that B. antennifera is pollinated by an assemblage of short-tongued hawkmoths with proboscides ~4 cm in length, corresponding to the spur length of the orchid. Pollinaria are attached between the palps of these hawkmoths via an intricate mechanism involving a wedge-shaped rostellum chamber and a tooth in the floral spur that deflect the head and proboscis of the hawkmoth to either side of the spur entrance. This functional subdivision of the flower results in removal of a single pollinarium per visit. Scent production is synchronized with the peak activity of hawkmoths at dusk, and electrophysiology experiments showed that hawkmoth antennae respond to several of the dominant volatile terpenoid compounds [e.g. R-(−)-linalool, caryophyllene epoxide, and (E)-nerolidol] emitted by the flowers. These results illustrate the functional significance of complex floral morphology and scent for pollen transfer in orchids.