Pollinia transfer on moth legs in Hoya carnosa (Apocynaceae).

Abstract

The genus Hoya (Apocynaceae; Asclepiadoideae) is characterized by a set of complex floral characters unique among the asclepiads, but their role in pollination is poorly understood. Here, we report a new mechanism of asclepiad pollination in the wax plant Hoya carnosa: the pollinaria are transferred on the legs of medium or large settling moths. Floral visitors and their behavior were observed on Amami-Oshima Island during 2013-2015, and the efficacy of different pollinators was determined by counting the pollinarium loads on different flower visitors. The floral characters were studied to establish the process of pollination in relation to pollinator behavior on the flower. Hoya carnosa was visited by various settling moths at night, but pollinia attachment was confirmed predominantly on the legs of the large moth Erebus ephesperis (Noctuidae) and less frequently on the legs of the medium-sized moths Bastilla arcuata (Crambidae) and Cleora injectaria (Geometridae). The moths walked around and searched for nectar on the inflorescence, and the corpusculum became clipped to the arolia (adhesive pads on moth tarsi) as they stepped on the pollinaria between the staminal corona. The downward spherical inflorescence of aggregated flowers with flat, velvety petals and a slippery corona provides restricted footholds for the visitors, which efficiently leads pollinator legs to the pollinaria. Although the pollination system of Hoya is largely unknown, pollination by insect legs may be a major pollination system in this genus because these basic floral characters are shared among many species.