Pollination of northeastern North American Spiranthes (Orchidaceae)

Abstract

Most northeastern North American Spiranthes are adapted to pollination by long-tongued bees (e.g., Bombus spp. and Megachilidae). The salient features of this adaptation are (i) a long, flat viscidium which attaches the pollinia readily to the flat rigid galea of the insect's proboscis, (ii) the nectar secreted into the base of the floral tube, (iii) the flowers are protandrous and sequential beginning at the base. In contrast, Spiranthes lucida apparently is pollinated largely by halictine bees. It differs from other northeastern taxa of Spiranthes in (i) having an oval viscidium which attaches the pollinia to the clypeus below the antennae, (ii) having the nectar available on the under side of the column behind the stigmatic surface, and (iii) in lacking protandry. In the characteristically Bombus-pollinated taxa, protandry is accomplished by a change in the position of the terminal portion of the column with respect to the lip, apparently due to cell elongation in both the column and the lip. The present investigation documents protandry in S. cernua var. cernua, S. lacera var. lacera, S. lacera var. gracilis, S. laciniata, S. magnicamporum, S. ochroleuca, S. romanzoffiana, S. tuberosa, and S. vernalis. Bees moving up the spike from the older female flowers to the younger male flowers act initially as pollen donors and later as pollen receivers; thus cross-fertilization is enhanced. Halictines occasionally act as pollinators of characteristically Bombus-pollinated taxa by visiting the flowers upside down so that the pollinia are inconspicuously attached to the lower side of the prementum. Data presently available for northeastern North American Spiranthes fail to establish pollinator specificity as significant in speciation except perhaps with respect to the separation of S. lucida from the other 14 northeastern taxa.