Pollination of the orchid Epipactis thunbergii by syrphid flies (Diptera: Syrphidae)

The behavior of visitors to the flowers of the orchid Epipactis thunbergii was studied, with special attention to the role of the epichile in the pollination process. Only four species of syrphid flies legitimately pollinated the flower, among which Sphaerophoria macrogaster was regarded as the most effective pollinator. The movable epichile, possessing a furrow at its base, played a critical role in the pollination process: it threw the syrphid fly onto the stigmatic surface when both sides of the basal slanting surface of the furrow were presumably pressed in the direction of the hypochile by the fore (and middle) legs of the retreating syrphid fly. At this moment, the fly received a set of pollinia on the thorax.