The Pollinators of the Malagasy Star OrchidsAngraecum sesquipedale, A. sororiumandA. compactumand the Evolution of Extremely Long Spurs by Pollinator Shift

Abstract

Abstract:The pollination process of the extremely long‐spurred orchidsAngraecum sesquipedaleandA. sororiumis described and documented here for the first time. The pollinaria and viscidia load was examined in moths captured in central and south Madagascar. Visits to orchids by hawkmoths were rarely observed in the field and were therefore systematically recorded in large flight tents using a night‐vision video technique and flashlight photography.Angraecum sesquipedalein Fort Dauphin is pollinated byXanthopan morgani praedictaandA. sororiumon Mt. Angavokely byCoelonia solani. By combining a deep nectar spur of extraordinary length with a protruding labellum functioning as a landing platform, these orchids overcome the moth's stereotypic swing‐hovering flight thus enabling full insertion of the long tongue.Angraecum compactumin Forêt d'Ambohitantely is pollinated by both the shorter and longer‐tongued forms ofPanogena lingenswhich never swing‐hover but is also exploited byX. morganiandC. solaniwith wastage of pollinaria. The duration of tongue insertion, nectar exploitation and tongue withdrawal were analyzed: legitimate and illegitimate visitors differ in their time budget and approach to the flower. Nectar volume, nectar level and sugar concentration ofA. sesquipedaleandA. sororiumwere compared with the nectar requirements of the pollinating hawkmoths. The evolution of very long spurs in these orchids is likely to have involved a series of pollinator shifts. The orchids adapted to different hawkmoth species with increasingly long tongues which primarily evolved to avoid predator attacks during visits to less specialized flowers. This “pollinator shift” model modifies the classical “coevolutionary race” model. The relevance of the taxonAngraecum bosseriSenghas is questioned.