The biological function of the wedge-like, dorsally adhesive viscidium of the neotropical orchid genera Cyclopogon, Pelexia and Sarcoglottis (grouped in the so-called 'Pelexia alliance') is elucidated by the study of the pollination biology of three species occurring in Sao Paulo State, southeastern Brazil. Cyclopogon congestus is pollinated by the bee, Pseudoaugochloropsis graminea (Halictidae), Pelexia oestrifera by workers of Bombus (Fervidobombus) atratus (Apidae) and Sarcoglottis fasciculata by males and females of Euglossa cordata (Apidae: Euglossini). These three species offer nectar as a reward and are self-compatible, though they need pollinators to set fruits. In spite of the difference in flower sizes and in their pollinators' taxonomic groups, the pollination mechanism is essentially the same for these species. The pollinarium adheres to the ventral surface of the bee labrum. The viscidium needs to be dorsally pressed in order to liberate a glue which fixes the pollinarium to a bee. Pollination is achieved by the interaction of the orchid column and the mouthparts of the bees. Fixation to the ventral surface of the labrum is advantageous for the orchid, since it is a difficult place for the bees to clean. Another advantage is that, since the labrum is articulated, when the bees fold and close their mouthparts, the pollinarium remains protected trader the bee's head, thus reducing the risks of pollen loss. Since the wedge-like, dorsally adhesive viscidium is a characteristic feature of Cyclopogon, Pelexia and Sarcoglottis, it is suggested that some kind of phylogenetic constraint may exist, impeding the occurrence of pollinators other than bees in these orchid genera. All other flower-visiting animals lack the labrum-like structure needed to fix the pollinarium.1313249262Ackermann, J.D., An orchid flora of Puerto Rico and the Virgin Islands (1995) Memoirs of the New York Botanical Garden, 73Balogh, P., Generic redefinitions in the subtribe Spiranthinae (Orchidaceae) (1982) American Journal of Botany, 69, pp. 1119-1132Burns-Balogh, P., Robinson, H., Evolution and phytogeny of the Pelexia alliance (Orchidaceae: Spiranthoideae: Spiranthinae) (1983) Systematic Botany, 8, pp. 263-268Calvo, R., Four-year growth and reproduction of Cyclopogon cranichoides (Orchidaceae) in South Florida (1990) American Journal of Botany, 77, pp. 736-741Catling, R.M., Breeding systems of northeastern North American Spiranthes (Orchidaceae) (1982) Canadian Journal of Botany, 60, pp. 3017-3039Catling, R.M., Pollination of northeastern North American Spiranthes (Orchidaceae) (1983) Canadian Journal of Botany, 61, pp. 1080-1093Catling, R.M., Notes on the breeding system of Sacoila lanceolate (Aublet) Garay (Orchidaceae) (1987) Annuals of the Missouri Botanical Garden, 74, pp. 58-68Catling, P.M., Catling, V.R., A synopsis of breeding systems and pollination in North American orchids (1991) Lindleyana, 6 (4), pp. 187-210Cocucci, C.A., Fertilización en orquídeas (1981) Anales de la Academia Nacional de Ciencias Exactas, 33, pp. 1-47Darwin, C.R., (1904) The Various Contrivances by Which Orchids Are Fertilized by Insects. 2nd Ed., , London: MurrayDressier, R.L., (1981) The Orchids: Natural History and Classification, , Cambridge, Mass.: Harvard University PressDressier, R.L., (1993) Phylogeny and Classification of the Orchid Family, , Portland, Oregon: Dioscorides PressRoubik, D.W., (1989) Ecology and Natural History of Tropical Bees, , Cambridge, Mass.: Harvard University PressRoubik, D.W., Ackermann, J.D., Long-term ecology of euglossine orchid-bees in Panamá (1987) Oecologia, 73, pp. 321-333Schlindwein, C., (1995) Wildbienen und Ihre Trachtpflanzen in Einer Südbrasilianischen Buschlandschaft: Fallstudie Guaritas, Bestaubung bei Kakteen und Loasaceen, , Stuttgart: GrauerSinger, R.B., Cocucci, A.A., Pollination mechanism in southern Brazilian orchids which are exclusively or mainly pollinated by halictid bees Plant Systematics and Evolution, , in press