P1. Syst. Evol. 217:101-117 (1999) Plant Systematics and Evolution © Springer-Verlag 1999 Printed in Austria Pollination mechanism in southern Brazilian orchids which are exclusively or mainly pollinated by halictid bees RODRIGO B. SINGER and ANDREA A. CocuccI Received October 27, 1997; in revised version March 5, 1998 Key words: Orchidaceae, Campylocentrum, Cyclopogon, Prescottia, Halictidae. - Bee pollination, flower morphology. Abstract: The pollination process is documented in three sympatric orchids found in Curitiba, Paran~ State, southern Brazil. In Campylocentrum aromaticum (Epidendroideae: Angraeciinae) and Prescottia densiflora (Spiranthoideae: Prescotiinae) halictid bees act as main pollinators with minor contributions from other insects, such as Syrphid flies. Because of the pollinarium and column structure in Cyclopogon diversifoIius. (Spiranthoideae, Spiranthinae), halictid bees are the only insects capable of performing pollination. In the three studied species pollinarium removal and deposition are achieved by interaction between the column and bee's mouth parts. Morphological and ecological flower features in agreement with this pollination mode are discussed and pollination by halictid bees is suggested to occur in a number of South American orchids. About 60% of the Orchidaceae are considered to be adapted to bee or wasp pollination (PI~ & DODSON 1966, DRESSLER 1981). Some groups of bees have particular importance in orchid pollination, specially the tribe Euglossini (faro. Apidae), the carpenter bees (faro. Anthophoridae, genus Xylocopa), the bumble- bees (fam. Apidae, genus Bombus) and some oil-gathering Anthophoridae (genera Centris, Paratetrapedia and others) (PIJL & DODSON 1966; DRESSLER 1981, 1993). Records of orchid pollination by halictid bees are scarce. For the Northern Hemisphere, there are records of halictid pollination in Spiranthes (CATLING1983), Isotria (MERHOFF 1983), Calopogon and Cephalanthera (PIJL & DODSON 1966). In Australia, this kind of pollination seems to be better represented and according to ADAMS and LAWSON(1993), halictid bees are confirmed as pollinators in species of Caladenia, Prasophyllum, Thelymitra and Dendrobium. There are also reports of halictid bees and Syrphid flies acting as co-pollinators of Phragmopedium longifolium in Colombia (PIJL & DODSON 1966), and this is, as far as we know, the only report of orchid pollination by halictids in South America. The aim of the present paper is to present evidence showing that three sympatric Brazilian orchid