BIOLOGICALLY ACTIVE COMPOUNDS IN ORCHID FRAGRANCES

Certain orchids in tropical America have become adapted to pollination by male euglossine bees. The bees are attracted by floral fragrances, and the chemical composition of the fragrances determines which species are attracted. The male bees collect the fragrance materials directly from the flower by rubbing the surface of the flower with special tarsal brushes. The bees launch into flight, transfer the fragrance materials, and store them in swollen glandular tibiae of the rear legs. The contents of the tibiae after floral visits were analyzed by gas chromatography; they were the same as the floral fragrance. Approximately 50 compounds are present in euglossine orchid fragrances, and some species may produce as many as 18 of the compounds. Other species produce fewer compounds. Certain of the compounds, when presented to the bees in tropical America, proved to attract many species of euglossine bees. Other compounds attracted only a few species although in some cases no bees were attracted. Combinations of the compounds attracted markedly fewer species than pure compounds. Appropriate combinations of compounds, in the proportions found in orchid fragrances, attract the same bees which are attracted by the flowers. Speciation and reproductive isolation in euglossine bee-pollinated orchids appears to be based dn specific attraction of pollinators to odors produced by the orchid flowers; the substances are believed to play a role in the life cycle of the bees.