Flowers of Oncidium sphacelatum and Trichocentrum oerstedii were observed during several days at two sites in the Soconusco region of Chiapas, Mexico, to obtain details of seed set and insect visitation and to capture and identify potential pollinators. Fragrance samples from flowers of both orchids were taken for chemical analysis. No insect activity was observed for T. oerstedii, and no seed capsules had been produced. The bee, Centris (Hemisiella) trigonoides, although the most consistent visitor to the flowers of O. sphacelatum, did not participate in pollination. Single Centris (Paracentris) mexicana bees visited a few flowers once a day, and the authors trapped one bee with an O. sphacelatum pollinia present on the dorsal surface of the thorax. This orchid species presented a single seed capsule at the end of the flowering period. Different pollination strategies proposed for the two genera in Oncidiinae are discussed. Analysis of hexane extracts of flowers revealed that the dominant component of both species was a mixture of unsaturated and saturated hydrocarbons (C25-C31) as wax constituents. Although the fragrance of O. sphacelatum was characterized by zingerone, and the distinguishing component of the fragrance of T. oerstedii was syringaldehyde, both species produced relatively little scent. The timing of peak scent production was variable and did not necessarily coincide with insect visitation, suggesting that fragrance is not an important factor in the process of pollination.
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