Species-specific antennal responses to tibial fragrances by male orchid bees

Male neotropical orchid bees (Euglossini) collect odoriferous substances from orchids and other sources and store them in tibial pouches, accumulating complex and species-specific bouquets. These fragrances are later exposed at display sites, presumably to attract females or conspecific males or both. We hypothesized that the necessity to detect and recognize specific fragrance bouquets has led to peripheral chemosensory specializations in different species of orchid bees. To test this, excised male antennae of four species of Euglossa were stimulated with complete tibial extracts of the same four species in a crosswise experiment. In the majority of the tested extracts, the amplitude of the electroantennogram (EAG) response was significantly different between species and always maximal in males of the extracted species. This effect did not appear to result from a given species' increased sensitivity toward certain attractive components: gas chromatography with electroantennographic detection (GC-EAD) of one extract of Euglossa tridentata evoked similar and generalized response patterns in all four species, encompassing a total of 34 peaks that elicited antennal responses. Therefore, the species effect in EAG responses to complete extracts likely resulted from species-specific interactions of compounds at the receptor level. Antennal specialization to conspecific bouquets adds additional strength to the argument that specificity is an important evolutionary aspect of euglossine tibial fragrances.