Sobralia Orchid

Photographed in nature [1]

Over 1400 orchid species are found in Costa Rica, sporting colors from red to yellow, pink to white, and almost any color in between. Some grow successively, with thickened parts of a central stem (called pseudobulbs) emerging one after the other— each capable of producing leaves and flowers. Others grow continuously, with alternate leaves projecting upwards as aerial roots protrude down to anchor the plant against the substrate. The latter, termed monopodial, is how the beautiful sobralia (Sobralia decora) pictured here grows. I spotted the orchid five meters off the ground as it splayed its roots haphazardly on a mossy trunk, and a rare ray of sunlight lit up the inflorescence in the otherwise dark understory. Flowers in this genus are ephemeral, lasting anywhere from a day to a mere few hours.

Orchid bees are the principal pollinators of neotropical orchids, and metallic blue-green bees in the genus Euglossa have been observed visiting Sobralia decora. Male bees hone in on orchids to collect volatile compounds. They will uptake the chemical flavors by combing surfaces with the forelimbs and mixing them with lipid secretions in their ‘saliva’, ultimately synthesizing a concoction that is stored along grooves on the hind tibia. Fragrances are not only obtained from orchids, but also from other flowers, fruits, fungi, and woody materials. While observing them in nature, I most often see orchid bees actively scrape plant surfaces while walking, whereas deposition onto the hindlegs is usually done during flight. Once enough compounds are acquired within the chemical bouquet, male bees will perform displays by posturing upwards, extending the hindlegs, and vibrating the wings. This behavior presumably broadcasts the fragrances for mate attraction and to repel rival males, though much empirical work needs to be done to support these hypotheses. Competition is fierce for desirable volatiles, and orchid bees can be seen fighting over orchids, bashing each other repetitively and even biting if given the opportunity. On one occasion in the Osa peninsula, I found a mob of over a dozen Euglossa orchid bees robbing a deceased black orchid bee (Eulaema meriana) of its floral volatile blends. Although it’s unlikely they took the larger bee down, this behavior lends support to the idea that volatiles are variable in composition and those of higher quality are likely scarce in the environment.

Robbing of floral volatile blends by a flurry of Euglossa sp. on a deceased Eulaema meriana. Note the yellow pollinia on the left! Photographed 07/07/2017, in situ [1]

Orchids allow the bees access to desirable fragrances, but not without something in return. As a bee attempts to extract the compounds from the orchid, it comes into contact with bags of pollen (called pollinia) that lie on the anther cap just above its head. The pollen itself is not adhesive, but there is a sticky pad at the opposite end of the pollinia structure that adheres to the bees’ exoskeletons. Luckily, pollinia is easily visible to the naked eye (if carefully inspected!), and it’s wonderful to see the diversity of forms and yellow-orange hues that adorn these already spectacular iridescent bees. So if the pollinia is very sticky, how can they be dropped off at another orchid destination? It turns out that orchids make it challenging for the bees to successfully navigate and enter the central column. The stigmatic surface is extremely slick, and the bees cannot land or walk with ease. What results is a seemingly clumsy bee fiddling in and out of the orchid, increasing the chance that pollinia becomes dislodged and pollination is achieved.

Schematic of pollination of an orchid (Stanhopea grandiflora) by Eulaema meriana; pollinia in yellow. Figure from Dressler, 1968

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