How orchids feed specialized bee pollinators
A significant proportion of orchids in the subtribe Oncidiinae produce floral oil as a food reward that attracts specialized bee pollinators. This oil is produced either by glands (epithelial elaiophores) or by tufts of secretory hairs (trichomal elaiophores). Although the structure of epithelial elaiophores has been well documented, trichomal elaiophores are less common and have not received as much attention.
The flowers of Lockhartia are 5–30 mm in length and lack fragrance perceptible to humans. Oil secretion by flowers of Lockhartia was first reported by Silvera (2002), but the morphology and anatomy of their elaiophores have not previously been studied in detail. A recent paper in Annals of Botany surveys the flowers of 16 species of Lockhartia and shows that all have elaiophores (oil glands) of the trichomal type.
Specialized hairs on the legs or abdomen (but not the mouthparts) of oil-gathering bees are used to collect oils, and the latter are then used as food for larvae. Pollinaria of Lockhartia are small (typically 0·7–1·3 mm long) and their attachment to the bodies of bees has not been reported. This may be due to the fact that the thin stipe collapses upon drying and this obfuscates identification of the pollinarium to generic level. The situation is further exacerbated by the fast-flying and extremely timid nature of oil-collecting bees. As a result, they are much more difficult to capture or observe from short distances than male euglossine bees, for which an abundance of observational data exists.
Blanco, M. A., Davies, K. L., Stpiczyńska, M., Carlsward, B. S., Ionta, G. M., & Gerlach, G. (2013). Floral elaiophores in Lockhartia Hook. (Orchidaceae: Oncidiinae): their distribution, diversity and anatomy. Annals of Botany, 112(9), 1775-1791.