Sexual deception of male Bradysia (Diptera: Sciaridae) by floral odour and morphological cues in Pterostylis (Orchidaceae)

Sexually deceptive orchids exploit the innate sexual preferences of their male insect pollinators to achieve pollen transfer. Although floral volatiles are critical for pollinator attraction in sexually deceptive systems, floral morphology is also expected to exploit the sexual preferences of the pollinator. Here, we investigate the pollination of the Australian orchid Pterostylis cycnocephala. We confirm that male fungus gnats of a single undescribed Bradysia sp. (Diptera, Sciaridae) act as pollinators and display sexual behaviour on flowers, including wing fanning, abdomen curling and genitalic clasping of the prominent labellum appendage. Gnats only triggered the labellum and became trapped in the flower after attempting pseudocopulation with the appendage, a process necessary for pollen removal and deposition. Male gnats located flowers hidden from view, suggesting long-range attraction is primarily due to floral odour. However, male gnats displayed reduced copulatory behaviour when the labellum was absent and when the labellum appendage was inaccessible, suggesting that appropriate morphology may be required to elicit the copulatory behaviour needed for pollination. Our study is the first detailed investigation of sexual deception of male Sciaridae in Australian orchids and represents an example of convergent evolution with some Neotropical Lepanthes orchids, which also sexually deceive male Bradysia.

Automated summary

This study investigates the pollination process of the Australian orchid Pterostylis cycnocephala and identifies an undescribed Bradysia sp. male fungus gnat as its main pollinator, which displays sexual behavior on the flowers. The study shows that floral odor is primarily responsible for long-range attraction, while floral morphology is necessary to induce the copulatory behavior required for pollination.