Morphology and sexual deception: are orchid labellum structures products of female wasp mimicry or mechanical fit with behaviourally different male pollinators? (#320)
Male wasps are common pollinators of sexually deceptive orchids which mimic their female’s sex pheromones. Although chemical mimicry is well characterised in this interaction, the role of floral morphology is unknown. We targeted Neozeleboria thynnine wasps that pollinate Chiloglottis orchids, as this group offers a unique opportunity to explore the influence of floral morphology on pollinator behaviour in the absence of chemical differences. C. trapeziformis and C. valida attract behaviourally distinct pollinators and exhibit divergent floral morphologies, but employ identical chemical compounds for pollinator attraction. This permits independent and experimental investigation of the importance of morphology in sexual deception. Neozeleboria wasps also display extreme size dimorphism, allowing us to make two clear predictions. If labellum structures are under selection via female mimicry, we expect trait overlap with the smaller female insects. Conversely, if labellum structures are under selection via mechanical fit to male wasps, we would expect trait overlap with the larger pollinating males instead. Results revealed some overlap between labellum structures and female wasps in terms of colour and size, indicating potential female mimicry. However, we also found that pollinator behaviour is strongly influenced by floral morphology and that labellum size is positively correlated with male pollinator size in some species. Furthermore, experimentally modifying labellum size in C. trapeziformis significantly reduced the duration of copulation attempts made by male wasps, indicating pollinator-mediated selection for mechanical fit. We thus find that sexual deception is governed not only by female mimicry, but also by morphological fit with the male pollinators and that differences in pollinator behaviour may be crucial for determining the strength of selection experienced.