Abstract:
Flowering plants that require insect vectors to complete pollination use signals to advertise rewards and lure in pollinators. Some plants, however, use a deceptive strategy, signalling the presence of a reward or resource, without possessing one. Deceptive signalling is more frequently found throughout the family Orchidaceae. One specialised form involves fooling one sex of the pollinating species. This type of sex-biased mimicry can be selective for males, as is the case with sexual deception, where males pollinate flowers whilst attempting to mate with them, or females as in brood-site mimicry, where females are fooled into pollinating flowers when attempting to lay eggs. There is growing evidence, however, that deception need not rely on mimicry but can instead involve a sensory trap. This occurs when a plant possesses a signal that causes the pollinator to exhibit an out of context reaction, due to its sensory biases, such as an innate colour preference. It may well be that sensory traps are a predecessor to more complex deceptive forms, however this is currently unknown. The New Zealand endemic helmet orchid, Corybas cheesemanii, was originally presumed to attract its proposed pollinators, female fungus gnats, through the method of brood-site deception. However, recent studies have found little evidence to support fungal mimicry in the orchid, but rather have found the orchid to strongly reflect ultraviolet (UV). Here I report on the potential impacts UV has as a pollinator-attracting signal and investigate whether UV may be working as a signal of brood-site deception, or rather as a sensory trap. I used a combination of field manipulations, spectral analyses, visual modelling, and lab-based behavioural assays. I found that UV as well as the brightness of floral colours impacted pollinator visitation to C. cheesemanii. Modelling of fly visual perception of the colours indicates a reduction in either would likely be perceived as noticeably different by the gnats. Additionally, both male and female fungus gnats showed a preference for UV over other similar colour signals, which does not support the idea of UV acting as a sex-biased signal. In conclusion, it is not likely that C. cheesemaniiis employing brood-site deception as previously suggested, but rather is using a more general form of deceptive signalling, such as a UV based sensory trap, in order to attract pollinators. This emphasises the importance of alternative, non-mimicry forms of deceptive signalling in general.