Flower-scent mimicry masks a deadly trap in the carnivorous plant Nepenthes rafflesiana

P>1. Nepenthes rafflesiana is a carnivorous vine from Borneo characterized by an ontogenetic pitcher dimorphism with aerial (upper) and ground (lower) pitchers of different morphologies. Previous studies have shown that fragrant upper pitchers of climbing parts of the plant are more effective in trapping flying insects than non-fragrant lower pitchers, which are essentially restricted to an ant diet. We tested the hypotheses that odours are effective cues for prey attraction in this carnivorous plant and that upper pitchers biochemically mimic flowers in their olfactory cues. 2. The visitor diversity and the scent composition of each pitcher type were determined for different sites and periods during field studies in Borneo. Olfactometer bioassays were conducted using fruit flies and ants as models for flying flower-visitors and non-flying visitors, respectively. 3. Fifty-four volatile compounds were identified and the analysis of their relative quantities in the blends showed significant differences between pitcher types. The blends of lower pitchers contained some aliphatics and terpenoids but were poor in benzenoids. Upper pitchers differed from lower ones in that they attracted a greater quantity and diversity of insects, including a guild of flower-visitors absent from the visitor spectrum of lower pitchers. Upper pitchers also emitted a greater quantity of odours and a larger spectrum of volatiles, including some terpenoids and benzenoids that often characterize the sweet scents classically found in flower blends. Choice bioassays showed that, in absence of any visual cue, the scents of the nectariferous pitcher rim (peristome) were particularly attractive to ants and flies, and those of upper pitchers were more attractive to flies than those of lower pitchers. 4. Synthesis. This study demonstrates the use of scent by Nepenthes carnivorous plants to mediate prey attraction. The climbing part of the plant produces pitcher-modified leaves that mimic flower olfactory cues and suggest an evolutionary convergent strategy with that of generalist pollination systems.