Specialized pollination by flies often involves specific floral mimicry of various foods and brood substrates. However, identifying a precise model is challenging due to the ecological diversity of flies, which subject plants to diverse selection pressures. The genus Vincetoxicum Wolf. (Apocynaceae) can be a promising taxon in which several plants use specific types of flies, but they share the basic floral morphology, implying specific attraction by floral scent.
　During exploration of pollinators of Japanese Vincetoxicum species, I happen to observe flies of the family Chloropidae gathering the inflorescence of Vincetoxicum nakaianum. Because some chloropids are kleptoparasitic and feed on the body fluid of the injured insects, I hypothesized that the flowers of V. nakaianum mimic injured insects. To test this hypothesis, I conducted filed observation of pollinators, investigation of the life history of pollinator flies, floral scent analysis, bioassays, and comparisons of odors between flowers and potential mimicry model insects.
　The field observations in the natural populations revealed that V. nakaianum is pollinated by four chloropid fly species, which were later experimentally confirmed to feed on the body fluids of injured ants. Chemical analyses of floral scent consistently identified five major components across all samples: nonane, undecane, octyl acetate, decyl acetate (10Ac), and methyl-6-methyl salicylate (6-MMS). A synthetic mixture of these five floral scent components was attractive to pollinating flies. However, in the absence of 10Ac or 6-MMS the mixture was unattractive to flies. Although neither 10Ac nor 6-MMS alone was attractive, their combination elicited attraction, indicating a synergistic effect.
　Volatile compositions of injured, mechanically stimulated, or freshly killed insects were analyzed for 38 taxa, including ants, true bugs, and ground beetles. The volatile profiles of Formica ants, particularly when attacked by spiders, most closely resembled the floral scent of V. nakaianum. Y-maze experiments confirmed that pollinators were attracted to Formica japonica following spider attack.
　These results demonstrate that V. nakaianum employs the olfactory mimicry of injured ants to attract pollinators. Although ants are ubiquitous and frequently interact with plants, no flowers have been reported to mimic ants, despite the fact that ant mimicry has evolved more than 70 times in arthropods. This study highlights the diverse evolutionary outcomes of adaptation to fly pollinators and the evolutionary capacity of ant mimicry in plants.