Ko Mochizuki of the University of Tokyo has discovered that Vincetoxicum nakaianum, a dogbane species native to Japan described for the first time by Mochizuki and his collaborators only a year ago, mimics the smell of ants attacked by spiders to attract flies that feed on such attacked insects, and in the process pollinate the flowers. This is the first case of a plant mimicking the odor of ants, revealing that the scope of floral mimicry is more diverse than previously imagined. The findings are published in the journal Current Biology.
The smell of warm, freshly baked bread has surely attracted many a customer to enter the shop as they pass by. Grass flies are no different: they are also attracted to the smell of their daily bread, injured ants. As they move from flower to flower in search of already injured prey, they also do flowers a great service: they pollinate them. Because ants are one of the most widespread species, and ant mimicry has independently evolved in many invertebrate species, it stands to reason that plants could have also evolved to mimic ants in one way or another. However, such cases had never been reported.
"I was working on another research project," says Ko Mochizuki, "and originally collected this species only as a 'reference' for comparison. By chance, I noticed chloropid flies gathering around its flowers in the nursery in the Koishikawa Botanical Gardens, and immediately realized that the flowers might be imitating dead insects."
This recognition was thanks to a string of thus far unrelated experiences. His participation in an intensive training course in 2019 helped him recognize the fly species swarming the flower. He also happened to be familiar with some previous studies that described plants pollinated by chloropid flies emitting odors resembling those of insects.
Following his hunch, Mochizuki set out to methodically observe the visitors on these flowers and compare the odors released by the flowers to odors released by various kinds of insects. He found that the smell of ants being attacked by spiders was the closest match. However, his hypothesis of ant mimicry was standing on frail legs: there had not been any official publications of chloropid flies, or any other similar fly species, targeting ants hunted and injured by other animals, such as spiders. So, Mochizuki turned to social media for more unconventional evidence. There, he found many amateur naturalists documenting what he had suspected: ants attacked by spiders, which then attracted kleptoparasitic (organisms that steal food from another) flies. This gave him confidence to test the hypothesis behaviorally and confirm whether chloropid flies were indeed more attracted to the smell of ants attacked by spiders than to other smells.
"That moment, when I saw the flies on the flowers, was truly one of inspiration," Mochizuki remembers, "a hypothesis suddenly taking shape. This experience taught me that unexpected discoveries often emerge from a combination of preparation and chance."
Talking of preparation… Mochizuki is already preparing for the next project.
"I would like to investigate the evolutionary background of ant mimicry by comparing the pollination systems, evolutionary history, and genetic makeup of Vincetoxicum nakaianum and its close relatives. In addition, since this study suggests that many forms of floral mimicry may remain hidden, I plan to explore other species, both within Vincetoxicum and in unrelated plant groups, to uncover further examples of potential mimicry."
