We often think of a flower’s scent as a single, integrated odor. Yet for an insect approaching a tubular flower, a different scent may await outside the entrance and farther inside.
A study published in 2025 by H. Lim and colleagues examined Isotrema manshuriense, a climbing plant in the family Aristolochiaceae. The researchers identified the flies that carry its pollen and analyzed how not only scent but also color and secretions change with place and time. They consider how the flower uses cues that insects seek when looking for food or places to lay eggs.
A Flower Without Insect-Trapping Hairs
Some related pipevines are known for flowers whose hairs inside the tube prevent insects from escaping. This species, by contrast, has no such trapping hairs. Although it has a vertical tube and slippery surfaces, how it attracts insects and gets them to transport pollen had not been fully understood.
The study took place in April–May 2023 in a single wild population in South Korea. The researchers labeled 536 flowers, tracked their flowering, and used subsets for the different analyses. Flowers progress from a female phase, when they first receive pollen, through a short transitional phase to a male phase, when they release pollen. The comparison was not between separate female and male flowers, but between different stages of flowers of the same species.
Most of the 171 insects collected by examining the interiors of 320 flowers were flies. Flies from families including Fanniidae were confirmed as pollinators bringing pollen into the flowers, with one species of Fannia and Delia platura particularly common. Many of the main pollinators were female.
None of the 14 flowers covered before opening to prevent insect entry set fruit, whereas approximately 18% of the 45 flowers open to natural visits did so. This showed that insect pollination supported reproduction in this population.
Sampling Outside and Inside the Flower Simultaneously
Scent analysis used a total of 36 flowers across combinations of female or male phase and day or night. Because many flowers grew high above the ground, they were cut, placed in containers, and sampled simultaneously for 10 hours for gases outside and inside the flower. Compounds from cut leaves and empty containers were excluded using these as controls.
The outside was characterized by aldehydes with nine carbon atoms, such as nonenals, which were especially prominent during the day. Inside, a different group of compounds stood out, including β-ionones and δ-cadinene. Composition differed not only between inside and outside but also between female and male phases.
At night, the number of compounds detected and the measured signals generally tended to be lower. However, differences in variability between samples also contributed to the day–night contrast, so the results do not mean that every compound decreased in the same way.
The values were instrument signals normalized using factors including the dry weight of the flowers. They did not directly measure absolute emission rates from flowers in nature or the intensity perceived by people or flies. Moreover, even with controls, effects of sampling cut flowers in containers for long periods remain a limitation.
Suggesting Two Kinds of Food: Fruit and Flowers?
The researchers also compared the scent with previously reported aroma compounds from other plants’ flowers, fruits, carrion, dung, fermented materials, and other sources. The basis of comparison was which compounds were present.
This flower’s combination of compounds lay apart from other flowers that mimic carrion or dung and closer to fruits such as cucumber and melon, and to flowers visited by bees and butterflies. The authors suggest that the external scent may evoke fruit, while the internal scent may evoke flowers offering rewards such as nectar, providing different cues for flies seeking food.
This is an interpretation based on shared compounds. Behavioral experiments did not establish that flies perceived the scents as the same as “fruit” or “a flower with nectar,” or that they approached because they were attracted to those scents.
Liquid at the Entrance and Color Inside Add Further Cues
Secretions appeared near the entrance on the first day of opening, during the female phase. Analysis of liquid collected from 13 flowers found that, rather than simply containing the simple sugars common in ordinary nectar, it consisted mainly of amino acids and organic acids. Flies were also observed exploring near the entrance, feeding on the liquid, and slipping inside in the process.
Color also differed spatially, with ultraviolet reflection observed in areas such as the upper tube during the female phase. The authors suggest that dark purple coloration and mottling inside may evoke egg-laying sites, while bright, translucent areas may look like exits and guide insects. Color measurements and existing models of fly color vision alone did not establish that this guidance was effective.
Eggs thought to belong to flies were occasionally found inside the flowers. However, no hatching was confirmed in this study, and no eggs or larvae were found in fallen flowers. The possibility of suggesting an egg-laying site must be distinguished from actually providing a place where larvae can develop.
Keeping Cues That Attract Insects as the Flower Changes Phase
More insects were present inside the flowers during the day than at night, and no difference between female and male phases was detected. This compared the numbers inside at the time of collection; it was not a choice experiment directly testing which phase the insects preferred.
The authors propose that entrance secretions and ultraviolet reflection during the female phase, and the stronger daytime internal scent during the male phase, may compensate for one another. Their explanation is that, with a weaker mechanism for prolonged confinement by hairs, the flowers need to attract insects and sustain their interest both when receiving pollen and when releasing it.
Summary
This flower’s scent, color, and secretion cues varied between outside and inside, day and night, and reproductive stages. The authors propose a system that mimics multiple targets by combining features suggestive of fruit and rewarding flowers, rather than simply imitating a decaying egg-laying site.
These results come from one population in South Korea, and the role of each cue still needs to be tested by examining insect perception and behavior. Even so, considering scent not as a single whole but along the process by which an insect approaches a flower, explores the entrance, and enters it can help explain why this group has evolved such diverse pollination mechanisms.
References
Lim, H., Kim, Y.-K., Lee, S., Choi, M.-S., Lee, D., Kim, D., Suh, S. J., & Kim, S.-G. (2025). Multimodal floral cues resembling both generalized food sources and oviposition sites in Isotrema manshuriense (Aristolochiaceae) pollinated by flies. Plant Biology, 27(6), 1058–1070. DOI: 10.1111/plb.70040 / Full original paper.
The research was supported by the National Research Foundation of Korea, the Rural Development Administration, and the KAIST Ecological Research Program. The authors declare no conflicts of interest. Original paper: © 2025 The Author(s), CC BY-NC-ND 4.0.
