Ceropegia gerrardii smells of pain to attract ‘blood’ drinking flies

Kleptomyiophily, a term that may sound unfamiliar, refers to a highly specialized form of floral mimicry where flowers imitate wounded insects to attract “kleptoparasitic” flies as pollinators. These flies typically steal food from other insects, such as the (not quite) blood of injured honey bees, rather than gathering their own. In a recent study published in New Phytologist, Annemarie Heiduk and colleagues have discovered a fascinating example of kleptomyiophily in the non-trapping flowers of Ceropegia gerrardii, a plant native to South Africa.

The clue that Ceropegia gerrardii was up to something odd came from observing milichiid flies visiting the plant’s flowers. Milichiidae are flies that feed on haemolymph, which is what insects have instead of blood. 

It’s well worth grabbing as it carries nutrients, hormones, and other vital substances. However, milichiidae are tiny, only a couple of millimetres long, so they don’t bring down insects themselves. Instead, they tend to turn up when something else, like a spider, kills an insect and then fly in to feed, which is why one site refers to them as freeloader flies.

The flies need to be fast to get a free lunch. Heiduk and colleagues say they’ve seen them turn up to an insect just a couple of minutes after injury. From this, they reasoned that some form of chemical effect must be happening, and plants are masters of chemicals. They also noticed that Ceropegia gerrardii secreted liquid drops, similar to another kleptomyiophilic flower. 

To understand the connection between the flies and the flower, the team set out to study if Ceropegia gerrardii is pollinated by the flies and if it needs them to produce seeds. They also examined if there was a visual attraction to the flowers for the flies or if it was the liquid drops or scent that attracted them.

The botanists found that Ceropegia gerrardii was pollinated by kleptoparasitic Desmometopa spp. (Milichiidae) flies. Excluding the flies from the flowers showed that these flies were doing a vital job for the plants in pollinating them. To attract these flies, the flower corollas extrude a protein- and sugar-containing secretion similar to the haemolymph of wounded insects on which the flies feed. But the similarity to wounded insects didn’t end with the reward.

Interestingly, the floral scent of Ceropegia gerrardii was chemically similar to that of injured honey bees. Through electrophysiological experiments, researchers were able to measure the electrical responses of the insects’ antennae to the volatile compounds emitted by the flowers. Four out of 24 electrophysiologically active compounds, all released by injured honey bees, were identified as key players in pollinator attraction. It opens the question of whether the plants are deceiving the flies. In their article, Heiduk and colleagues write:

The fly-pollination system of C. gerrardii clearly involves mimicry in the sense that it promotes cognitive misclassification by the signal receiver (Johnson & Schiestl, 2016), yet it is not entirely deceptive in the sense that flies do obtain a reward. Whether the floral exudate is as nutritious to flies as actual haemolymph remains to be established, although the presence of protein in the secretion suggests that it provides more than just an energy source.

…

In the case of C. gerrardii, the floral exudate seems to have a function of detaining flies on flowers for long periods so that they eventually probe between the corona lobes and acquire or deposit pollinaria (a function replaced by the trap mechanism in C. sandersonii).

Heiduk et al. 2023

From the insects’ point of view, the plants are dripping with blood and smell of pain, which is perfect for a kleptoparasitic fly. This strategy allows the plant to ensure pollination, as the flies visit the flowers to feast on the ‘blood’, unknowingly transferring pollen in the process.

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Heiduk, A., Brake, I., Shuttleworth, A. and Johnson, S.D. (2023) “‘Bleeding’ flowers of Ceropegia gerrardii (Apocynaceae-Asclepiadoideae) mimic wounded insects to attract kleptoparasitic fly pollinators,” New Phytologist. Available at: https://doi.org/10.1111/nph.18888.