Orchids mimic alarm pheromones of bees to attract wasps

August 26, 2009 • 7:00 am

At the end of The Origin, Darwin constructed a justly famous metaphor of nature as a “tangled bank.” In the case of mimicry, the metaphor might be Walter Scott’s “tangled web,”* since the network of interspecific interaction and deception can become quite intricate. Some of the most famous examples of mimicry are those of orchids that mimic bees and wasps.

Natural selection has molded the flowers of these orchids (many in the genus Ophyrys) into mimics of the insects that pollinate them. Horny male insects, thinking that the petals are a female, land on them and engage in fruitless attempts to copulate (“pseudocopulation”). During the barren act, the insects’ heads or bodies contact the orchids’ pollen sacs, which break off and attach to the insect. The frustrated insect flies off, but soon tries to copulate with another orchid, which puts the hitchhiking pollen in contact with the new orchid’s stigma. In such a way the bees/wasps serve as “flying penises,” helping the orchids have sex. Here are some specimens:

Ophrys insictiferaFig. 1. Ophyrys insectifera (fly orchid), which deceives male digger wasps.

Mirror orchid

Fig. 2. Mirror orchid (Ophrys speculum), which attracts scolid wasps.

By adopting the “female mimic” strategy, the orchid sacrifices half its potential pollinators (the female bees/wasps), but there’s no obvious way to attract a coy female insect.

Here’s a David Attenborough video showing pseudocopulation of a mirror orchid by a wasp. As Attenborough notes, the orchids also have olfactory mimicry of the wasp: they produce a chemical similar to the mating pheromone of the female wasp, further increasing the flower’s allure. (This form of mimicry was recognized only recently since it is far less obvious than the visual similarity).

In a new paper in Current Biology, Jennifer Brodman and her coauthors show that a Chinese orchid, Dendrobium sinese, has an even more intricate strategy for attracting wasp pollinators. Rather than mimicking the wasp’s mating pheromone, the flower produces a chemical that mimics the alarm pheromone of two species of honeybees that are likely to be the wasp’s prey. (I posted on such bee-eating wasps a few days ago.) The paper is short and easy to read; it should be accessible to the non-scientist. And the abstract says it all:

Approximately one-third of the world’s estimated 30,000 orchid species are deceptive and do not reward their pollinators with nectar or pollen. Most of these deceptive orchids imitate the scent of rewarding flowers or potential mates. In this study, we investigated the floral scent involved in pollinator attraction to the rewardless orchid Dendrobium sinense, a species endemic to the Chinese island Hainan that is pollinated by the hornet Vespa bicolor. Via chemical analyses and electrophysiological methods, we demonstrate that the flowers of D. sinense produce (Z)-11-eicosen-1-ol and that the pollinator can smell this compound. This is a major compound in the alarm pheromones of both Asian (Apis cerana) and European (Apis mellifera) honey bees and and is also exploited by the European beewolf (Philanthus triangulum) to locate its prey. This is the first time that (Z)-11-eicosen-1-ol has been identified as a floral volatile. In behavioral experiments, we demonstrate that the floral scent of D. sinense and synthetic (Z)-11-eicosen-1-ol are both attractive to hornets. Because hornets frequently capture honey bees to feed to their larvae, we suggest that the flowers of D. sinense mimic the alarm pheromone of honey bees in order to attract prey-hunting hornets for pollination.

This is the kind of new result that keeps us evolutionists juiced up: you can never predict what kind of bizarre adaptation will crop up in the next issue of a journal.

And if you look at the D. sinense flower, it doesn’t look like a bee or a wasp:


Figure 3. Dendrobium sinense Flower and Vespa bicolor Forager. D. sinense flower (A) and V. bicolor forager with pollinia stuck onto the thorax (B). (Figure from Brodman et al.)

Apparently the fragrance alone is enough to attract the wasp. And, as predicted from the scent mimicry, the wasp doesn’t try to copulate with the flower; instead, it “pounces” on it, exactly as a wasp pounces on an alarm-pheromone-emitting bee. But that pouncing is enough to stick the pollen sac onto the wasp. As the old saying goes, “Natural selection is cleverer than you are.”

h/t: Matthew Cobb


Brodman, J. R. Twele, W. Francke, L. Yi-Bo, S. Xi-quiang, and M. Ayasse. 2009. Orchid mimics bee alarm pheromone in order to attract hornets for pollination. Current Biol. 19:1368-1472.

*Oh! What a tangled web we weave
When first we practice to deceive!

Walter Scott, Mamion

A bizarre case of sound mimicry involving caterpillars and ants

February 7, 2009 • 7:26 am

Mimicry was one of the first pieces of evidence supporting Darwinism, as it wasn’t immediately obvious why a celestial creator would make one creature mimic another, while natural selection could easily explain it if the mimic gained protection or resources by resembling its “model.” The most recent issue of Science–an issue loaded with fascinating studies of evolution–has a really interesting article involving mimicry of sounds.

Ant colonies have been invaded by many species of arthropods who take advantage of the ants’ food stores or presence to gain food or protection from predators. Often natural selection has molded these arthopods to mimic the appearance of the ants. In this issue of Science, however, Francesca Barbero and his colleagues describe a caterpillar that has gone farther–it mimics not only the chemicals of the ants, but also the sounds produced by the ant queen to subdue and gain sustenance from her workers.

Caterpillars of the lycaenid butterfly Maculinea rebeli are carried by one species of ants (Myrmica schencki) into the ant nest, where they are fed by the workers, who normally feed larvae of the ants. Why do the workers feed the alien species? They are deceived by the caterpillar in two ways: the caterpillars mimic the ants both chemically and through sound!

First, the caterpillars secrete chemicals that resemble the chemicals on the surface of ant larvae. Chemical mimicy is not so unusual in insects. What is more remarkable is that both the caterpillars AND PUPAE (the next life stage of the caterpillar) of the butterfly are able to produce sounds similar to those produced by the ant queens (how the caterpillars do this is unknown.) These queens have “stridulatory organs” which they rub to produce special “queeny” sounds that induce the workers to tend and feed them. (The ant workers also make sounds, but they differ form those of the queen). Sure enough, both caterpillar and pupae are able to produce sounds more similar to those of the queen than to those of the workers. Playback experiments of recorded caterpillar and pupal calls demonstrated that these sounds induce tending behavior in the ant workers. This is simply astounding–I am not aware of any other case in which a pupa is able to produce sounds, much less sounds that mimic those of another species. It is this kind of bizarre adaptation, resulting from selection on the butterflies to get free food in the juvenile stages, that gets the juices of evolutionary biologists flowing. The bounty of natural selection is endless: you can never predict what it will come up with.

You can listen to the sounds of the ant queens and workers, and of the larvae and pupae of the butterfly here.

Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite.  Francesca Barbero, Jeremy A Thomas, Simona Bonelli, Emilio Balletto, and Karsten Schönrogg, Science 6 February 2009: Vol. 323. no. 5915, pp. 782 – 785