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:
Fig. 1. Ophyrys insectifera (fly orchid), which deceives male digger wasps.
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