Summoned by screams

Come into my parlour

THE botanical kingdom is rife with deceivers. Carrion flowers mimic the smell of rotten meat in order to attract scavenging beetles and flies and then cover them in pollen. Passion vines, beloved by some butterflies as food for their caterpillars, have yellow spots on their leaves that make them look as if they have already had an egg-laying visit from a gravid female. And numerous carnivorous plants lure insects with sweet odours, only to devour them. Now Stefan Dötterl and Annemarie Heiduk, of the Universities of Salzburg and Bayreuth respectively, think they have uncovered yet another form of deception. As they describe in Current Biology, a vegetable called the parachute plant uses chemical signals to trick carnivorous flies into believing the insects those flies prey on are lying wounded inside it.

At first sight parachute plants, which have cone-shaped flowers (see picture) decorated inside with needlelike inward-pointing hairs, look as though they might be carnivorous themselves. They are not, though they come close to it. Insects that enter a parachute-plant flower fall into a pit of pollen and cannot escape past the needle-hairs until the flower begins to wilt—by which time they are thoroughly covered in the stuff.

That raises the question of why those insects go inside in the first place, for parachute-plant flowers do not smell of any of the conventional odours, sweet or rank, that plants employ to attract attention. To answer that, the two researchers asked themselves two other questions: exactly which insects do parachute flowers attract and what volatile chemicals, if any, do those flowers produce?

By collecting the victims of many wild parachute plants Dr Dötterl and Ms Heiduk discovered that most of the trapped insects were flies of the genus Desmometopa, a group with a predilection for sucking up vital fluids that leak out of honey bees as their bodies are pierced by the fangs of spiders. Meanwhile, collecting chemicals given off by the flowers and running them through a gas chromatograph showed a combination of four molecules—2-heptanone, geraniol, 2-nonanol and (E)-2-octenyl acetate—previously unknown in plants. When Dr Dötterl and Ms Heiduk caught some foraging bees, stuck them in test tubes and poked them with the tip of a glass pipette to mimic a spider attack, the bees produced exactly these four compounds. And when the two researchers set traps containing the four chemicals out in the wild, they instantly attracted a goodly haul of Desmometopa.

An examination of past apiological research showed that the compounds in question are already known to students of bees. The insects’ mandible glands produce 2-heptanone when they are attempting to bite predators, and this chemical has a debilitating effect on such threats to the hive as predatory moth larvae and mites. The other three compounds, meanwhile, are signal molecules released by bees fighting for their lives, to notify colony members of the danger. Together, then, these four substances are a good indication of a honey bee in the sort of trouble that is a dinner gong to Desmometopa. That parachute plants have evolved to mimic this gong is yet another example of the deceptive power of plants.

 


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