Paphiopedilum rothschildianum, Orchid of the Month, July 2017

Paphiopedilum rothschildianum flower, which can be 20 to 30 cm across

Paphiopedilum rothschildianum is a commanding plant with an extraordinary flower and is much beloved of Paphiopedilum growers, but it also has other charms that make for a good story. The plant has long glossy strap-like leaves and a tall multi-flowered spike with one of the most distinctive flowers in the plant world. It is terrestrial and grows in Borneo (more about that later) on steep slopes on rocks or among plant detritus that isn't quite soil, usually above running water, often exposed to unusually sunny conditions for an orchid.

Paph. rothschildianum plants in situ in Borneo

If growing only in Borneo isn't enough to make it feel exotic to you, this plant has been found only on the lower slopes of a single mountain in Borneo, Mt. Kinabalu. Okay, it is a really big wide mountain, but still this is one rare plant in nature. There is presumably something special about this location, because this is not a plant struggling to make it by - it is locally abundant. When it has the right conditions it thrives. It could be the microclimate or the soil/rock type is just right, or it could be that its coevolved syrphid fly pollinator is found only in that area. Tropical climates generally support more biological diversity, in part because species are native to a smaller area, but this example is extreme. This gradient of biological diversity is called, cleverly enough, the latitudinal diversity gradient, and it well worth your time to read about - for one thing nobody is sure exactly why it happens.

Speaking of syrphid flies, also known as hoverflies, these are thought to be the pollinators of Paph. rothschildianum. These flies are familiar to those in temperate climate because many species look rather like a bee, with black and yellow stripes (hoverflies don't sting, and their coloration is a case of Batesian mimicry, but the full story takes us too far afield). The larvae of many syrphid flies eat other insects, and mother fly provides for her young by laying eggs among their preferred dinner. In the case of the fly that pollinates Paph. rothschildianum, dinner is aphids and the staminode of the flower has two parts: one part packed with bristles with a fat white tip that apparently looks like a bad aphid infestation to mother fly, and the other part waxy and presumably slippery to the fly. The fly lands to lay eggs (and often does lay eggs, thus dooming her aphid-eating larvae), slips and falls into the bucket-like lip below, and the rest of the process is the same as for all of the Lady Slipper orchids: the slippery walls, the escape route, the pollinia - described many times all over the web. Apparently the trick of luring unwary syrphid flies by mimicking aphids is shared by some other Paphiopedilum and Phragmipedium species, though I am not sure how many. The staminode bristles don't look quite like aphids to me, but then again I don't know what kind of aphid the fly is looking for, and there may be olfactory and tactile cues as well. As for why the rest of the flower is so large and boldly marked, I confess I have only a poor idea. To me it says "beware syrphid fly, here be fake aphids", but perhaps the fly can't figure this out, but is smart enough to learn that huge aphid colonies (appear to) infest these places. This should be an evolutionarily stable strategy if the frequency of real aphid infestations is much higher than Paph. rothschildianum flowers.

Close up of the staminode (the V-shaped structure near the middle), with the white-tipped aphid mimic bristles covering all but the outer (leftward) face, photo from IOSPE.

And finally, another picture showing the strap leaves and the bold flowers as they might appear to a female syrphid fly looking for a great place to lay eggs. Oh yeah, and they look pretty awesome to us humans too.