Darwin's "Orchis Bank" at Downe

by Irene Palmer.

'Kent appears to be the most favourable county in England for the order, and within a mile of my house nine genera, including thirteen species grow . . .'

Skipper butterfly on a Pyramidal Orchid, Anacamptis pyramidalis.
Photo by John Palmer.

Downe Bank is owned by the Kent Wildlife Trust. It was purchased on the advice of Dr Francis Rose in 1962, when it became the first of some 50 nature reserves. It was extended by the acquisition of Hangrove Wood in 1982. Downe Bank is a uniquely important site, despite its small size, due to the fundamental plant studies Charles Darwin carried out there.

This orchid-rich site, close to Darwin's home, was called "Orchis Bank" by the Darwin family who enjoyed picnicking there. Downe Bank is currently playing a key role in Darwin's Landscape Laboratory, which has been nominated as a potential World Heritage Site by the UK government. Wild orchids provided Charles Darwin with a eureka moment in the summer of 1860, a year after he published the Origin of Species. Darwin was not the first biologist to realise that flowers had not been created for our aesthetic pleasure or for their honey but his opinion was still controversial. While working on the Origin of Species he realised the significance of cross-fertilisation, and understood that most flowers didn't pollinate themselves but had evolved colours, shapes and scents to attract insects to do the work for them.

A Brimstone butterfly, Gonepteryx rhamni, attempting to feed from a Pyramidal Orchid, Anacamptis pyramidalis. Photo by Bill Welch.

In May 1860 Darwin discovered orchid flowers are uniquely constructed to ensure they are cross-fertilised. Instead of stamens, orchid flowers have a pair of club-shaped sacs, called pollinia, which contain pollen. He decided to mimic an insect seeking nectar and probed an orchid flower with a pencil. As it was withdrawn he saw the pollinia had become firmly attached and watched with amazement as he saw them tilt forward into the correct position to adhere to the stigma of the next orchid flower. He concluded this ensured orchid pollen was efficiently transported by insects and such precision instruments must have evolved over a long period of time. He immediately put his other work aside to work on orchids as he was desperate to find evidence to support his theory of evolution by natural selection.

Flower of a Broad-leaved Helleborine, Epipactis helleborine. Photo by John Palmer.

His studies revealed many highly specialised adaptations between orchids and insects. He observed that each species of orchid relied on different insects, sometimes on a single species. Orchid pollinia are precisely aligned to stick onto their heads, tongues, abdomens or legs, depending on the species. For example, the pollinia of the common spotted orchid (Dactylorhiza fuchsii) become attached to insects' heads, while the pollinia of the pyramidal orchid (Anacamptis pyramidalis) clip onto the tongues of butterflies and day-flying moths.

In 1862 Darwin published Fertilisation of Orchids, comparing British and foreign orchids, claiming orchids provided a supreme example of co-adaptation with their insect pollinators. His observations provided considerable weight of evidence to support his theory. It is widely agreed that the Origin of Species would have attracted far less controversy if his orchid book had been published first. Subsequently Darwin published five more books on plants and Downe Bank provided a valuable source of plants for some of these studies. Notable species included Oxalis acetosella, Euonymus europaeus and Viola spp. However, orchids continue to attract most interest from visitors. The second, expanded edition of Darwin's orchid book was published in 1877 and is the usual source of reference. The 150th anniversary of Darwin's orchid book was celebrated in 2012.

Common Twayblade, Neottia ovata.
High Elms Country Park.
Photo by Bill Welch.

Ichneumon Wasp on a Common Twayblade, Neottia ovata.
Photo by John Palmer.

Since Darwin's time our knowledge of orchid pollination has advanced considerably. We've learned that orchids have evolved two main strategies to attract insect pollinators: the majority offer a nectar reward but about a third of the world's orchids are nectarless and deceive insects.

The common twayblade (Neottia ovata) is the most abundant species on Downe Bank and tempts insects with a supply of freely available nectar on its elongated lip. The broad-leaved helleborine (Epipactis helleborine) similarly offers freely available nectar; both orchids attract different species of wasps. Insects often visit several flowers on the same spike of species that provide nectar, which is a genetic disadvantage for the orchid, although more of their flowers generally get pollinated. However, about a third of all orchids world-wide are deceivers.

Deceptive species found on Downe Bank include the common spotted orchid (Dactylorhiza fuchsii) and the pyramidal orchid (Anacamptis pyramidalis). These orchids deceive insects by mimicking nectar-bearing flowers; an energy saving strategy that is remarkably effective. Although deceptive species attract fewer insects, each fruit capsule has thousands of seeds. Biologists have recently discovered that seeds produced by deceptive orchids are more likely to have been produced by cross-fertilisation from different plants that also grow at some distance from each other. As these orchids are generally pollinated by naive, newly emerged insects searching for a reliable food source that get frustrated when they fail to find nectar, they fly some distance away before entering another flower.

Greater Butterfly Orchid, Platanthera chlorantha. Pollinia are splayed apart and attach to the eyeballs of Noctuid moths.
Photo by John Palmer.

Lesser Butterfly Orchid, Platanthera bifolia. Pollinia are parallel and attach to the tongues of Sphingid moths.
Photo by John Palmer.

In the Origin of Species Darwin highlighted one of the most remarkable examples of a highly specialised relationship. The Madagascar comet orchid has night-scented white flowers and a nectar tube eleven inches long. Our two species of native butterfly orchids are miniature replicas and after studying their pollination mechanisms Darwin deduced the Madagascar comet orchid must be pollinated by a moth with a tongue eleven inches long. (Wallace agreed with him). The moth that pollinates this orchid was discovered long after Darwin died and was called Xanthopan morgani praedicta, recognising that Darwin had predicted it.

Recently, the bee orchid was selected to illustrate a series of stamps celebrating the work of Charles Darwin, along with the finches, a chimp and iguana, creatures that are usually associated with Darwin in most people's eyes. This marked welcome recognition of the importance of Darwin's local plant studies..

'. . . no single point in natural history interests me so much as the self-fertilisation of the Bee-orchis.'

Flower of a Bee Orchid, Ophrys apifera.
Photo by Bill Welch.

Our native bee orchid is perhaps the most charismatic of all our wild flowers. Darwin also fell under its spell. Although this orchid closely mimics a bee, no one had ever seen it being pollinated by a bee; instead its structures ensure it is self-pollinated. He was so perplexed by this orchid that towards the end of his life he confided to his physician Dr Norman Moore, that 'one of the things that made him wish to live a few thousand years was his desire to see the extinction of the Bee-orchis.'

Flowers of a Fly Orchid, Ophrys insectifera.
Photo by John Palmer.

Darwin wasn't harbouring a desire to rid the planet of bee orchids but logic suggested it had evolved to attract bees and he found it impossible to understand why this species had evolved such accurate mimicry. He believed in-breeding would lead to its extinction in the long term, as it was ill fitted to evolve under changing circumstances. Today, it is believed the true pollinator has become extinct. However, the bee orchid has proved capable of mutating and producing hybrid swarms in some circumstances, so new pollinators may be attracted in time. This is a safe guess because of the great range of different bee orchids found on the continent that frustrate taxonomists.

Male Digger Wasp exchanging pollinia on a fly orchid, Ophrys insectifera. Photo by John Palmer.

Members of the bee orchid family are deceivers that have evolved elaborate strategies of sexual mimicry to ensure pollination. The flowers of these orchids mimic female bees or wasps. For example, the fly orchid (Ophrys insectifera), secretes a perfume containing pheromones (sex hormones), similar to those that a tiny female digger wasp employs to attract a mate. The flowers of these orchids resemble a small female wasp, they have silvery patches on their lips that mimic a female wasp's folded wings, while their slender upper petals resemble the wasp's antennae, thus completing the deception.

Male Digger Wasp on a Fly Orchid, Ophrys insectifera.
Photo by John Palmer.

Male digger wasps are completely fooled by this deception and attempt to mate with the orchid flowers. 'Pseudocopulation', as this behaviour is now known, was discovered in 1920 by two Frenchmen called Pouyanne and Correvon while studying bee orchids in Algeria. More than twenty years passed before they dared publish their shocking discovery that insects were attempting to mate with orchids.

Three male Digger Wasps on a Fly Orchid, Ophrys insectifera.
Photo by Grant Hazlehurst.

When orchid expert David Lang published his popular books only one photograph of fly orchid pollination existed in the UK. Darwin never saw an insect visiting the fly orchid. Some years ago, in my ignorance, I recall brushing a tiny wasp off a fly orchid flower that we were trying to photograph. Photographs of this event eluded us until a lucky day when persistence was rewarded.

My late husband John finally photographed this remarkable event and luckily a film crew that was on hand on Downe Bank to film it. Since then our assistant warden Grant Hazlehurst has taken some remarkable and unique photographs of a double-decker and a triple-decker of ardent male wasps attempting to mate with a fly orchid flower.

One further challenge remained, as none of these photographs showed wasps with pollinia attached to their heads. Then one day John finally succeeded in photographing the rare event of a male wasp actually exchanging pollinia. I would give anything to see Charles Darwin's expression, if we could tell him these stories.