Wallace, Darwin and the night shift

Last week I had the great honour of attending the Royal Entomological Society’s annual conference, this year hosted by Edge Hill University, to accept the 2017 Alfred Russel Wallace Award. The award is given for the PhD thesis “judged to make the most significant contribution to entomology in the year”, and after several rounds of selection my own thesis, “The role of moths as pollinators, and the effects of environmental change”, was chosen. At the conference, I said that I was particularly proud to receive an award in Wallace’s name, because of his own connections to the subject matter of my thesis – nocturnal pollination by moths. In this blog, I re-tell the story of that connection…

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It is 25th January, 1862, and Charles Darwin is writing to his friend and confidante Joseph Dalton Hooker. Earlier that same day, Darwin had taken delivery of a box of specimens of exotic orchids from James Bateman, a Staffordshire-based orchid specialist.1 Among these, Darwin found one in particular (Angraecum sesquipedale, a species from Madagascar) to be remarkable, due to the great length of the nectary, extending 11.5 inches below the flower. “Good Heavens”, wrote Darwin, “what insect can suck it”.

Angraecum_sesquipedale.png
(L-R) a drawing of Angraecum sesquipedale by W.H. Fitch, included in: James Bateman (1876) A second century of orchidaceous plants. Curtis’s Botanical Magazine; two flowers of A. sesquipedale (© Michael Wolf | Wikimedia Commons | CC BY-SA 3.0); a plate commissioned by A.R. Wallace depicting the hypothetical hawkmoth pollinator of A. sesquipedale, drawn by T.W. Wood and included in: Alfred Russel Wallace (1867) Creation by Law. The Quarterly Journal of Science, 477.

Darwin must have acted quickly. In the same letter to Hooker he had described his “infinite satisfaction” at the prospect of correcting the proofs of his next books, Fertilisation of Orchids, “in 2 or 3 weeks”. Yet when Orchids emerged less than four months later on 15th May, Darwin had revised the text, incorporating a discussion of A. sesquipedale and a rough hypothesis as to its insect pollinator. Reasoning that the longest tongues among English insects belonged to hawkmoths (family Sphingidae), he wrote:

in Madagascar there must be moths with probosces [sic] capable of extension to a length of between ten and eleven inches

Darwin’s prediction sparked debate; among those who sprung to his defence was Alfred Russel Wallace. Contrary to popular opinion, correspondence between Darwin and Wallace was friendly and reasonably frequent after they jointly proposed evolution by natural selection (van Wyhe & Rookmaaker 2015). Five years after the publication of Orchids, Wallace’s Creation by Law asserted not only that Darwin’s predicted moth could safely be assumed to exist, but further, that naturalists visiting Madagascar “should search for it with as much confidence as astronomers searched for the planet Neptune”. Wallace picked out one moth in particular to back up this statement – Xanthopan morganii, then known from tropical Africa (as Macrosila morganii), which had an exceptionally long proboscis.

Xanthopan_morgani.png
Xanthopan morgani; a specimen from continental Africa. Malagasy X. m. praedicta have proboscides several inches longer still!

Several decades later, Walter Rothschild (2nd Baron Rothschild) and Karl Jordan described a newly-discovered subspecies of X. morganii, from Madagascar. Remarkably, the subspecies had a tongue that was, by several inches, longer than that of its continental cousins. In honour of the predictions made by first Darwin and later Wallace, the new subspecies was named X. morganii praedicta.

DarwinWallaceRothschild
(L-R) Charles Darwin, Alfred Russel Wallace, and Walter Rothschild

At this point, the trail went cold for the best part of the 20th century. We had the flower, and we had found the moth with a proboscis to match it, but the final proof of their association was lacking because nobody had ever seen the two interact. That state of affairs lasted until the early 1990s, when Professor L.T. Wasserthal (then of the Friedrich-Alexander University Erlangen-Nuremburg) finally photographed, beyond doubt, the pollinator of A. sesquipedale. It was, indeed, X. morgani.

BotanicaActa
Making the front cover, a photograph of Xanthopan morgani praedicta visiting the flower of Angraecum sesquipedale finally resolves the mystery!

As a final chapter, the video below is well worth a watch: both for its remarkable footage, shot in 2004 by Dr Philip DeVries of the University of New Orleans, of this pollination interaction in action, and for the sheer joy and excitement shown by Dr DeVries and his colleagues when their hard work finally paid off!

If you want to learn more about this story, I owe a debt of gratitude to the review of Arditti and colleagues (2012), on which I drew heavily whilst researching this blog.

1: The relationship between Darwin and Bateman must have been complex. Bateman was, at this time, one of Britain’s foremost experts on, and cultivators of, orchids, and it is hard not to assume that he and Darwin corresponded while the latter prepared Fertilisation of Orchids. However, this box of specimens, and a brief subsequent exchange of letters, appears to be the only surviving evidence of direct interaction between the two (Darwin Correspondence Project). Bateman lived at Biddulph Grange in Staffordshire, designing (with his wife Maria) its famous landscape gardens, which are now owned by the National Trust. The gardens featured a Geological Gallery, seen as a direct attempt to refute Darwin’s theory of evolution by natural selection by drawing connections between evidence from the fossil record and the account of creation given in the book of Genesis.

References

Arditti, J., Elliot, J., Kitching, I.J. & Wasserthal, L.T. (2012) ‘Good Heavens what insect can suck it’–Charles Darwin, Angraecum sesquipedale and Xanthopan morganii praedicta. Botanical Journal of the Linnean Society169, 403-432.

Darwin Correspondence Project, “Letter no. 3356,” accessed on 3 April 2018, http://www.darwinproject.ac.uk/DCP-LETT-3356. Also published in The Correspondence of Charles Darwin, vol. 10.

Van Wyhe, J. & Rookmaaker, K. (2015) Alfred Russel Wallace: Letters from the Malay Archipelago. Oxford University Press, Oxford.

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One thought on “Wallace, Darwin and the night shift

  1. Thanks. What an amazing story. Seems that there are quite a few of these long-tongued sphingid moths in Madagascar. Probably lots more research couldbe carried out? Great video. Came across this paper but have not read it yet. Cheers, Ray
    Netz, C., & Renner, S. S. (2017). Long-spurred Angraecum orchids and long-tongued sphingid moths on Madagascar: a time frame for Darwin’s predicted Xanthopan/Angraecum coevolution. Biological Journal of the Linnean Society, 122(2), 469-478.

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