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Darwin Correspondence Project

From E. J. Johnston   16 March 1875

Sir,

Having gathered from notices and articles in several periodicals, that you were engaged upon the investigation of insect-capturing plants, I am reminded of a very curious case which came within my own knowledge many years ago.1 My observations should long ago have been anticipated; but, so far as I can find, no account of the matter has hitherto been published. I therefore proceed to place the facts before you; my object in so doing being to have the whole matter thoroughly examined into and elucidated.

The plant I alluded to is the Araujia sericofera of Brotero, Trans. Linn. Soc. xii.–xiii.—1818. p. 62.2 In the north of Portugal, where I was residing at the time I refer to, (viz., the summer of 1856) this plant grows very well in the open air, and produces seeds in abundance. The flowers have a rather heavy, sweetish scent, and are frequently visited by butterflies and moths, which are attracted by the melliferous poses of the corolla. In passing by the plant, it struck me that the insects remained rather longer than usual upon particular flowers, and that they did not fly away when I came nearer. Upon this, looking more closely, I found that they were caught by their trunks3 between the two horns of the stigma, which, diverging in the middle, meet again near their apices, and then turn back, so as to form a figure something like a lyre. On separating these horns a little, they show their elasticity by springing back to their original position of contact, as soon as released, with a slight but distinctly audible snap.

I found cabbage butterflies, hawk moths, and sometimes the Gamma moth,4 caught in the above mentioned way, and vainly fluttering or struggling to break loose. I recollect on one occasion having plucked a flower in which a hawk moth was caught, and having carried it away and shown it to some relatives with its captive caught by the trunk, but suspended in the air (above the flower I held) by the vibration of the wings.

How the insects were caught, is the interesting question, to which, not having been able to pursue the matter otherwise than imperfectly, I cannot give a definitive answer. My conjecture is, that the insects, after alighting on the flower and extracting the honey from the four nearest pores, passed their trunks through the opening between the two stigmatic horns, in order to reach the fifth pore without the trouble of going round; and that in withdrawing their trunks obliquely upwards, they became caught like a wedge in a tree.

Knapp, in his “Journal of a Naturalist”, says that the Apocynum androsæmifolium catches flies by their trunks.5 According to his illustrative plates, the two stigmas, (or two divisions of one stigma), diverge at a considerable angle, and close upon the trunk of the insect when it gets between them. But no such movement was detected in the Araujia sericofera. The divisions of the stigma were never found apart, but always closely pressed together. (I ought to add that some of the flowers had very large stigmas, while in others they were very small. Possibly the latter were abortive.) However, as no dead insects were found on the flowers, it would seem that the stigmas after a time lose their elasticity. This of course is merely conjecture. The case would appear to be different with the Apocynum androsæmifolium, which is described by Knapp as not relaxing its hold upon its insect captives until they are dead from sheer exhaustion.

I have mentioned the case of this Apocynum, because it belongs to the same natural order as the Araujia;6 and because, in spite of my negative results, their botanical affinity leads me to suspect that there may be something common to these two plants in their mechanism of capture.

Brotero’s figure does not exactly correspond with the plant I saw, as it is taken from a branch in which none of the flowers were more than half expanded.7 The leaves of my plant were mostly long, and almost triangular, but sometimes they were heart-shaped at the base, as in his figure.

I suppose, though it did not occur to me at the time, that the detention of the insects would in some way or other contribute to the fertilization of the flowers.

In this country the Araujia sericofera would doubtless require a hothouse or conservatory. That would in a great measure prevent the visits of Lepidoptera. But if grown in a pot or box, and placed in the open air when in flower, the sight of the captive and struggling insects would certainly attract attention. Should you feel disposed to examine into this matter, my friends in Portugal will, I am sure, gladly co-operate with me in rendering any assistance in their power. I apprehend, however, that there would be no difficulty in procuring living specimens.

A rough sketch which I made bears the date of August, 1856; so that the season of flowering would allow ample time for preparation. The plant I saw flourished without cultivation, being merely planted at the foot of a wall with a western aspect, and, after a little preliminary training, left to attach itself to other plants that grew out of the stones above it.

I am, Sir, | Your obedient servant, | Edwin J. Johnston Jr.

Charles Darwin Esq.

My address is—

14 Wycliffe Grove, | Lavender Hill, | Wandsworth Road | S.W.

16th March 1875.

CD annotations

Top of letter: ‘Plant which catches moths like Apocynumblue crayon

Footnotes

Although CD had not yet published any of his research on insectivorous plants, references to his ongoing work had appeared in science journals. See, for example, Hooker 1874b and Burdon Sanderson 1874a.
Félix de Avellar Brotero had first described Araujia sericofera in a paper read on 7 November 1815 (Brotero 1815); in the published paper in the Transactions of the Linnean Society of London 12 (1817–18): 62–70, the spelling of the species epithet was given incorrectly. The accepted name is Araujia sericifera (common moth-vine or cruel plant; for more on the synonymy of this species, see Forster and Bruyns 1992).
Johnston uses the word ‘trunk’ to refer to the proboscis of the insects.
The cabbage white butterfly is Pieris brassicae; the gamma moth, now more commonly known as the silver Y, is Autographa gamma. Hawk moths belong to the family Sphingidae.
John Leonard Knapp described the destructive powers of Apocynum androsaemifolium (fly-trap dogbane) in [Knapp] 1829, pp. 81–2, and figured its capture mechanism in plate I, fig. 5.
Both Araujia sericifera and Apocynum androsaemifolium belong to the family Apocynaceae. Apocynaceae is described as a natural order of Gentianales in Lindley 1853, p. 594.
For Brotero’s figure, see Brotero 1815, tab. 4, facing p. 69.

Bibliography

Brotero, Félix de Avellar. 1815. Descriptions of a new genus of plants named Araujia, and of a new species of Passiflora. [Read 7 November 1815.] Transactions of the Linnean Society of London 12 (1817–18): 62–75.

[Knapp, John Leonard.] 1829. The journal of a naturalist. London: John Murray.

Lindley, John. 1853. The vegetable kingdom; or, the structure, classification, and uses of plants, illustrated upon the natural system. 3d edition with corrections and additional genera. London: Bradbury & Evans.

Summary

Reports an Araujia in Portugal that captures various insects on the horns of its stigma. Relates this to another asclepiad, Apocynum, which also captures insects. Is this "insectivory" or insect fertilisation?

Letter details

Letter no.
DCP-LETT-9890
From
Edwin John Johnston
To
Charles Robert Darwin
Sent from
London, Wycliffe Grove, 14
Source of text
DAR 168: 74
Physical description
8pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9890,” accessed on 24 November 2020, https://www.darwinproject.ac.uk/letter/DCP-LETT-9890.xml

Also published in The Correspondence of Charles Darwin, vol. 23

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