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Darwin Correspondence Project

From Francis Walker   9 December 1867

Elm Hall | Wanstead

Decber. 9th. 1867

My dear Sir,

I have looked at the Homopterous Insects armed with spines or with horns, & I believe that there is no difference in this respect between male & female. In the Homoptera there is no contrast between the sexes, but only the general difference between one sex & the other.1 This rule excludes the Aphididæ & the Coccidæ, the latter being most remarkable in the want of resemblance between the sexes. I have, as yet not published on variation in Insects except a few remarks on the question of species in my list of the Diptera of the Eastern Isles in the Journ Linn. Soc—2 I am about to publish some notes on the Aphididæ—a translation of Passerini’s synopsis of the species & remarks on the nomenclature, & I have introduced here & there some words on the mutability of species.3 I will write to you all that may appear from me on the latter subject. I have made only one experiment on Aphides. The Aphis of the holly & the Aphis of the ivy seem to be exactly alike tho’ they have been described as two species. I took some shoots of ivy infested by Aphides & tied them with thread to shoots of holly, but tho’ the Aphides wandered to the holly they did not feed on it & soon perished. Probably the Aphis requires the same kind of food from its birth for tho’ some species migrate to a great variety of plants they hardly feed after they have migrated & pass out of existence when they have brought forth a new generation4   I believe that the migrations initiate yearly many species (so called) which have no continuity, but pass away before winter. I am just now making out a list of the Blattidæ or Cockroaches— this tribe is remarkable in four particulars— 1st.—the great difference in some species between male & female   2nd the incomplete metamorphose of some species or their perpetuity in the apparently larva-state   3rd. the facility with which some species accomodate themselves to artificial circumstances & thus become cosmopolitan—& 4th. the disguises which they put on or their mimicry of the Coleoptera, Myriapoda & terrestrial Crustacea with which they are associated in their natural habitations— They thus seem to epitomize some tribes that are inferior to them in structure & to indicate others of higher organization—5 Some insects are said to photograph the colours near them— those that are disguised may be said to morphograph other tribes with which they dwell or to be examples of morphography.6

I enclose my Carte de visite— may I ask for the favor of yours?—7

With my best wishes, | Yours truly | F. Walker

P.S. Your Carte de Visite has just arrived— many thanks for it.8

CD annotations

1.1 I have … female. 1.2] scored red crayon
1.2 In the … the sexes. 1.5] ‘Homoptera’ added and underl blue crayon
1.5 I have, … winter. 1.20] crossed ink
1.21 1st.... larva-state 1.23] scored red crayon
1.21 difference.... & female 1.22] ‘I suppose (not in ornament)’ added ink
1.23 3rd.... best wishes, 3.1] crossed ink
Top of letter: ‘Perhaps serve as defences like Bates’ Ants’9 ink


Walker made an extensive study of the Homoptera collected by Alfred Russel Wallace in the Malay Archipelago; his catalogue of the collection had been read at the Linnean Society in February 1867 (Walker 1867).
Walker refers to his synopsis of the Diptera discovered by Wallace in the Malay Archipelago (Walker 1865); in it, he commented on similarities between species and on geographical distribution, questioning whether some species of Laphria, for example, should be considered as varieties (Walker 1865, pp. 1–6). In Descent 1: 348–9, CD cited Walker on differences between the sexes in the dipterous genus Bibio.
Giovanni Passerini published two works on aphids (Passerini 1860 and Passerini 1862–3); Walker refers to the English publication of Passerini’s systematic arrangement of the aphids, supplemented by Walker’s notes (Walker 1868–70). For more on Walker’s work on the taxonomy of aphids, see Doncaster 1961.
Walker described Aphis ilicis, the holly aphid, and A. hederae, the ivy aphid, and the experiment of changing the food source, in Walker 1868–70, p. 1998.
Walker’s Catalogue of the specimens of Blattariae in the collection of the British Museum included representatives of the family Blattidae (Walker 1868). Walker reiterated his third and fourth points in Walker 1868, p. 233.
Walker’s neologism has not persisted in the sense in which he uses it.
Walker’s photograph has not been found in the Darwin Archive–CUL. The photographic ‘carte’ or ‘carte de visite’, smaller than a conventional portrait, became popular in the 1860s and several were made of CD between 1864 and the end of his life (see Browne 1998, pp. 253–80).
It is not clear which ‘carte de visite’ CD sent. It may have derived from an 1864 photograph of CD by his son William Erasmus Darwin (see Correspondence vol. 12), or from photographs taken by Ernest Edwards, probably in November 1865 or April 1866 (see Correspondence vol. 13, letter from E. A. Darwin to Emma Darwin, 25 [November 1865], n. 3). See also letter from E. Schweizerbart’sche Verlagsbuchhandlung, 22 March 1867, n. 8.
CD may refer to Henry Walter Bates’s account of differences in the sizes of mandibles in a number of Brazilian ant species; different types of worker ants of the same species exhibited these differences (Bates and Smith 1855). There is an annotated copy of Bates and Smith 1855 in the collection of unbound journals in the Darwin Library–CUL.


Browne, Janet. 1998. I could have retched all night. Darwin and his body. In Science incarnate. Historical embodiments of natural knowledge, edited by Christopher Lawrence and Steven Shapin. Chicago and London: University of Chicago Press.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.

Doncaster, John Priestman. 1961. Francis Walker’s aphids. London: British Museum (Natural History).

Passerini, Giovanni. 1860. Gli afidi con un prospetto dei generi ed alcune specie nuove italiane. Parma: Tipografia Carmigani.

Passerini, Giovanni. 1862–3. Aphididae italicae hucusque observatae. Archivio per la zoologia, l’anatomia e la fisiologia 2: 129–212.

Smith, Frederick. 1855. Catalogue of British Hymenoptera in the collection of the British Museum. Pt 1. Apidæ–bees. Edited by John Edward Gray. London.

Walker, Francis. 1865. Synopsis of the Diptera of the Eastern Archipelago discovered by Mr. Wallace, and noticed in the "Journal of the Linnean Society." [Read 1 June 1865.] Journal of the Linnean Society (Zoology) 9 (1868): 1–30.

Walker, Francis. 1867. Catalogue of the homopterous insects collected in the Indian archipelago by Mr. A. R. Wallace, with descriptions of new species. [Read 7 February 1867.] Journal of the Linnean Society (Zoology) 10 (1870): 82–193, 276–330.

Walker, Francis. 1868–70. Notes on Aphides. Zoologist 2d ser. 3 (1868): 1048–53, 1118–23, 1296–1301, 1328–33; 5 (1870): 1996–2001.

Walker, Francis. 1868. Catalogue of the specimens of Blattariae in the collection of the British Museum. London: Trustees of the British Museum.


The horns and spines of homopterous insects do not vary between sexes. Sexual differences in Blattidae.

Letter details

Letter no.
Francis Walker
Charles Robert Darwin
Sent from
Source of text
DAR 82: A48–9
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5718,” accessed on 6 March 2021,

Also published in The Correspondence of Charles Darwin, vol. 15