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Darwin Correspondence Project

From John Blackwall   12 February 1868

Hendre House,

February 12th, 1868.

Dear Sir,

I wish it were in my power to give satisfactory answers to the questions contained in your letter of the 10th instant.1 Adult spiders of the same species differ so remarkably in size and colour, and that independently, to all appearance, of the situations in which they are found, that I am unable to assign an adequate cause for this extraordinary fact. At one time I was disposed to suspect that a deficiency of light might influence the colour of such species as habitually pass their lives in obscure gloomy places; but I have observed such marked differences in individuals of the same species inhabiting the same or similar localities that I refrain from expressing a decided opinion on the subject. The several striking varieties of Theridion lineatum to which you allude, I have frequently met with in close proximity, and that they are specifically identical cannot be doubted, as they entirely coincide in their external structure, habits and economy.2 Foreign spiders seem to present variations in colour as conspicuous as those observed in our indigenous species, but it commonly happens that very little is known of their habits and haunts.

In numerous instances the sexes of spiders differ greatly in size and colour, and the males in some cases bear a very small proportion, apparently, to the females in point of numbers.3 Among the spiders provided with spines on the abdomen, belonging to the genus Gasteracantha, I have not yet seen a male; and Walckenaer and Koch4 do not appear to have been more fortunate than myself in that respect; the only descriptions of this sex with which I am acquainted are very brief ones of two species given by Thorell, the Swedish arachnologist, from which I infer that the abdominal spines do not constitute a sexual characteristic.5

The courtship of spiders is usually a very prolonged and tedious affair; I have frequently observed two or more males on the same web with a female, but I do not recollect having seen them fight for the possession of her; they are generally extremely cautious in making their advances.6

Spiders are subject to frequent variations in the number and size of their eyes, a circumstance of much importance when considered in relation to their systematic arrangement. In the spring of last year I received an undescribed species of Pholcus7 from India, in which the two sexes were united, the left side being that of an adult male, and the right side that of a female; this must be a case of exceedingly rare occurrence, as it is the only instance of the kind that has come under my observation, and I have not met with any notice of a similar one in the course of my reading.

Regretting that the information in my possession relative to the particulars which form the subjects of your inquiry is so very scanty, | I am, dear Sir, | very truly yours | John Blackwall.

CD annotations

2.1 and the … numbers. 2.3] scored red crayon
3.1 I have … of her; 3.3] scored red crayon
4.1 Spiders … their eyes, ] scored red crayon
4.3 I received … united, 4.4] scored red crayon


CD’s letter to Blackwall has not been found.
Theridion lineatum (now Enoplognatha ovata) is described in Blackwall’s A history of the spiders of Great Britain and Ireland (Blackwall 1861–4, 1: 176–7), a copy of which is in the Darwin Library–Down. Blackwall noted that two varieties of T. lineatum had been described as distinct species (T. redimium and T. ovatum) based on patterns of coloration on the abdomen. In Descent 1: 337–8, CD cited Blackwall 1861–4 for information on the different coloration of male and female spiders, and mentioned T. lineatum as illustrating the extreme variability in colour in some species.
Information from Blackwall on the proportion of the sexes in spiders is reported in Descent 1: 314–15.
Ludwig Carl Christian Koch and Charles Athanase Walckenaer. Gasteracantha is a genus in the family Araneidae, orbweavers.
Blackwall refers to the descriptions of Gasteracantha parvula (now G. hasselti) and G. modesta (now Isoxya tabulata) by Tord Tamerlan Teodor Thorell (see Thorell 1859, pp. 303–4).
Information from Blackwall on the courtship of spiders and on the fighting of males for females is given in Descent 1: 338–9.
Pholcus is a genus in the family Pholcidae, daddy long-legs spiders.


Blackwall, John. 1861–4. A history of the spiders of Great Britain and Ireland. 2 vols. London: Ray Society.

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

Thorell, Tord Tamerlan Teodor. 1859. Nya exotiska Epeirider. Öfversigt af Kongl. Vetenskaps-Akademiens Förhandlingar 16: 299–304.


Variability of sizes and colours in spiders; on proportion of sexes; courtship of spiders.

Letter details

Letter no.
John Blackwall
Charles Robert Darwin
Sent from
Hendre House
Source of text
DAR 82: A78–9
Physical description
ALS 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5871,” accessed on 1 October 2023,

Also published in The Correspondence of Charles Darwin, vol. 16