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Darwin Correspondence Project

From John Blackwall   8 September 1869

Hendre House,

September 8th, 1869.

Dear Sir,

In fulfilment of my promise, I write a few lines to inform you that the result of my observations on the young of the varieties of Theridion lineatum on quitting the cocoons that I had collected for the purpose of ascertaining whether on that occasion and for some time afterwards they would present any characteristics by which they might be distinguished from each other or not, completely confirms my opinion previously expressed, that no such characteristics can be perceived; and this remark applies also, as might be expected, to the young spiders in their customary haunts.1 It would be well to extend the observations through the several stages of growth in the young of these varieties, but the undertaking is surrounded by so many and great difficulties as to render it scarcely practicable.

The colours on the abdomen of Epëira diadema2 present considerable variety in adult specimens, but the design formed by their distribution is pretty constant; the young, however, on quitting the cocoon differ decidedly from the parents and cannot be distinguished from each other.

An additional motive for bestowing my tediousness upon you has been induced by meeting with a passage in Westring’s “Araneæ Svecicæ” having relation to the stridulous sound produced by several species of Theridia.3 At the conclusion of the description of Theridion serratipes,4 p. 175, he adds the following “Obs. Maris abdomen circa nervum, quo thoraci affixum est, valvula denticulata munitum, cujus usum sæpe animo perpendi, et tandem mihi fortuito contigit comperire hanc valvulam instrumentum stridoris esse, et quod thoracis basi, sub oculo armato transverse subtilissime rugulosa, ab animalculo applicatum, stridorem fere ut in Cerambycinis, Reduvio personato et multis aliis insectis reddebat. Hunc sonum audiebam, cum aranea, acu affixa, pulvillo acuum vel segmento papyri usque ad pectus apprimebatur, et tunc aranea, aut sua sponte, aut digitis erga anum attacta, abdomen sursum deorsum movebat, hujusque basin vel valvulam serratam erga basin thoracis affricabat. Femina organis iis caret. Idem post tempus plures mares Theridiorum stridentes mihi obvenerunt.”5

I may remark also that M. Westring terminates his description of the male of Theridion castaneum,6 p. 184, with this statement. “Instrumenta stridentia in basi abdominis et thoracis ut in Th. serratipede, 4-punctato, hamato, guttato et albomaculato adsunt.”7

Hoping that the above particulars may interest you, | I am, dear Sir, | very truly yours | John Blackwall.

CD annotations

3.2 “Araneæ Svecicæ”] underl blue crayon
3.4 Theridion serratipes, p. 175,] underl blue crayon
4.2 castaneum] underl blue crayon
Top of letter: ‘I must add that it is a singular circumstance, that the young of Theridion, do not acquire their [darker] [interl] colour until late in life; a circumstance which I shd have expected only in secondary sexual characteristics’8 ink; ‘(& Stridulation)’ blue crayon


See letter from John Blackwall, 10 August 1869. The spider Theridion lineatum is now Enoplognatha ovata (family Theridiidae, cobweb weavers).
Epeira diadema is now Araneus diadematus, the cross orbweaver.
The reference is to Niklas Westring and Westring 1861.
Theridion serratipes is now Steatoda phalerata.
The Latin may be translated as follows: ‘Observation. The abdomen of the male is next to a sinew, by which it is attached to the thorax, and which is protected by a tooth-edged flap, the use of which I have often debated; at length it happened by chance to become clear to me that this flap is the instrument of stridulation, because when applied by the insect to the very finely horizontally wrinkled base of the thorax under the armoured eye, it yielded a noise almost as in the Cerambycinis, the Reduvio personato, and many other insects. I have heard this sound when a spider, affixed with a pin either to a pin-cushion or to a sheet of paper, was pressed on the breast. Then the spider, either spontaneously or from the touch of my finger against its rear, moved its abdomen up and down, and the base of this or the serrated flap was rubbing against the base of the thorax. The female lacks those organs. Likewise, in time many stridulating males of the Theridiorum have come before me.’ Westring refers to the longhorn beetle tribe Cerambycini. ‘Reduvio personato’ may refer to the family Reduviidae (assassin bugs) in the order Hemiptera.
The species described in Westring 1861 is Theridium castaneum (now Steatoda castanea). Theridium was an invalid synonym of Theridion.
The Latin may be translated as follows: ‘Instruments of stridulation are present in the base of the abdomen and thorax as in Th. serratipede, 4.-punctatum, hamatum, guttatum, and albomaculatum.’ CD referred to this discussion in Descent 1: 339 n. 16. Theridion guttatum is now Crustulina guttata; T. albomaiculatum is now Chrysso albomaculata; T. 4.-punctatum and T. hamatum have not been identified.
In Descent 1: 338, CD reported on Blackwell’s authority that in spiders both sexes usually resembled each other when young, but underwent great changes in colour as they matured.


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

Westring, Niklas. 1861. Araneae svecicae: descriptae. Gothenburg: D. F. Bonnier.


His observations on young of Theridion lineatum reveal no characteristics distinguishing one from another;

quotes N. Westring on stridulation in Theridion serratipes [see Descent 1: 339].

Letter details

Letter no.
John Blackwall
Charles Robert Darwin
Sent from
Hendre House
Source of text
DAR 82: A80–1
Physical description
3pp †

Please cite as

Darwin Correspondence Project, “Letter no. 6884,” accessed on 23 September 2021,

Also published in The Correspondence of Charles Darwin, vol. 17