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Darwin Correspondence Project

From Robert McLachlan   21 February 1868

1 Park road terrace | Forest Hill

21 Feby. 1868.

My dear Sir

I will now endeavour to answer some of your questions as far as I am able; but must premise that I am not able to give very much information, nor can I obtain much satisfaction from those friends to whom I have put some of your queries.1

Taking the Neuroptera generally I have not observed any great numerical disparity in the sexes, & think that the ♀♀ rather exceed the ♂♂; but this is speaking in the broad sense— In some cases I can positively affirm that the ♀ is by far the most abundant; & in one Brit. Phryganidan (Apatania muliebris mihi) I have been unable to discover the ♂ after the most careful search among 2 or 300 ♀♀.2 And there is no reason to suspect, in this case, any peculiar habit or structure in the ♂. I am moreover convinced that ordinarily throughout the insect-world the ♀ is the most abundant, only that, as it is usually more retired in its habits, it is less frequently observed. Such a question can only be decided by breeding in confinement, & all results tend to prove the superabundance of the ♀.

I am aware that it is generally believed that a male insect after having once performed its reproductive function is unfitted for a repetition of the same act, but it has been proved in certain Phryganidae (& I believe the same thing will be found pretty common in other orders) that the male is to a certain extent polygamous, & able to impregnate several females.

I do not think that Dragon-flies will give any important results as to protective sexual peculiarities. I think I may say that they are of all insects the least liable to the attacks of birds &c. In themselves they are the tyrants of the insect world, & have few enemies.3 Universal aggressors yet seldom aggrieved. Very many species, European & exotic, vary in colour in the sexes. In the true Libellulidæ, when this obtains, the male is usually blue & the female yellow, yet I see not how the latter & more conspicuous colour can protect. It is to be remarked also that these insects take a considerable time to attain their full coloration & that at first both sexes are like coloured. Thus the ♂ of Libellula depressa does not acquire its blue tint of body until nearly a fortnight after its first appearance as an imago; & it is probable that until its period of puberty, so to speak, has arrived, it is unfit for reproduction.4 I look upon the colours of Dragon flies more in the light of sexual attractions than protections.

In the Agrions I think I have observed the males fighting, but I cannot remember to have observed this in any of the larger species.5 From the extraordinary arrangement of the generative organs, the male has to impregnate himself (pardon the expression) before he seeks the female, & it is possible that conflicts may take place between two males simultaneously excited, but I have never observed it in the larger species. In some Neuroptera, e.g. Ephemeridae,6 the eggs are deposited in one mass, & with such force as to rupture the ventral integuments; & in these insects the parts of the mouth unfit them to take food, & consequently the whole aim of their existence is reproduction which once set in a fair way, the parent dies. In the Dragon flies this is different, & I believe the eggs are deposited slowly. With a mouth so highly organised, & with muscular power sufficient to baffle ordinary enemies, it seems reasonable to suppose that existence should have charms which the others cannot feel, & that boxes sexes should be long lived; the same remark will apply to most Coleoptera—such as Carabidae &c.7

In some Dragon flies the rule of Dimorphism in the ♀ is known to obtain. You have no doubt observed among a host of blue Agrions hovering over a pond, several individuals of an orange-colour— these are always ♀.8 This is not confined to one species but is found in many; yet I know not the reason of this difference in the colour of certain individuals. So far as I know they are equally fertile with their blue sisters. It may be owing to the existence of a family tendency. In the true Libellulas where differences of coloration exist, the female is always yellow or orange, & so in the Agrions a tendency to assume the same tint may be more or less latent.

But a few days since I received a paper from my friend Herr Brauer of Vienna, the well known Neuropterist, in which he proves the occurrence of actual dimorphism (not differences of colour but of structure) in an exotic genus. I send you this paper & beg you will return it when you shall have mastered his full meaning.9 I leave you to draw your own deductions from this most singular fact.

I know little of Orthoptera beyond that there is a certain tendency in our common grasshoppers to assume the tint of the soil they frequent. Mr Bates10 will no doubt be able to furnish you with information on this Order.

I fear you will find this letter a jumble of disconnected paragraphs, & that I have in some cases avoided direct answers. And pray please receive my assurance that I have done my best to give you information (most of it probably not new to you) on some points in Neuropterology, & I leave it to you to extract from it anything that may serve to elaborate any theoretical views. You are aware that I do not follow you to the full extent, yet I believe that you have accounted for many of the hitherto inexplicable phenomena of nature, & that instead of lessening our respect for the Great First Cause, you have given reason for increased admiration of the laws whereby the whole System is sustained.

Mr Wallace seems to me to ride his hobby too hard, & to risk causing a reversion to old fashioned ideas in the minds of many.11

I beg you will not hesitate to apply to me at any time for any hints I may be able to give, & remain | Yours most faithfully | R. McLachlan

C. Darwin Esq.

CD annotations

1.1 I will … queries. 1.3] crossed blue crayon
2.1 I have … the sexes, 2.2] double scored red crayon
2.3 I can … abundant; 2.4] double scored red crayon
2.4 I have … ♀♀. 2.5] scored blue crayon
2.7 throughout … in confinement, 2.9] scored blue crayon
3.2 but it has … Phryganidae 3.3] double scored blue crayon
3.3 (& … females. 3.5] double scored red crayon
3.4 that the male … females. 3.5] scored red crayon; double scored blue crayon
4.2 they are … of birds &c. 4.3] triple scored blue crayon
4.6 the male … protect. 4.7] double scored blue crayon
4.9 Thus … imago; 4.10] scored blue crayon
4.9 until … imago; 4.10] underl blue crayon
4.11 I look … protections. 4.12] double scored blue crayon
5.1 In the … fighting, ] reversed question mark added blue crayon
5.1 In the … long lived; 5.13] crossed pencil
5.1 I think] underl blue crayon
5.1 the males fighting,] underl blue crayon
5.10 I believe … slowly. ] underl blue crayon
6.2 blue Agrions] underl blue crayon; ‘Dimorphic’ added blue crayon
6.3 orange-] underl blue crayon
6.6 In the … latent. 6.8] double scored blue crayon
7.1 But … to give, 11.2] crossed blue crayon
Top of letter: ‘McLachlan’ blue crayon; ‘Inequality of sexes— Polygamy & Neuroptera’ pencil, scored blue crayon; ‘Red number of sexesred crayon


No letter from CD to McLachlan before this date has been found. McLachlan had been among those who offered information when Henry Walter Bates announced CD’s interest in the proportion of the sexes in insects at a meeting of the Entomological Society of London (see letter from H. W. Bates, 18 February 1868).
McLachan had given this information at the Entomological Society. Apatania muliebris is now in the family Limnephilidae. The family Phryganidae (now Phryganeidae) is now in the order Trichoptera, caddisflies. The modern order Neuroptera is restricted to lacewings and their kin.
CD quoted McLachlan on this point in Descent 1: 362.
McLachlan’s family ‘true Libellulidae’ corresponds roughly to the modern suborder Anisoptera (dragonflies). Libellula depressa (the broad-bodied chaser) is now in the modern family Libellulidae (common skimmers or perchers; see Silsby 2001). McLachlan is cited on the change in male colour in Descent 1: 363.
In Descent 1: 364, CD noted that male Agrions were the only dragonflies that had been observed fighting. Agrion was the name applied to the group of narrow-winged damselflies, roughly equivalent to the modern family Coenagrionidae in the suborder Zygoptera (damselflies). On the historical use of the taxa Agrion and Agrionidae, see Silsby 2001, p. 190.
The family Ephemeridae is now in the order Ephemeroptera (mayflies).
Carabidae is the family of ground beetles. McLachlan wrote ‘boxes’ instead of ‘both’.
CD cited McLachlan on dimorphism in females of several species of Agrion in Descent 1: 363–4.
CD discussed Friedrich Brauer’s paper on female dimorphism in Neurothemis (Brauer 1867), citing the abstract in the Zoological Record for 1867, p. 450, in Descent 1: 363. CD’s annotated copy of the Zoological Record is in the Darwin Library–CUL.
Henry Walter Bates.
McLachlan may refer to Alfred Russel Wallace’s recent criticism of arguments for design in nature presented by George Douglas Campbell, the duke of Argyll, and others (see A. R. Wallace 1867c, and Correspondence vol. 15, letter from A. R. Wallace, 1 October [1867], and letter to A. R. Wallace, 12 and 13 October [1867]).


Brauer, Friedrich. 1867. Ueber den Dimorphismus der Weibchen in der Libellulinen-Gattung Neurothemis. Verhandlung der kaiserlich-königlichen zoologisch-botanischen Gesellschaft in Wien 17: 971–6.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 26 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.

Silsby, Jill. 2001. Dragonflies of the world. London: Natural History Museum in association with CSIRO Publishing.


On numerical proportions of sexes in insects; coloration. Dimorphism in dragonflies (Agrion) in which usual coloration is reversed in sexes [see Descent 1: 362–4].

Wallace seems to ride his hobby too hard.

Letter details

Letter no.
Robert McLachlan
Charles Robert Darwin
Sent from
Forest Hill
Source of text
DAR 86: A8–9, DAR 82: A88–9
Physical description
7pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5910,” accessed on 12 December 2019,

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