Protective role of colours in caterpillars and butterflies. Sexual differences in colours of butterflies.
9, St. Mark's Crescent | N.W.
I saw Bates a few days ago & he mentioned to me this difficulty of the catterpillars. I think it is one that can only be solved by special observation. The only probable solution I can imagine is something like this Catterpillars are very similar in form & there are hundreds of species that are only to be distinguished by colour.
Now great numbers are protected by their green colours assimilating with foliage or their brown colours resembling bark or twigs. Others are protected by prickles and long hairs—which no doubt render them distasteful to birds, especially to our small birds which I presume are the great destroyers of catterpillars. Now supposing that others, not hairy, are protected by a disagreeable taste or odour, it would be a positive advantage to them never to be mistaken for any of the palatable catterpillars, because a slight wound such as would be caused by a peck of a bird's bill almost always I believe kills a growing catterpillar. Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable catterpillars, would enable birds to recognise them easily as a kind not fit for food, & thus they would escape seizure which is as bad as being eaten.
Now this can be tested by experiment, by any one who keeps a variety of insectivorous birds. They ought as a rule to refuse to eat and generally refuse to touch gaudy coloured catterpillars, & to devour readily all that have any protective tints. I will ask Mr. Jenner Weir of Blackheath about this, as he has had an aviary for many years & is a very close and acute observer, & I have no doubt will make the experiment this summer.
When our discussion on Mimicry took place a most interesting little fact was mentioned by Mr. Stainton. After mothing he is accustomed to throw all the common species to his poultry & once having a lot of young turkeys he threw them a quantity of moths which they eat greedily, but among them was one common white moth (Spilosoma menthastri) One of the young turkeys took this in his beak, shook his head & threw it down again, another ran to seize it and did the same, and so on, the whole brood in succession rejected it! Mr. Weir tells me that the larva of this moth is hairy & is also rejected by all his birds, which sufficiently accounts for the insect being very common. But what is still more curious, another moth much less common (Diaphora mendica) has the female also white, (although the male is quite different) and might at night be easily mistaken for the other! So here we have a case of British mimicry exactly analogous in all its details to that of the Heliconidæ & Danaidæ; and it is particularly valuable because it is a direct proof that Lepidoptera do differ in flavour, & that certain flavours are distasteful to birds.
My female mimetic butterfly is much more beautiful than the male, being metallic blue while the male is dull brown. I sometimes doubt whether sexual selection has acted to produce the colours of male butterflies. I have thought that it was merely that it was advantageous for the females to have less brilliant colours, & that colour has been produced merely because in the process of infinite variation all colours in turn were produced. Undoubtedly two or three male butterflies do often follow a female, but whether she chooses between them or whether the strongest & most active gets her is the question. Cannot this also be decided by experiment? If a lot of common butterflies were bred, say our ``brimstone'' or better, the ``orange tip'', & the males and females separated & then a certain number of the males discoloured by rubbing the wings carefully;—and we were then to turn out a female along with a coloured and a discoloured male into a room or greenhouse, would the female always or in the majority of cases choose the best coloured male.? A series of experiments of this kind carefully carried out would I think settle the question. I will suggest these two classes of experiment at the next meeting of the Entomological & perhaps some country residents may be induced to carry them out.
I hope you will take care of your health, & not work too hard when you get a little better.
I often wish I lived in the country, & was able to carry out some of these most interesting observations but I do not know whether I shall be able to manage it.
Yours very faithfully | Alfred R. Wallace
C. Darwin Esq.
- f1 5416.f1The year is established by the relationship between this letter and the letter to A. R. Wallace, 23 February 1867.
- f2 5416.f2Henry Walter Bates had advised CD to pose his question on colourful caterpillars to Wallace (see letter to A. R. Wallace, 23 February 1867).
- f3 5416.f3In Descent 1: 416, CD paraphrased Wallace's remarks and quoted from the similar statement made by Wallace at the 4 March 1867 meeting of the Entomological Society of London (see Transactions of the Entomological Society of London (Journal of the Proceedings) 3d ser. 5 (1865--7): lxxx).
- f4 5416.f4Though there is no further extant correspondence in 1867 regarding these experiments, John Jenner Weir conducted them in the summers of 1867 and 1868 (see Correspondence vol. 16, letters from J. J. Weir, 24 March 1868 and 31 March 1868). See also letter from A. R. Wallace, 10 March 1869 (Calendar no. 6651). In 1869 and 1870, Weir read a paper describing his experiments with insectivorous birds to the Entomological Society (Weir 1869--70); see Descent 1: 417.
- f5 5416.f5Wallace refers to a discussion held at the Entomological Society on 3 December 1866 (see Transactions of the Entomological Society of London (Journal of the Proceedings) 3d ser. 5 (1865--7): xliv--xlviii); Henry Tibbats Stainton's case of the Spilosoma and Weir's comment were recorded on p. xlv. The general discussion on mimicry and CD's theory was continued from the Entomological Society meeting of 19 November 1866, reported in ibid., pp. xxxvi--xli (see also Correspondence vol. 14, letter from A. R. Wallace, 19 November 1866). CD reported the case in Descent 1: 398 n. 16, and Wallace in [A. R. Wallace] 1867a, p. 25. A synonym for Spilosoma menthastri (white ermine moth) is S. lubricipeda, and the caterpillar is the woolly-bear.
- f6 5416.f6In his study of the Amazonian Heliconidae and their mimics, Bates argued that natural selection accounted for the phenomenon of mimicry; both male and female Heliconidae are brightly coloured, and Bates surmised that they were protected by a secretion or odour unpalatable to predators, and that their mimics were protected by their resemblance to Heliconidae (Bates 1861, pp. 502--15). Wallace had discovered cases where only the females of some species of Diadema (now Hypolimnas) mimicked species of Euploea of the family Danaidae, while females of several species of Pieris resembled Heliconius species of the family Heliconidae (see A. R. Wallace 1866a, p. 186; see also n. 7, below). CD mentioned Wallace's consideration of Diaphora mendica in Descent 1: 398 n. 16, giving the earlier generic name of Cycnia. See also ibid., pp. 412--14. Diaphora mendica is the muslin moth.
- f7 5416.f7See letter to A. R. Wallace, 23 February 1867 and n. 5.
- f8 5416.f8CD had asked Wallace how he would defend the view that the bright colours of male butterflies were due to sexual selection (see letter to A. R. Wallace, 23 February 1867).
- f9 5416.f9Wallace refers to Gonepteryx rhamni (brimstone butterfly), and to Anthocharis cardamines (orange-tip butterfly). In Descent 1: 409, CD stated that the male brimstone butterfly had probably acquired his bright colours as a result of sexual selection, but did not mention experiments. There is no mention in the transactions of the meeting of the Entomological Society of 4 March 1867 of Wallace's asking for the experiments with discoloured males to be carried out. He did, however, ask for observations on which caterpillars were eaten by birds (see Transactions of the Entomological Society of London (Journal of the Proceedings) 3d ser. 5 (1865--7): lxxx, and Descent 1: 417.
- f10 5416.f10CD refers to James Philip Mansel Weale's observation of caterpillars mimicking thorns (see letter from J. P. M. Weale, 9 January 1867 and n. 7). In Descent 1: 416--17, CD referred to Weale's observation in his discussion of Wallace's opinion regarding conspicuous colour in caterpillars.