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Darwin Correspondence Project

From Mary Treat   20 December 1871

Vineland, New Jersey,

Dec. 20, 1871.

Mr. Darwin:

Dear Sir,

Experimenting with Papilio asterias, Cramer,1 I learned to distinguish the sex in the larva state—the female being larger than the male—and this led me to try to control the sex.

My first experiments were a a year ago last summer, some three or four hundred miles inland, where I had much better success than I had here last summer near the coast.

The larvae of my first experiment were of the first brood,2 so that I only had to wait a few days for their final transformation. These larvae fed on two quite dissimilar Umbelliferous plants—the Poison Hemlock (Conium maculatum), and the Caraway of the gardens (Carum Carni). I could always distinguish the larvae that fed on the Poison Hemlock from those that fed on the Caraway; but with the butterflies there was little or no marked difference in their general appearance.

I noticed that the female larva fed longer than that of the male. So taking several larvae of the same age, I found some specimens were inclined to leave their food several days earlier than others, and these always proved to be males. It then occurred to me to try to induce some of these male larvae to feed longer; so, after they had wandered from their food, and even selected places for their transformations, (of course not fixed), if I disturbed them, made them leave their places, and coaxed them with a fresh supply of their favorite food, I could almost invariably induce them to eat from ten days to two weeks longer, when all such ones would be females— If it was a larva that fed on Caraway, I tempted it with the tenderest, and freshest leaves and flowers of the same plant. It was never hungry enough at that stage to be induced to change its food, though they will change in their earlier stages rather than starve, but evidently they do not like to change even then, and frequently fail to transform when their food is thus changed. They can be induced to change their food to the nearest allied species of plants with less difficulty—

On the other hand, when a larva had become the right size to produce a male, if I cut off its supply of food, even when it was eating greedily, it would wander about perhaps a little longer, as if in search of food, but finally it almost always changed to the chrysalis, and such a chrysalis always produced a male butterfly.

Last summer a fit of sickness prevented my experimenting with the first brood of these butterflies; and with the second brood—although I procured many specimens before the first moult—I was only able to rear about one in ten. An unnamed tiny Microgaster was their fatal enemy.3 But the few larvae that I succeeded with, behaved precisely as those of the previous year. Their food-plant was Archangelica hirsuta.4 I have the chrysalids of my last experiment carefully marked, and am fully confident that I know which will produce the different sexes.

Now that I am writing I will give you my observations on Drosera, which have escaped the notice of botanists. I had two or three species of these pretty plants growing for window ornaments; and soon saw that D. longifolia was a fly-trap of considerable power. The unlucky fly—a common house fly—would no sooner be caught by the sticky glands of the leaf, than the blade would at once commence to fold about its victim; it folded from the apex to the stem of the leaf, after the manner of its vernation.5 Closer and closer it held the poor fly in its embrace, until it ceased its struggles, when it soon became partly absorbed by the plant.— Prof. Gray will give my observations on this plant in his new edition of “How Plants Grow”.6

I do not know that my experiments can be of use to you, but I thought perhaps they might interest you. A life time of observation and experiments could not repay the debt of gratitude we owe you.

Yours most respectfully, | Mrs. Mary Treat.

CD annotations

Top of letter: ‘Mrs Treat’ blue crayon
End of letter: ‘Treat Drosera’ blue crayon
Verso of last page: ‘Drosera’ blue crayon


Papilio asterias is now P. polyxenes asterius, the black swallowtail butterfly.
The black swallowtail produces two or three broods between April and October (West et al. 1972).
Microgaster is a genus of braconid wasps (family Braconidae).
Archangelica hirsuta is now Angelica venenosa (hairy angelica).
CD had discussed the incurvation of the leaves of some species of Drosera with Daniel Oliver (see Correspondence vol. 8, letter from Daniel Oliver, 25 September 1860, and letter to Daniel Oliver, 27 [September 1860]). In his letter to Oliver of 27 [September 1860], CD had noted that in Drosera longifolia, the incurvation was ‘always terminal’ (that is, beginning from the apex of the leaf). In Insectivorous plants, p. 278, CD cited Treat’s observations on D. longifolia from her paper, ‘Observations on the sundew’ (Treat 1873b); CD referred to the species by its modern name, D. anglica.
Asa Gray’s How plants grow, originally published in 1858 (A. Gray 1858) and reissued several times, appeared under the series title Botany for young people and common schools (shortened in some later editions to Botany for young people). Treat’s observations on Drosera longifolia were mentioned in the second book in the series, How plants behave (A. Gray 1872, pp. 43–4; Gray referred to her observations as those of ‘an intelligent lady’ (ibid., p. 44).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Insectivorous plants. By Charles Darwin. London: John Murray. 1875.


Describes fly-catching activity of Drosera longifolia.

Experiments on Papilio asterias; sex of adult determined by length of larval feeding time.

Letter details

Letter no.
Mary Lua Adelia (Mary) Davis/Mary Lua Adelia (Mary) Treat
Charles Robert Darwin
Sent from
Vineland, N.J.
Source of text
DAR 58.1: 33
Physical description
ALS 5pp †

Please cite as

Darwin Correspondence Project, “Letter no. 8113,” accessed on 20 April 2024,

Also published in The Correspondence of Charles Darwin, vol. 19