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Fritz Müller on Flowers and Insects

THE enclosed letter from that excellent observer, Fritz Müller, contains some miscellaneous observations on certain plants and insects of South Brazil, which are so new and curious that they will probably interest your naturalist readers.¹ With respect to his case of bees getting their abdomens dusted with pollen while gnawing the glands on the calyx of one of the Malpighiaceæ,² and thus effecting the cross-fertilisation of the flowers, I will remark that this case is closely analogous to that of Coronilla recorded by Mr. Farrer in your journal some years ago, in which parts of the flowers have been greatly modified, so that bees may act as fertilisers while sucking the secretion on the outside of the calyx.³ The case is interesting in another way. My son Francis⁴ has shown that the food bodies of the Bull's-horn Acacia, which are consumed by the ants that protect the tree from its enemies (as described by Mr. Belt),⁵ consist of modified glands; and he suggests that aboriginally the ants licked a secretion from the glands, but that at a subsequent period the glands were rendered more nutritious and attractive by the retention of the secretion and other changes, and that they were then devoured by the ants. But my son could advance no case of glands being thus gnawed or devoured by insects, and here we have an example.

With respect to Solanum palinacanthum, which bears two kinds of flowers on the same plant, one with a long style and large stigma, the other with a short style and small stigma, I think more evidence is requisite before this species can be considered as truly heterostyled, for I find that the pollen-grains from the two forms do not differ in diameter. Theoretically it would be a great anomaly if flowers on the same plant were functionally heterostyled, for this structure is evidently adapted to insure the cross-fertilisation of distinct plants. Is it not more probable that the case is merely one of the same plant bearing male flowers through partial abortion, together with the original hermaphrodite flowers? Fritz Müller justly expresses surprise at Mr. Leggett's suspicion that the difference in length of the pistil in the flowers of Pontederia cordata⁶ of the United States is due to difference of age; but since the publication of my book⁷ Mr. Leggett has fully admitted, in the Bulletin of the Torrey Botanical Club, that this species is truly heterostyled and trimorphic.⁸ The last point on which I wish to remark is the difference between the males and females of certain butterflies in the neuration of the wings, and in the presence of tufts of peculiarly-formed scales. An American naturalist has recently advanced this case as one that cannot possibly be accounted for by sexual selection. Consequently, Fritz Müller's observations, which have been published in full in a recent number of Kosmos, are to me highly interesting, and in themselves highly remarkable.⁹

CHARLES DARWIN

Down, Beckenham, Kent, November 21

You mention ("Different Forms of Flowers," page 331), the deficiency of glands on the calyx of the cleistogamic flowers of several Malpighiaceæ, suggesting, in accordance with Kerner's views, that this deficiency may be accounted for by the cleisto gamic flowers not requiring any protection from crawling insects Now I have some doubt whether the glands of the calyx of the Malpighiaceæ serve at all as a protection. At least, in the one species, the fertilisation of which I have very often witnessed they do not. This species, Bunchosia gaudichaudiana, is regularly visited by several bees belonging to the genera Tetrapedia and Epicharis. These bees sit down on the flowers gnawing the glands on the outside of the calyx, and in doing so the under side of their body is dusted with pollen, by which, afterwards, other flowers are fertilised.

There are here some species of Solanum (for instance S. Palina canthum) bearing on the same plant long-styled and short-styled flowers. The short-styled have papillæ on the stigma and apparently normal ovules in the ovary, but notwithstanding they are male in function, for they are exclusively visited by pollen-gathering bees (Melipona, Euglossa, Augochlora, Megacilissa, Eophila n. g., and others), and these would probably never insect then proboscis between the stamens.

In a few moths I hope to the able to send you seeds of out white-flowered violet with subterranean cleistogamic flowers I was surprised at finding that on the Serra (about 1, 100 metre above the sea) this violet produced abundant normal fruits a well as subterranean ones, while at the foot of the Sèrra, though

¹ Müller's letter discusses the pollination of flowers by insects and certain butterflies with scent-producing scales thought to attract females.

² Malpighiaceae, a family of tropical shrubs.

³ Farrer 1874.

⁴ F. Darwin 1877.

⁵ Thomas Belt (1832-1878), geologist and mining engineer. Belt 1874, p. 218.

⁶ William Henry Leggett (1816-1882), American botanist. Leggett 1875. See Calendar 10790. Pontederia cordata = Pickerel weed, an aquatic plant.

⁷ Forms of flowers, p. 187.

⁸ Leggett 1877.

⁹ Müller 1877.

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it had flowered profusely, I could not find a single normal fruit, and subterranean ones were extremely scarce.

According to Delpino the changing colours of certain flowers would serve to show to the visiting insects the proper moment for effecting the fertilisation of these flowers. We have here a Lantana the flowers of which last three days, being yellow on the first, orange on the second, purple on the third day. This plant is visited by various butterflies. As far as I have seen the purple flowers are never touched. Some species inserted their proboscis both into yellow and into orange flowers (Danais erippus, Pieris aripa),others, as far as I have. hitherto observed, exclusively into the yellow flowers of the first day (Heliconius apseudes, Colœnis julia, Eurema leuce. This is, I think, a rather interesting case. If the flowers fell off at the end of the first day the inflorescence would be much less conspicuous; if they did not change their colour much time would be lost by the butterflies inserting their proboscis in already fertilised flowers.

In another Lantana the flowers have the colour of lilac, the entrance of the tube is yellow surrounded by a white circle; these yellow and white markings disappear on the second day.

Mr. Leggett's statements about Pontederia cordata appear to me rather strange, and I fear that there is some mistake. In all the five species of the family which I know the flowers are so short-lived, lasting only one day, that a change in the length of the style is not very probable. In the long-styled form of our highland Pontederia the style has its full length long before the flowers open. In my garden this Pontedaria is visited by some species of Augochlora collecting the pollen of the longest and mid-length stamens; they are too large to enter the tube of the corolla, and have too short a proboscis to reach the honey; they can only fertilise the long-styled and mid-styled forms, but not the short-styled.

Among the secondary sexual characters of insects the meaning of which is not understood, you mention ("Descent of Man," vol. i, p. 345) the different neuration in the wings of the two sexes of some butterflies. In all the cases which I know this difference in neuration is connected with, and probably caused by, the development in the males of spots of peculiarly-formed scales, pencils, or other contrivances which exhale odours, agreeable no doubt to their females. This is the case in the genera Mechanitis, Dircenna, in some species of Theela, &c.

FRITZ MÜLLER

Blumenau, St. Catharina, Brazil, October 19