Thanks for German version of Origin .
Dipterous insects are adapted to eating pollen rather than only to sucking nectar. He describes such adaptations in two dipteran species.
I am very much obliged to you for your kindness in having sent me the German version of the forth edition of your work on species. I have read several months ago the original of this forth edition and have been as much pleased as surprised at the plenty of new important evidences your theorie has gained since the first German edition has been published. It has been a great pleasure to me to receive now out of your own hand your work together with your portrait. Being by no means able to return my thanks to you by any however unimportant own work I can only send to you my portrait.
A single observation made by me several weeks ago perhaps could be of some interesting to you. I have never heard or read that Dipterous insects take other than fluid food. Therefore I was surprised by seing that Eristalis tenax, Rhingia rostrata, different species of Syrphus and many other Dipterous insects eat enorm quantities of pollen grains.
With Eristalis tenax I was allowed long time to look at its eating in immediate nearness, and to convince myself that the complicate structure of its underlip is admirably adapted as well to this foot as to the sucking of nectar.
The two flaps on the free end of the underlip clasp little clusters of pollen grains between them and grind them behind by quick alternate porrections and retractions. The numerous horny cross-ledges on the inner side of the two flaps serve to seize, to hold and to shove behind the single pollengrains The distance of these horny ledges apparently is adapted to the greatness of the pollengrains. For I found the stomach of Eristalis tenax filled for the greatest part with many hundred thousands of the large pollen grains of an Oenothera cultivated in my garden; whilst the stomach of Syrphus balteatus (which species has narrower cross ledges on the underlip) contained only much lesser pollengrains
When sucking the fly put the ends of their bristle shaped mandibles (not represented in my drawing) between the two flaps, thus giving to them a fixed direction and then draws back the lateral portions C.
The cold season has finished to early my just begun observations. It will be, I believe, an interesting proposing to compare accurately the structure of the underlip of stinging and exclusively sucking Dipterons with those who besides eat pollen and to search generally what details of structure the mouth of the flower-visiting insects of all orders has gained in the struggle for existence, in the purpose to attain better the food offered by the flowers. For the complet understanding of the complicate structure of the mouth of Dipterons it is, I believe, indispensable to take notice of its adaption to pollen-food.
If you know any observations published on this object I whould be glad of hearing where they are published.
With the greatest respect and my sincere thanks I remain, dear Sir, | yours very faithfully | Hermann Müller.
- f1 5657.f1The reference is to the third German edition of Origin (Bronn and Carus trans. 1867), which was translated from the fourth English edition. No presentation list for the third German edition has been found.
- f2 5657.f2Müller refers to the fourth edition of Origin and Bronn trans. 1860.
- f3 5657.f3CD may have sent a copy of a photograph of himself taken by Ernest Edwards during his visits to London in November 1865 or April 1866 (see Correspondence vol. 13, letter from E. A. Darwin to Emma Darwin, 25 [November 1865] and n. 3), or a copy of one taken by his son William Erasmus Darwin in 1864 (see Correspondence vol. 14, letter from W. E. Darwin, 8 May  and n. 10).
- f4 5657.f4Müller's photograph has not been found in the Darwin Archive--CUL.
- f5 5657.f5Eristalis tenax (the drone fly), Rhingia rostrata (the long-tongued hoverfly), and Syrphus all belong to the family Syrphidae (hoverflies or flower flies), dipterous insects that often resemble bees or wasps (F. S. Gilbert 1993).
- f6 5657.f6In modern terminology, the `two flaps' that Müller refers to are the labella, the `underlip' is the prementum, and the `horny cross-ledges' are the pseudotracheae. The `bristle-shaped mandibles' are probably the labrum-epipharynx (personal communication, Francis Gilbert). Syrphus balteatus is now Episyrphus balteatus. For more on the structure and function of mouthparts in the Syrphidae, see F. S. Gilbert and Jervis 1998. In a paper on the application of Darwinian theory to flowers and flower-visiting insects, Müller described his observations, made in the summer of 1867, of the behaviour of Eristalis tenax visiting flowers of Oenothera media (now O. fruticosa; H. Müller 1869, p. 59). CD's heavily annotated copy of the paper is in the Darwin Pamphlet Collection--CUL.