Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/250

230 of the orifices close to the anthers; for I found that when a bristle was thus inserted the glutinous pollen adhered to it, and could afterwards be left on the stigma; but this latter part of the operation was not well effected. After the publication of my book Professor Asa Gray wrote to me that he was convinced from an examination of several American species that the flowers were fertilised by small insects entering the labellum through the large opening on the upper surface, and crawling out by one of the two small orifices close to the anthers and stigma. Accordingly I first introduced some flies into the labellum of C. pubescens, through the large upper opening, but they were either too large or too stupid, and did not crawl out properly. I then caught and placed within the labellum a very small bee which seemed of about the right size, namely, Andrena parvula, and this by a strange chance proved, as we shall presently see, to belong to the genus on which in a state of nature the fertilisation of C. calceolus depends. The bee vainly endeavoured to crawl out again the same way by which it had entered, but always fell backwards, owing to the margins being inflected. The labellum thus acts like one of those conical traps with the edges turned inwards, which are sold to catch beetles and cockroaches in the London kitchens. It could not creep out through the slit between the folded edges of the basal part of the labellum, as the elongated, triangular, rudimentary stamen here closes the passage. Ultimately it forced its way out through one of the small orifices close to one of the anthers, and was found when caught to be smeared with the glutinous pollen. I then put the same bee back into the labellum; and again it crawled out through one of the small