Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/196

176 of the pollen-mass glued to its back. The insect then generally gets through the passage, and comes out with this peculiar appendage, to return nearly immediately to its feast, when it is generally precipitated a second time into the bucket, passing out through the same opening, and so inserting the pollen-masses into the stigma while it forces its way out, and thereby impregnating either the same or some other flower. I have often seen this; and sometimes there are so many of these humble-bees assembled that there is a continual procession of them through the passage specified."

There cannot be the least doubt that the fertilisation of the flower absolutely depends on insects crawling out through the passage formed by the extremity of the labellum and the over-arching column. If the large distal portion of the labellum or bucket had been dry, the bees could easily have escaped by flying away. Therefore we must believe that the fluid is secreted by the appendages in such extraordinary quantity and is collected in the bucket, not as a palatable attraction for the bees, as these are known to gnaw the labellum, but for the sake of wetting their wings, and thus compelling them to crawl out through the passage.

I have now described, perhaps in too much detail, a few of the many contrivances by which the Vandeæ are fertilised. The relative position and shape of the parts—friction, viscidity, elastic and hygrometric movements, all nicely related to one another—come into play. But all these appliances are subordinate to the aid of insects. Without their aid, not a plant belonging to this tribe, in the species of the twenty-nine genera examined by me, would set a seed. It is also certain in a majority of the cases, that insects withdraw the pollinia only when retreating from the flower, and