Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/140

120 and firmly cemented to their heads. As soon as the insect flies away, it withdraws the pollinia, carries them to another flower, and there leaves masses of the friable pollen on the adhesive stigma.

In order to witness what I felt sure would take place, I watched for an hour a group of plants on three occasions; each time I saw numerous specimens of two small Hymenopterous insects, namely, a Hæmiteles and a Cryptus, flying about the plants and licking up the nectar; most of the flowers, which were visited over and over again, already had their pollinia removed, but at last I saw both these species crawl into younger flowers, and suddenly retreat with a pair of bright yellow pollinia sticking to their foreheads; I caught them, and found the point of attachment was to the inner edge of the eye; on the other eye of one specimen there was a ball of the hardened viscid matter, showing that it had previously removed another pair of pollinia, and in all probability had subsequently left them on the stigma of a flower. As these insects were captured, I did not witness the act of fertilisation; but Sprengel saw a Hymenopterous insect leave its pollen-mass on the stigma. My son watched another bed of this Orchid at some miles' distance, and brought me home the same Hymenopterous insects with attached pollinia, and he saw Diptera also visiting the flowers. He was struck with the number of spider-webs spread over these plants, as if the spiders were aware how attractive the Listera was to insects.

To show how delicate a touch suffices to cause the rostellum to explode, I may mention that I found an extremely minute Hymenopterous insect vainly struggling to escape, with its head cemented by the hardened viscid matter, to the crest of the rostellum