Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/95

. II. I felt convinced, from the position of the viscid discs, that it would be fertilised in a different manner from the Larger Butterfly Orchis; and now, owing to the kindness of Mr. F. Bond, I have examined two moths, namely, Agrotis segetum and Anaitis plagiata, one with three pollinia, and the other with five pollinia, attached, not to the eyes and side of the face as in the last species, but to the base of the proboscis. I may remark that the pollinia of these two species of Habenaria, when attached to moths, can be distinguished at a glance.

Professor Asa Gray has described the structure of no less than ten American species of Platanthera. Most of them resemble in their manner of fertilisation the two British species; but some of the species, in which the viscid discs do not stand far apart, have curious contrivances, such as a channelled labellum, lateral shields, &c., compelling moths to insert their proboscides directly in front. P. hookeri, on the other hand, differs in a very interesting manner: the two viscid discs stand widely separated from each other; consequently a moth, unless of gigantic size, would be able to suck the copious nectar without touching either disc; but this risk is avoided in the following manner:—the central line of the stigma is prominent, and the labellum, instead of hanging down, as in most of the other species, is curved upwards, so that the front of the flower is made somewhat tubular and is divided into halves. Thus a moth is compelled to go to the one or other side, and its face will almost certainly be brought into contact with one of the discs. The drum of the pollinium, when removed, contracts in the same manner as I have described under P. chlorantha.