Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/215

. VII. brane formed of polygonal cells, resting on and adhering to a thick cushion, formed of irregular rounded balls of brown matter, separated from each other and embedded in a transparent, structureless, highly elastic substance. This cushion towards the posterior end of the disc graduates into viscid matter, which when consolidated is brown, translucent, and homogeneous. Altogether the disc of Catasetum presents a much more complex structure than in the other Vandeæ.

I need not further describe the present species, except as to the position of the antennæ. They occupied exactly the same position in all the many flowers which were examined. Both lie curled within the helmet-like labellum; the left-hand one stands higher up, with its inwardly bowed extremity in the middle; the right-hand antenna lies lower down and crosses the whole base of the labellum, with the tip just projecting beyond the left margin of the base of the column. Both are sensitive, but apparently the one which is coiled within the middle of the labellum is the more sensitive of the two. From the position of the petals and sepals, an insect visiting the flower would almost certainly alight on the crest of the labellum; and it could hardly gnaw any part of the great cavity without touching one of the two antennæ, for the left-hand one guards the upper part, and the right-hand one the lower part. When either of these is touched the pollinium is ejected and the disc will strike the head or thorax of the insect.

The position of the antennæ in this Catasetum may be compared with that of a man with his left arm raised and bent so that his hand stands in front of his chest, and with his right arm crossing his body lower down so that the fingers project just beyond his left side. In Catasetum callosum both arms are held lower down