Afarensis

Darwin performed a wide variety of experiments and each one had a different purpose. In my first post in this series I used an example where Darwin had a hypothesis about the intellectual abilities of insects and had derived some implications from it. The experiment was designed to see if those implications were correct. In the second post I mentioned that comparative anatomy was an important tool for Darwin. In this, long example, we see Darwin using comparative anatomy and combining it with some experiments to try and figure out how one, particularly difficult plant, was pollinated. We also see a successful prediction (in bold) based on Darwin's knowledge of orchid and insect anatomy.

The Angræcum sesquipedale [picture to left - afarensis], of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar, must not be passed over. A green, whip-like nectary of astonishing length hangs down beneath the labellum. In several flowers sent me by Mr. Bateman I found the nectaries eleven and a half inches long, with only the lower inch and a half filled with nectar.

What can be the use, it may be asked, of a nectary of such disproportionate length? We shall, I think, see that the fertilisation of the plant depends on this length, and on nectar being contained only within the lower and attenuated extremity. It is, however, surprising that any insect should be able to reach the nectar. Our English sphinxes have proboscides as long as their bodies; but in Madagascar there must be moths with proboscides capable of extension to a length of between ten and eleven inches! This belief of mine has been ridiculed by some entomologists, but we now know from Fritz Müller* that there is a sphinxmoth in South Brazil which has a proboscis of nearly sufficient length, for when dried it was between ten and eleven inches long. When not protruded it is coiled up into a spiral of at least twenty windings. [emphasis mine - afarensis]

The rostellum is broad and foliaceous, and arches rectangularly over the stigma and over the orifice of the nectary: it is deeply notched by a cleft enlarged or widened at the inner end. Hence the rostellum nearly resembles that of Calanthe after the disc has been removed (see fig. 26, C). The under surfaces of both margins of the cleft, near their ends, are bordered by narrow strips of viscid membrane, easily removed; so that there are two distinct viscid discs. A short membranous pedicel is attached to the middle of the upper surface of each disc; and the pedicel carries a pollen-mass at its other end. Beneath the rostellum a narrow, ledge-like, adhesive stigma is seated.

I could not for some time understand how the pollinia of this Orchid were removed, or how the stigma was fertilised. I passed bristles and needles down the open entrance into the nectary and through the cleft in the rostellum with no result. It then occurred to me that, from the length of the nectary, the flower must be visited by large moths, with a proboscis thick at the base; and that to drain the last drop of nectar, even the largest moth would have to force its proboscis as far down as possible. Whether or not the moth first inserted its proboscis by the open entrance into the nectary, as is most probable from the shape of the flower, or through the cleft in the rostellum, it would ultimately be forced in order to drain the nectary to push its proboscis through the cleft, for this is the straightest course; and by slight pressure the whole foliaceous rostellum is depressed. The distance from the outside of the flower to the extremity of the nectary can be thus shortened by about a quarter of an inch. I therefore took a cylindrical rod one-tenth of an inch in diameter, and pushed it down through the cleft in the rostellum. The margins readily separated, and were pushed downwards together with the whole rostellum. When I slowly withdrew the cylinder the rostellum rose from its elasticity, and the margins of the cleft were upturned so as to clasp the cylinder. Thus the viseid strips of membrane on each under side of the cleft rostellum came into contact with the cylinder, and firmly adhered to it; and the pollen-masses were withdrawn. By this means I succeeded every time in withdrawing the pollinia; and it cannot, I think, be doubted that a large moth would thus act; that is, it would drive its proboscis up to the very base through the cleft of the rostellum, so as to reach the extremity of the nectary; and then the pollinia attached to the base of its proboscis would be safely withdrawn.

I did not succeed in leaving the pollen-masses on the stigma so well as I did in withdrawing them. As the margins of the cleft rostellum must be upturned before the discs adhere to a cylindrical body, during its withdrawal, the pollen-masses become affixed some little way from its base. The two discs did not always adhere at exactly opposite points. Now, when a moth with the pollinia adhering to the base of its proboscis, inserts it for a second time into the nectary, and exerts all its force so as to push down the rostellum as far as possible, the pollen-masses will generally rest on and adhere to the narrow, ledge-like stigma which projects beneath the rostellum. By acting in this manner with the pollinia attached to a cylindrical object, the pollen-masses were twice torn off and left glued to the stigmatic surface.

This particular experiment has an interesting follow up which will be the subject of today's Darwin Quote. On an administrative note, I do not wish to bore my readers by giving too many orchid examples in a row, so my next experiment will come from a different book. Never fear, though, I have plenty of examples from the Orchid book and will get to them all.