ORCHIDS, FORMS OF FLOWERS, POLLINATION
Angraecum sesquipedale, often called Darwin’s Comet Orchid, is one of 220 Angraecum species with star-shaped flowers bearing long nectar spurs; many of the species are found in Madagascar. The flowers are considered a spectacular example of coevolution, with pollinators adapted to pollinate specific species. Darwin received a specimen of A. sesquipedale in January 1862 from Staffordshire banker and orchid enthusiast James Bateman. Marveling at the length of its nectar spur, Darwin wrote to Joseph Hooker, “I have just received such a Box full from Mr Bateman with the astounding Angraecum sesquipedalia with a nectary a foot long.—Good Heavens what insect can suck it.”22 Darwin did tests with long thin rods to remove pollen and hypothesized that this species must be pollinated by a moth with a proboscis long enough to reach the nectar at the bottom of the long spur.
The nectar-secreting organs of the Orchideae present great diversities of structure and position in the various genera; but are almost always situated towards the base of the labellum. …
The Angraecum sesquipedale, 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 sphinx-moth 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.
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 viscid 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.
If the Angraecum in its native forests secretes more nectar than did the vigorous plants sent me by Mr. Bateman, so that the nectary ever becomes filled, small moths might obtain their share, but they would not benefit the plant. The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop. If such great moths were to become extinct in Madagascar, assuredly the Angraecum would become extinct. On the other hand, as the nectar, at least in the lower part of the nectary, is stored safe from the depredation of other insects, the extinction of the Angraecum would probably be a serious loss to these moths. We can thus understand how the astonishing length of the nectary had been acquired by successive modifications. As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angraecum and other deep tubular flowers, those individual plants of the Angraecum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their proboscides up to the very base, would be best fertilised. These plants would yield most seed, and the seedlings would generally inherit long nectaries; and so it would be in successive generations of the plant and of the moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angraecum and the proboscis of certain moths; but the Angraecum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as deeply as possible in order to drain the last drop of nectar.
In response to Darwin’s critics who doubted such a close relationship could evolve by natural selection, his friend and colleague Alfred Russel Wallace had undertaken a search for the theorized pollinator. Scouring the British Museum’s insect collection for candidates, ruler in hand, Wallace had found sphinx moths of the genus Macrosila (now Xanthopan) from tropical Africa and South America with probosces as long as 9¼ inches. Writing in an 1867 review, Wallace declared, “A species having a proboscis two or three inches longer could reach the nectar in the largest flowers of Angraecum sesquipedale, whose nectaries vary in length from ten to fourteen inches. That such a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune,—and they will be equally successful!”23 The predicted moth, Xanthopan morgani praedicta, discovered in 1903, is a subspecies of one of the moths mentioned by Wallace in his article. It is fitting that Darwin’s Comet Orchid should be pollinated by Wallace’s Hawkmoth.
Arachis hypogaea. Hand-colored engraved plate drawn by M. M. Payerlein from Christoph Jacob Trew, Plantae Rariores Quas Maximam Partum.