The special genius of Darwin is evidenced here not so much by the fact that he realized that only a moth with an outsized proboscis would be attracted to an Angraecum sequipedale flower--any clever naturalist with a basic knowledge of pollination biology could have intuited the characteristics of the sort of critter that would be needed to pollinate such a bloom--but in that he provided a clear explanation for the history and peculiar details of the Angraecum story. Here's the man himself:
If the Angræcum in its native forests secretes more nectar than did the vigorous plants sent me by Mr. Bateman, so that the nectary 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 The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop.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 Angræcum would become extinct. On the other hand, as the nectar, at least in the lower part of the nectary, is stored safe from depredation by other insects, the extinction of the Angræcum would probably be a serious loss to these moths. We can thus partially understand how the astonishing length of the nectary may have 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 Angræcum and other deep tubular flowers, those individual plants of the Angræcum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed, and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis of certain moths; but the Angræcum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as far as possible in order to drain the last drop of nectar.Darwin, C. R. 1862. On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing. pp. 201-203. London: John Murray. [Darwin Online link]
The orchid is not providing nectar out of some vegetable sense of charity, it is making the moth struggle for every drop and still holding a little more just out of reach, maximizing the chances that the moth will get stuck with pollinia (specialized adhesive pollen bodies) to carry to the stigma of the next Angraecum flower. The average nectar spur is a bit longer than the average proboscis. The moth, for its part, just wants a meal, and may in fact be actively trying to avoid getting a package of orchid pollen glued to its mouth parts, but is forced to cram its head all the way into the flower by the lure of the nectar at the bottom of that overly long spur.
The seemingly absurd length of nectar spur and moth proboscis is the outcome of an evolutionary arms race. Moths with slightly longer proboscises got more nectar, and prospered. Orchids with slightly longer spurs were more effective at foisting pollen onto moths and getting moths to deposit pollen onto their stigmas. Any angraecums with spurs shorter than a moth proboscis were unable to force the moths into the proper position to pick up or drop off pollen, and would have found themselves on the wrong end of the process of natural selection. The arms race may very well be ongoing to this day, unless one or other of the combatants has come up against structural limitations to the length of tube it can support.
A. sequipedale is on display in the EEB greenhouses for as long as the flower holds up (probably a week or so), and anyone who's in the area is welcome to stop in and see it in person.