sitors is the
welcome extended. What, then, are the conditions embodied? The insect
must have a tongue of such a length that, when in the act of sipping,
its head must pass beyond the anther well into the opening of the
flower. Its body must be sufficiently large to come in contact with the
anther. Such requisites are perfectly fulfilled by the humblebee, and
we may well hazard the prophecy that the Bombus is the welcomed affinity
of the flower.

[Illustration: Fig. 4]

The diagrams (Fig. 4) sufficiently illustrate the efficacy of the
beautiful plan involved. At A the bee is seen sipping the nectar. His
forward movement thus far to this point has only seemed to press the
edge of the anther inward, and thus keep it even more effectually
closed. As the bee retires (B), the backward motion opens the lid, and
the sticky pollen is thus brought against the insect's back, where it
adheres in a solid mass. He now flies to the next Arethusa blossom,
enters it as before, and in retiring slides his back against the
receptive viscid stigma, which retains a portion of the pollen, and thus
effects the cross-fertilization (C). Professor Gray surmised that the
pollen was withdrawn on the insect's head, and it might be so withdrawn,
but in other allied orchids of the tribe Arethusae, however, in which the
structure is very similar, the pollen is deposited on the thorax, and
such is probably the fact in this species. In either case
cross-fertilization would be effected. Nothing else is possible in the
flower, and whether it is Bombus or not that effects it, the method is
sufficiently evident.

Having thus had one initiation into this most enticing realm of riddles,
each successive orchid whose structure we examine from this stand-point
becomes a most interesting, perhaps a fresh, problem, whose assumed
solution may often be verified by studying the insect in its haunts.
Darwin thus foretold the precise manner of the cross-fertilization of
_Habenaria mascula_, and also the insect agent, simply by the structural
prophecy of the flower itself.

Suppose, for example, an unknown orchid blossom to be placed in our
hands. Its nectary tube is five inches in length, and as slender as a
knitting-needle. The nectar is secreted far within its lip. The
evolution of the long nectary implies an adaptation to an insect's
tongue of equal length. What insect has a tongue five inches long, and
sufficiently slender to probe this nectary? The sphinx-mot