It will be seen by a reference to [Plate XIX],[144] figs. A and A 1, that in addition to the cylindrical branch, which mounts upwards, there is a shorter branch which leaves the main tube on the opposite side (on the left as seen in the Plate), and takes a downward course. Now this descending branch, which is barely more than an inch in length, is a cavity of variable form, being sometimes cylindrical, and sometimes egg- or even watch-shaped,[145] but there is one particular in which it never varies, and that is the position of its elliptic orifice. This orifice is always situated on the opposite side of the main tube to that on which the ascending branch leaves this latter, so that the whole nest, when seen in section, presents the figure of a St. Andrew's cross, only with arms of unequal length.
[144] A nest of a scarcely half-grown spider is here represented in order that sufficient space might be gained to show the lower door in its two positions. The perfect cavity is still found in nests of much larger dimensions, and occasionally, indeed, in nests of almost the maximum size.
[145] I take the liberty of coining a word to replace "lenticular," the form of a watch being more familiar than that of a lens.
But the most remarkable point is that, when the lower door is pushed across so as to close the main tube (as shown in fig. A, [Plate XIX]), it will invariably be found to lie in such a position that its lower extremity exactly meets the lower lip of the orifice of the descending cavity, when it will be seen that the semi-cylindrical surface of the lower door then coincides with, and appears to continue and form part of, the lower wall of the descending cavity on the one side, and of the corresponding wall of the main tube on the other. When the upper portion of the main tube is thus united to the cavity the two combine to form what appears like a short, independent unbranched nest.
Now, if we fancy ourselves an insect entering the nest in search either of the spider, her eggs, or young, I think it is plain that, when the lower door is in this position (fig. A), we should probably walk straight down to the bottom of the cavity, expecting to find our prey there, and should then return by the way we came, impressed with the belief that we had explored the whole nest, the secret of the lower door remaining undiscovered.
Whether this imaginary case may, or may not, represent what really takes place, is of course mere conjecture; but the constant occurrence of this beautiful adaptation of the various parts to one another, surely points to the conclusion that this is no mere coincidence, but rather a subtle contrivance having some very definite use and meaning.
We must admit, however, that it is difficult to conceive why, if this structure is of such great utility, it should be abandoned by the oldest and largest spiders.
Among the possible answers to this question I think that one of the more probable is that this arrangement may have been specially devised for protection against some enemy which the aged spiders have ceased to fear.