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. Indeed it is not unlikely that these aged spiders may have come to a time of life when they no longer lay eggs, and so do not need to keep up all the defences which they employed when they had families to protect. Since my attention was drawn to the existence of this cavity in the dwellings of _N. Manderstjernæ_ I have never noted the presence of young in those nests in which the cavity was filled up and disused; but then I have only exact records with reference to this point in the case of seven nests. In these seven nests, however, there was no free cavity, and there were no young spiders, though it was at the season when it was common to find young in the nests. The question, therefore, remains open, and further observations on this head would be very acceptable. I detected the _débris_ of insects, and especially the horny coats of ants, in the descending cavity, in many nests; and in some of the oldest, where it had become completely blocked up, these remains still indicated its former outlines and position.