Evidence of the enlargement of the door is not very rare to meet with, though, as a rule, the new piece is woven on to the old with such neatness as more or less to obscure this. In fig. B, [Plate X.], p. 100, the old and smaller surface-door of a nest of Nemesia meridionalis is seen partially attached to the larger new door, which has been constructed below it; while in fig. C of the same plate, three doors, or rather three enlargements of one door, may be traced. It is this, I believe, that gives rise to the tiled appearance which these trap-doors sometimes present, and which has caused them to be compared to oyster-shells. Something similar may also be occasionally seen in doors of the cork type, as, for example, in that figured at A and A 1 in [Plate VIII.], p. 94, where the old and smaller door is seen partially raised above the surface of the new one. This I imagine to be merely an example of rather clumsy workmanship, as, if I am right, a full-sized cork door usually incloses within itself several lesser doors, which formerly fitted the tube and have had to be enlarged.
This is borne out by the fact that such a door will, on examination, be found to consist of several layers of silk, with more or less earth between each, these layers decreasing in size from without inwards, and together forming a sort of saucer in which the small central mass of earth lies. Thus by moistening a series of the cork doors of Nemesia cæmentaria, I have been able to detach, in one of medium size, from six to fourteen circular patches of silk, of which the outermost, or that which forms the lower surface of the door, is the largest, and the innermost the smallest, the others being intermediate in size as in position. Perhaps if I had had larger doors at my disposal for examination I might have found more layers, as other authors[84] speak of a much greater number of layers in the cork doors of Cteniza fodiens. Be this as it may, I am confirmed in my opinion that the layers of silk mark the successive enlargements of the nest by the additional fact that in very small doors the layers of silk are few or single, and that a proportion is observable as a rule between the size of the door and the number of layers of which it is composed.[85]
[84] M. de Walckenaer seems to have found more than thirty alternate layers of silk and earth in one of the doors of Cteniza fodiens, as we may gather from the following:—"Quoique cette porte n'ait guère que trois lignes d'epaisseur, elle est formée par la superposition de plus de trente couches de terre séparées les unes des autres par autant de couches de toile. Toutes ces assises successives s'emboitent les unes dans les autres comme les poids de cuivre à l'usage de nos petites balances. Les couches de toile se terminent au pourtour de la porte." Walckenaer, Histoire des Insectes Aptères (Suites à Buffon), vol. i. p. 238 (Paris, 1837).
I have not found the regular layers of earth and silk of which M. de Walckenaer speaks, the silk layers being usually in contact at their centres and only separated by a little ring of earth interposed between their edges, this earth being thickest towards the circumference of the layers of silk.
[85] This may be seen by the comparison of the composition of doors of different sizes, given in [Appendix H].
Another proof that enlargement takes place, may at times be found in the nests of N. Eleanora, where one, or even two useless doors may be detected behind the lower door.
Now when there are three lower doors in this way the one which is in use is the largest, and the door lying nearest to this one the next in size, while the hindmost is the smallest of all. But though those abandoned doors are now too small to fit the existing tube, they did so, no doubt, in their day, for they are exact copies in miniature of the ordinary horse-shoe shaped lower doors. The lower door actually in use may sometimes be found to have two separable cases of thick silk enclosing the central mass of earth, and this also, very probably, represents enlargement. In the nests of N. meridionalis I have never found any of these abandoned doors behind the one in use, nor should I expect to find any, for if they were present they would permanently obstruct the entrance from the main tube to the branch.
It is clear that it is better economy on the part of the spider to enlarge its nest rather than build a new one each time. If we compare the infant spider and its nest (fig. B, [Plate IX.], p. 98) with the full-grown creature and its nest (fig. A, [Plate IX.]), it becomes evident that the growing spider must either construct many nests of intermediate size, or frequently enlarge the original domicile. And we do in fact find nests of all sizes between the two extremes.
I cannot help thinking that these very small nests, built as they are by minute spiders probably not very long hatched from the egg, must rank among the most marvellous structures of the kind with which we are acquainted. That so young and weak a creature should be able to excavate a tube in the earth many times its own length, and know how to make a perfect miniature of the nest of its parents, seems to be a fact which has scarcely a parallel in nature.
When we remember how difficult a thing it is for even a trained draughtsman to reduce by eye a complicated drawing or model to a greatly diminished scale, we must own that the performance of this feat by a baby spider is so surprising as almost to exceed belief.