To form the wafer door, the spider covers the entrance to the tube with a closely-woven layer of silk, which it afterwards bites away at the edge, except at the point where the hinge is to be. Doors of the cork type consist of alternate layers of silk and earth. After weaving a covering of silk, the creature brings earth in its jaws and lays it on the top, binding it down with a second layer of silk, and the process is repeated until the requisite thickness is attained. The nests are exceedingly difficult to detect, as the spiders take the precaution of attaching leaves, moss, or small twigs to the outer surface of the doors. This does not appear to be the result of intelligence, but a mere instinctive habit; for if a door be removed and the surrounding earth denuded of moss, the spider will render the new door conspicuous by bringing moss from a distance, and thus making a green spot in the bare patch of earth.[278] The cork doors fit with great exactness, and there is always to be found on their under surface a notch by which they are held down by the fore-legs of the spider against any attempt to open them from without. Many nests with trap-doors of the wafer type are found to have a second and more solid door within the tube. This serves to shut off the lower part of the nest as a still more secure retreat. This second door opens downwards, and the Spider, getting beneath it, is effectually shielded from an enemy which may have mastered the secret of the outer barrier. The nests of some species present still further complications in the way of lateral branches from the main tube. In one case (_Nemesia congener_) the burrow becomes =Y=-shaped, and the second door hangs at the fork of the =Y= in such a manner as to connect the bottom chamber either with the entrance or with the branch, which does not reach the surface, but ends blindly. Trap-door Spiders are greatly attached to their tubes, which they enlarge and repair at need. They begin burrowing very early in life, and their tiny tubes resemble in all respects those of their parents. Their habits are nocturnal, and little is known of them; an observation, however, on a species inhabiting the island of Tinos in the Grecian Archipelago (_Cteniza ariana_), by Erber,[279] must not be omitted. This spider leaves its tube at night and spins a web near at hand and close to the ground. It carries captured insects into its tube, and in the morning clears away the net, adding the material of it, M. Erber believes, to the trap-door.