When a caterpillar cannot find earth sufficiently moist to bear kneading into the requisite consistence, it has the means of moistening it with a fluid which it ejects for the purpose; and as soon as it has thus prepared a small pellet of earth, it fits it into the wall of the vault, and secures it with silk. As the little mason, however, always works on the _inside_ of the building, it does not, at first view, appear in what manner it can procure materials for making one or two additional walls on the inside of the one first built. As the process takes place under ground, it is not easy to discover the particulars, for the caterpillars will not work in glazed boxes. The difficulty was completely overcome by M. Réaumur, in the instance of the caterpillar of the water-betony moth (_Cucullia scrophulariæ_, Schrank), which he permitted to construct the greater part of its underground building, and then dug it up and broke a portion off from the end, leaving about a third part of the whole to be rebuilt. Those who are unacquainted with the instinct of insects might have supposed that, being disturbed by the demolition of its walls, it would have left off work; but the stimulus of providing for the great change is so powerful, that scarcely any disturbance will interrupt a caterpillar in this species of labour. [Illustration: Earth-mason Caterpillars' Nests, with the perfect Moth, &c.] The little builder accordingly was not long in recommencing its task for the purpose of repairing the disorder, which it accomplished in about four hours. At first it protruded its body almost entirely beyond the breach which had been made, to reconnoitre the exterior for building materials. Earth was put within its reach, of the same kind as it had previously used, and it was not long in selecting a grain adapted to its purpose, which it fitted into the wall and secured with silk. It first enlarged the outside of the wall by the larger and coarser grains, and then selected finer for the interior. But before it closed the aperture, it collected a quantity of earth on the inside, wove a pretty thick net-work tapestry of silk over the part which remained open, and into the meshes of this, by pushing and pressing, it thrust grains of earth, securing them with silk till the whole was rendered opaque; and the further operations of the insect could no longer be watched, except that it was observed to keep in motion, finishing, no doubt, the silken tapestry of the interior of its little chamber. When it was completed, M. Réaumur ascertained that the portion of the structure which had been built under his eye was equally thick and compact with the other, which had been done under ground.