Inaccurate observers have inferred that these earthen structures were formed by a very rude and unskilful process--the caterpillar, according to them, doing nothing more than roll itself round, while the mould adhered to the gluey perspiration with which they describe its body to be covered. This is a process as far from the truth as Aristotle's account of the spider spinning its web from wool taken from its body. Did the caterpillar do nothing more than roll itself in the earth, the cavity would be a long tube fitted exactly to its body (Fig. c): it is essentially different. [Illustration: Nests, &c., of an Earth-mason Caterpillar.] It does not indeed require very minute observation to perceive that every grain of earth in the structure is united to the contiguous grains by threads of silk; and that consequently, instead of the whole having been done at once, it must have required very considerable time and labour. This construction is rendered more obvious by throwing one of these earthen cases into water, which dissolves the earth, but does not act on the silk which binds it together. To understand how this is performed, it may not be uninteresting to follow the little mason from the beginning of his task. When one of those burrowing caterpillars has done feeding, it enters the earth to the depth of several inches, till it finds mould fit for its purpose. Having nowhere to throw the earth which it may dig out, the only means in its power of forming a cavity is to press it with its body; and, by turning round and round for this purpose, an oblong hollow is soon made. But were it left in this state, as Réaumur well remarks, though the vault might endure the requisite time by the viscosity of the earth alone, were no change to take place in its humidity, yet, as a great number are wanted to hold out for six, eight, and ten months, they require to be substantially built; a mere lining of silk, therefore, would not be sufficient, and it becomes necessary to have the walls bound with silk to some thickness.