within the space of thirty days. From five they extend to nine or ten, the latter number having been observed in the tiger moth, (Chelonia caja.) But the great majority of insects do not undergo this operation oftener than three or four times. The caterpillars of butterflies are usually limited to the former of these numbers, those of moths to the latter, but among moths many other exceptions occur besides the one mentioned above. The proximate cause, as already intimated, of this moulting, is the more rapid expansion of the body than of the skin, which, in consequence, soon exceeds the capacity of its envelope: the latter, it is true, admits of a certain degree of distention, but a few fixed points on its surface, as well as the rigidity of the part enclosing the head necessary to give support to the masticating organs, prevent it keeping pace with the growth of the body. The larva ceases to eat when the change is approaching, and, during this temporary suspension of its ordinary employment, the fat lying immediately beneath the outer skin is absorbed, a circumstance which greatly hastens the crisis by abstracting the moisture from the skin and shrivelling it, while, at the same time, it tends still further to dilate the internal parts. The colours, being dependent in a great measure on the freshness and moisture of the skin, also become dull and confused. The only motions made by the larvæ are occasional contortions and undulatory movements of the segments, which ultimately produce a complete separation between the exterior skin and the new
