of Dipterous insects have the power of introducing air into their pupa-case for the purpose of forcing a passage out of it. The air is said to be introduced under the middle part of the head, which becomes inflated into a membranous vesicle, and thus acts upon the end or lid of the pupa-case, and, in time, forces it open. This singular lever is necessary in the case of the flies alluded to, in consequence of the substance of the puparium being so hard as to offer more than usual resistance.
Many subterraneous pupæ assume the perfect form beneath the ground, and others make their way to the surface before undergoing that change. In the former case, the insect remains where it was disclosed till it acquires sufficient strength and hardness to render it safe to force a passage upwards; the rosechafer (Cetonia aurata,) continues about fifteen days, the cockchafer (Melolontha vulgaris,) and Oryctes nasicornis, nearly a month. This, however, would obviously be impracticable for soft winged insects, (such as moths, two-winged flies, &c.) and it is, therefore, necessary that they should reach the surface (whether it be of the ground or the rotten trunk of a tree,) while yet pupæ. We have mentioned the means by which the pupa of the goat-moth effects this, and a similar plan is followed by most others so circumstanced. Such pupæ as are enclosed in cocoons are provided with means for forcing a passage through this additional obstacle of which an account has been given when treating of the insects by which cocoons are usually constructed. Other peculiarities
