eye-case is called Ophthalmotheca, the antennæ-cases, Cerathecæ, &c.
The length of time insects pass in this stage of their existence varies from a few days to nearly two years. Each species, however, has in general a definite period assigned it from which there is no material deviation, unless under very peculiar circumstances. Perhaps the most general duration is from two to four weeks, but, even in the same species, this depends upon the season of the year, for a pupa which would disclose the perfect insect in a few weeks during the summer, will frequently lie dormant throughout the entire winter. Unless a provision of this kind obtained, it is obvious that many insects would infallibly perish from being brought into existence at a time when it is impossible to find the means of maintaining life. The immediate cause of this prolongation of their quiescent condition is to be found in the effects of the winter's cold, the more remote one in the wise ordination of providence. Artificial heat, as has been already stated in a former volume, will mature the perfect insect and make it burst from its prison at any period of the year; and, in like manner, artificial cold will retard its birth. From these considerations, it is obvious that the evolution of the imago depends on the evaporation and assimilation of the fluids, and this takes place in a more speedy or tardy manner, according to the greater or less degree of heat to which the pupa is exposed; it does not, however, happen that pupæ of the same species, placed in precisely the same circumstances,
