ENTOMOLOGY. 
93 
eye-case is called Ophthalmo theca, the antemue-cases, 
Cerathecse, &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 de- 
finite period assigned it from which there is no mate- 
rial deviation, unless under very peculiar circumstances. 
Perhaps the most general duration is from two to four 
weeks, hut, 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 in- 
fallibly 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 pro- 
longation 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 assimi- 
lation 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 pupae of the same 
species, placed in precisely the same circumstances, 
