ENTOMOLOGY. 
95 
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 pupae 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 rose- 
chafer ( Cetonia aurata,) continues about fifteen days, 
the cockchafer ( Mdolontha vulgaris ,) and Oryctes 
nasicornis, nearly a month. This, however, would 
obviously be impracticable for soft winged insects, 
(such as moths, two-winged flies. See.) and it is, 
therefore, necessary that they should reach the sur- 
face (whether it be of the ground or the rotten trunk 
of a tree,) while yet pup®. 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 pupae as are enclosed in co- 
coons 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 ar^ usually constructed. Other peculiarities 
