26 INTERNAL ANATOMY OF INSECTS. 
very minute. In Hypogymna dispar, we learn from Cu- 
vier, this orifice is of that description, and of a trian- 
gular shape*. 
It can admit of no reasonable doubt that one of the 
principal intentions of these changes is to accommodate 
the nervous system to the altered fuuctions of the ani- 
mal in its new stage of existence, in which the antennas, 
eyes, and other organs of the senses, as well as the limbs 
and muscles moving them, and the sexual organs, being 
very different from those of the larva, and if not wholly 
new, yet expanded from minute germs to their full size, 
may well demand corresponding changes in the struc- 
ture of the nervous system by which they are acted 
upon. 
But are these changes also concerned, as Dr. Virey 
conjectures, in producing that remarkable alteration 
which usually takes place between the instincts of the 
larva and imago ? In order to answer this question, it 
will be requisite first to quote the ingenious illustration 
with which this able physiologist elucidates his ideas on 
this point. " The more readily," he observes, " to com- 
prehend the action of instinct, let us compare the insect 
to one of those hand-organs in which a revolving cylin- 
der presents different tunes noted at its surface, and 
pressing the keys of the pipes of the organ, gives birth to 
all the tones of a song : if the tune is to be changed, the 
cylinder must be pulled out or pushed in one or more 
notches, to present other notes to the keys. In the same 
manner let us suppose that nature has impressed or en- 
graved certain determinations or notes of action, fixed in 
ii Anat. Comp. ii. 348. 
