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 functions of the ani- 

 mal in its new stage of existence, in which the antennae, 

 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 



a Anat. Comp. ii. 348. 



