28 INTERNAL ANATOMY OF INSECTS. 
ture of the nervous chord is not changed ; and yet we 
know that many tribes of these orders acquire instincts 
in their imago state altogether different from those which 
directed them in their state of larvae. A perfect Locust, 
for instance, acquires the new instincts of using its wings ; 
of undertaking those distant migrations of which so many 
remarkable instances were laid before you in a former 
letter a ; and, if a female, of depositing its eggs in an ap- 
propriate situation. But if such striking changes in the 
instinct of these tribes can be effected without any per- 
ceptible alteration in the structure of the nervous chord, 
it is contrary to the received rules of philosophical in- 
duction to refer to this alteration the changes in the in- 
stincts of other tribes where it is found. Is it not far 
more probable that this alteration has in fact no con- 
nexion with the changes of instinct, but is solely con- 
cerned with those remarkable changes in the organs of 
sense and motion, which occur in the larva and imago 
states of the orders in which it is observed ? In a com- 
mon caterpillar, the form of the body, the legs, the eyes, 
and other organs of the senses, all strikingly differ from 
those of the imago ; whereas, with the exception of the 
acquisition' of new wings, a perfect locust differs little 
from its larva : so that we may reasonably expect a cor- 
responding change, such as we find it, in the structure 
of the nervous chord of the lepidopterous insect, not 
called for in that of the neuropterous species, in which 
accordingly it does not take place. 
This reasoning, in opposition to Dr. Virey's theory, 
that the changes of instinct depend on the altered struc- 
ture of the nervous system, becomes greatly strengthened 
' Vol. I. p. 217—. 
