LETTER XLIII. 
INTERNAL ANATOMY AND PHYSIOLOGY 
OF INSECTS, CONCLUDED. 
MOTION. 
YvE have seen upon a former occasion the great variety 
of movements that insects can perform, and of the ex- 
ternal organs with which they perform them a : but we 
are now to consider the internal apparatus, by the im- 
mediate action of which they take place — their system of 
muscles. When we reflect upon the wonderful velocity, 
their size considered, with which many insects move, and 
the unparalleled degree of muscular force that many ex- 
ert b , we feel no small degree of curiosity to know some- 
thing of that part of their internal structure that pro- 
duces these almost incredible effects. I shall in the 
present letter endeavour in some degree to gratify that 
curiosity, and give you an account of the muscles of these 
little animals, — first considering them in general ,• and 
then, as far as my information goes, adverting to those 
in particular that move the different parts and organs of 
an insect's body. 
* Vol. II. Letter XXII. Vol. III. Letteus XXXIV— XXXVI. 
" Vol. II. p. 280, 295—, 306, 31 0-. &c. 
