4.04 EXTERNAL ANATOMY OF INSECTS. 
The number of articulations or pieces that form the 
integument and its members in these animals, varies 
greatly in different tribes, genera, &c. ‘Thus, in the 
common louse they scarcely reach fifty, while in some 
cockroaches they amount to more than eight times that 
number. 
Having premised these observations on the external 
anatomy of the body in general, in the remainder of the 
present letter I shall confine myself to the consideration 
of the head and its parts. 
I. The Head of insects, as the principal seat of the 
organs of sensation, must be regarded in them, as well 
as in the vertebrate animals, as the governing part of the 
body. It may be considered with respect to its sub- 
stance, figure, composition, superficies, proportion, direc- 
tion, articulation with the trunk, motions—and more 
particularly as to its parts and appendages. 
i. With regard to its swbstance—-the head may be said 
in general to be the hardest part of the crust: and it is 
so for very good reasons. In the first place, as it has to 
make way for the rest of the body when the animal moves, 
it is thereby best fitted to overcome such resistance as 
may be opposed by the medium through which it has to 
pass ; in the next, as it bears the organs of manducation, 
it was requisite that it should be sufficiently firm and so- 
lid to support their action, which is often upon very 
hard substances; and besides this, as no motion of its 
parts znter se, as in the case of the trunk, is requisite to 
facilitate the play of its organs, a thin integument was 
not wanted. 
