EXTERNAL ANATOMY OF INSECTS. 405 



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 com- 

 mon louse (Pediculus humanus) they scarcely reach fifty, 

 while in some cockroaches (Blatta) 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 siib- 

 stance,Jigure, composition, superficies.) proportion, direc- 

 tion, artimlation with the trunk, motions — and more par- 

 ticularly as to its parts and appendages. 



i. With regard to its substance — 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 

 inter se, as in the case of the trunk, is requisite to fa- 

 cilitate the play of its organs, a thin integument was 

 not wanted. 



