324 THE MECHANICAL FUNCTIONS. 



are joined together at the sides of the trunk ; 

 these again admit of further subdivision ; but for 

 the names and descriptions of these smaller 

 pieces I must refer the reader to works on Ento- 

 mology. The parts of the thorax to which the 

 wings are attached indicate the situation of the 

 centre of gravity of the whole insect ; a point, 

 which being in the line of the resultant of all the 

 forces concerned in the great movements of the 

 body, requires to be sustained by the moving 

 powers under all circumstances either of action 

 or repose. 



Victor Audouin, who has made extensive re- 

 searches on the comparative forms of all these 

 parts in a great variety of insects, appears to have 

 satisfactorily established the general proposition 

 that, amidst the endless diversity of forms exhi- 

 bited by the skeleton of insects, they are invari- 

 ably composed of the same number of elements, 

 disposed in the same relative situations and order 

 of arrangement : and that the only source of dif- 

 ference is a variation in the proportional deve- 

 lopement of these elements. He has also ob- 

 served that the great expansion of one part is 

 generally attended by a corresponding diminu- 

 tion of others. 



The third division of the body is termed the 

 Ahdomen (b) ; it is composed of all the remaining 

 segments, which join to form a cavity enclosing 



