230 THE MECHANICAL FUNCTIONS. 



transverse line. Each of these three segments is divisible 

 into an upper, a lower and two lateral portions, which are 

 joined together at the sides of the trunk; these again admit 

 of farther subdivision; but for the names and descriptions of 

 these smaller pieces I must refer the reader to works on 

 Entomology. 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 re- 

 sultant of all the forces concerned in the great movements 

 of the bod}^, requires to be sustained by the moving powers 

 under all circumstances either of action or repose. 



Victor Audouin, who has made extensive researches on 

 the comparative forms of all these parts in a great variety 

 of insects, appears to have satisfactorily established the ge- 

 neral proposition that, amidst the endless diversity of forms 

 exhibited by the skeleton of insects, they are invariably 

 composed of the same number of elements, disposed in the 

 same relative situations and order of arrangement: and that 

 the only source of difference is a variation in the propor- 

 tional development of these elements. He has also observed 

 • that the great expansion of one part is generally attended 

 by a corresponding diminution of others. 



The third division of the body is termed the Mdomen 

 (b;) it is composed of all the remaining segments, which join 

 to form a cavity enclosing the viscera subservient to nutri- 

 tion, respiration, and reproduction. The number of these 

 abdominal segments is very various in different genera of 

 insects. Sometimes there appear to be but three or four; 

 while, in other cases, there are twelve or even a greater 

 number. In the Calosoma (Fig. 150, b,) the abdomen has 

 six complete, followed by three imperfect segments. Not 

 being intended to carry any of the organs of progressive 

 motion, they retain the form of single hoops, which is the 

 primitive type of the segments of annulose animals. Each 

 segment has a ligamentous connexion with the next, which 

 is often so close, as hardly to admit of any motion between 

 them; but in other instances It is more lax, and allows of 

 the abdomen being flexible. In the former case, which is the 



