290 THE MECHANIC AL FUNCTIONS. 



the same relative situations and order of arrange- 

 ment; and that the only source of difference is a 

 variation in the proportional developement 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 

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

 segments, which join to form a cavity enclosing the 

 viscera subservient to nutrition, 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 six or 

 seven, or even a greater number. In the Calosoma 

 (Fig. 150, b), the abdomen has six complete, fol- 

 lowed by three imperfect segments. Not being 

 intended to carry any of the organs of progressive 

 motion, they retain the form of simple hoops, which 

 is the primitive type of the segments of annulose 

 animals. Each segment has a ligamentous con- 

 nexion 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 construction in all the Coleoptera, or beetles, 

 the rings have an imbricated arrangement ; that 

 is, each overlaps the next, often to the extent of 

 two-thirds of its breadth : so that they present a 

 succession of spheroidal hoops, capable of being 

 drawn out, to a certain extent, like the tubes of 

 a telescope. This very artificial construction is 

 manifestly designed to allow of a great variety of 

 movements, determined by the position of the mus- 



