326 THE MECHANICAL FUNCTIONS. 



admits of a twisting motion; and the latter seg- 

 ment may be pushed more or less into the cavity 

 of the former, either generally, or on one side. 



Each segment, besides being separate from 

 the rest, is further divided into an upper, or 

 dorsal, and a lower, or ventral portion ; each 

 portion having the form of a semicircle, or rather 

 of an arch of a circle. These are connected at 

 the sides by a ligamentous band, which runs the 

 whole length of the abdomen. Great advantage 

 results from this division of the circles, allowing 

 of the upper and lower portions of the abdominal 

 covering being at one time separated, and at 

 another brought nearer together ; for thus the 

 cavity is capable of being enlarged or contracted 

 in its dimensions, and adapted to the variable 

 bulk of its contents. It is deserving of notice 

 that, during the process of transformation, some 

 of the abdominal segments, which are present 

 in the larva, disappear entirely, or leave only 

 imperfect traces of their former existence. 

 Sometimes the posterior segments become so 

 exceedingly contracted in their diameter as to 

 give rise to the appearance of a tail : this is 

 exemplified in the Panorpa. 



The junction of the abdomen with the trunk 

 is effected in various ways. In all the Coleoptera, 

 it is united by the whole margin of its base, 

 without having a narrower part : in other tribes 

 there is a visible diminution of diameter, forming 



