STRUCTURE OF INSECTS. 231 



construction in all the Coleopiera, or heeilcs, the rings have 

 an imbricated arrangement; that is, each overlap the next, 

 often to the extent of two-thirds of its breadth: so that they 

 present a succession of spheroidal hoops, capable of beino- 

 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 muscles they enclose: for since the surfaces 

 which receive, as well as those which are received, are seg- 

 ments of spheroids, this structure admits of a twistino- mo- 

 tion; and the latter segment 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 far- 

 ther divided into an upper, or dorsal, and a lower, or ventral 

 portion; each portion having the form of a semicircle, or ra- 

 ther 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 ab- 

 dominal covering being at one time separated, and at ano- 

 ther brought nearer together; for thus the cavity is capable 

 of being enlarged or contracted in its dimensions, and adapt- 

 ed to the variable bulk of its contents. It is deservino- of 

 notice that, during the process of transformation, some of 

 the abdominal segments, which are present in the larva, dis- 

 appear entirely, or leave only imperfect traces of their for- 

 mer 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 Pa- 

 Tiorpa. 



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, form- 

 ing a groove all round, or an incision^ as it is technically 

 termed. In the Ilymenoptera, this incision is so deep as to 

 leave only a narrow pedicle, like a neck, connecting these 



