]08 PARTIAL ORISMOLOGY. 



III. THE ABDOMEN. 



84. 



THE third and last chief division of the body of an insect bears the 

 general name of abdomen. Notwithstanding its being very variable in 

 form, it does not exhibit an equal difference of structure, but consists 

 of several consecutive horny rings or segments, in some cases merely 

 following upon, in others, retractile within each other. These rings 

 vary much in number, but never exceed nine. Upon consideration, we 

 shall remember that the body of all caterpillars and undeveloped insects 

 consists of thirteen segments (53). Of these, one constitutes the 

 head, three the thorax, and, consequently, nine remain for the abdomen. 

 These, however, are not always present; frequently, several appeal- 

 united in one; and, more frequently, the last are so completely covered 

 by the preceding, as wholly to escape observation upon a superficial 

 examination. Sometimes, also, the back presents more divisions than 

 the belly; indeed, in Carabus we observe nine distinct dorsal divisions, 

 whereas, we can distinguish in the venter but five. The belly very 

 generally presents one less than the back. We observe, also, a 

 difference of number in the sexes; for, in many Hymenoptera*, the 

 males have seven and the females but six segments. These segments 

 are either simple horny rings, or else each consists of a superior and 

 inferior half ring, which are connected together at the sides by means 

 of a delicate membrane ; sometimes a longer or shorter free process of 

 the superior half segment projects over this point of union, thus 

 covering this delicate part from all exterior injury. In the Coleoptera, 

 and, in general, in such insects as are provided with hard superior 

 wings, this structure is not to be perceived, but the soft uniting 

 membrane lies exposed, and even the horny substance of the superior 

 half segment is very small and soft, from the very natural cause of the 

 hard elytra supplying the place of all other modes of protection. In 

 these orders, therefore, the ventral portion of the segments acquire 



* All the Aculeate Hymenoplcra. Tr. 



