531< EXTERNAL ANATOMY OF INJECTS. 



out, not only by the different direction of the first pair of 

 legs from that of the two last, but also in a large pro- 

 portion by a deep incisure ; and in all it is further mani- 

 fested by the anterior segment having a motion distinct 

 from that of the rest of the trunk, and separating readily 

 from it; and this not only where it is large, as in insects 

 that have a thoracic shield, but also in those in which 

 the prothorax is less apparent : whereas the other two 

 pedigerous segments have little or no distinct motion, 

 will not readily separate from each other, and in some 

 cases exhibit no pectoral suture between them. Some- 

 times, however, these two last segments are more promi- 

 nently distinguished : in Lytta, Mylabris, and other ve- 

 sicatory beetles, they are separated below by an incisure, 

 or rather the first or mid-leg segment, is not nearjy so 

 elevated as that of the hind-legs. In some ants (Atta 

 Latr.), in the neuters, there is no distinction of segments 

 in the trunk; but in others (Formica Latr.) it follows the 

 general law, and consists of three. In the Arachnida, 

 with the exception of Galeodes, in which the head is di- 

 stinct, and the three segments of the trunk may be traced, 

 these parts together form only a single segment. In- 

 duced by these reasons, I consider the trunk as consist- 

 ing in general of two primary segments, the manitrunJc 

 and alitrunk : the latter resolvable into two secondary 

 ones. 



* Manitruncus a . — The manitrunk, then, is the ante- 

 rior section of the trunk, which bears the arms and con- 

 tains the muscles that move them. This part has free 

 motion, or a motion independent of that of the rest of the 



» Plates VIII. & IX. a. 



