122 INTRODUCTION TO 



the base appears united by its whole circumference 

 to the metathorax, a suture alone indicating the point 

 of junction ; in such cases the abdomen is said to be 

 sessile. A very narrow point, in other instances, 

 forms the whole bond of connection, the base being 

 contracted into a slender trumpet-shaped tube, which 

 scarcely appears of adequate dimensions to transmit 

 the vessels requisite for maintaining life ; such an 

 abdomen is said to be petiolated. 



The segments of the abdomen may be regarded as 

 composed each of two arches, a dorsal and a ventral 

 one; but analogy inclines, us to believe that thes 

 are made up of several subordinate parts, althoug 

 it is often impossible to point out their boundaries 

 In the StaphylinidoB) for example, there is a latera 

 portion, in the shape of a parallelogram, on the uppe 

 side of each of the ventral arches, united by a line 

 or articulation to the membranous part. These pieces, 

 which M. Strauss was the first to notice, and which 

 he named lombar pieces, (pieces lombaires,) are pro- 

 bably analogous to some of the lateral plates of the 

 thorax. The segments articulate with each other in 

 two principal ways. In the first, the superior arches 

 cover each other more or less, or simply touch at the 

 edges, while the lower ones are soldered together by 

 the middle, and the sides alone left free. The result 

 of this arrangement necessarily is, that the former 

 alone are susceptible of dilatation, and the abdomen 

 greatly restricted in its powers of expansion and 

 movement. In the second, each segment is covered 

 by that which precedes it without any union at any 

 part, so that they slide into each other like the tubes 



