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 these 
are made up of several subordinate parts, although 
it is often impossible to point out their boundaries. 
In the Staph ylinidce, for example, there is a lateral 
portion, in the shape of a parallelogram, on the upper 
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 
