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 Staphylinidæ, 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, (pièces lombaires,) are probably 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
