ELEMENTS OP ENTOMOLOGY. 
32 
Diptera, the triangular contour is still more observable under variott® 
modifications, and most commonly with the posterior tip rounded on- 
Sometimes, as in several of the hymenopterous insects, the posterior 
end is armed with spines or denticulations; this is, however, not usua • 
The scutel in the far greater number of insects, whether terminating 
in a point or rounded, is commonly unarmed. In point ot size th< 
scutel is more variable than in figure: in some it is so small as almos' 
to escape notice, merely forming a point at the extremity ol the tho- 
rax, as we observe in certain kinds ot the beetle tribe ; in others it i® 
very conspicuous, being sometimes so large as to cover the middle o' 
the back; and in others, as the scutellale kinds of Cimices and a fevt 
of the genus Acridium, it expands over the back, entirely concealing 
the wings and wing-cases, and covering the margin of the abdomen. 
ABDOMEN. The third principal division, or posterior part of the 
body, is connected with the breast, either closely or at a distance, by 
means of a fillet. The abdomen is composed of annular joints of 
segments, the number of which vary in different insects. The upper 
part of the abdomen is called by entomologists, tergum ; the inferior 
or belly, venter. The opening at the posterior part of the abdomen i® 
the vent; and the extremity in most insects contains the organs of ge- 
neration : there are exceptions to the latter. 
The total movement of tire abdomen is not very obvious, except n® 
insects which have that portion of the body pediculated, as in many o* 
the hymenopterous genera. It lias then a real joint, in which the first 
annulation is indented above, and receives a projecting process from the 
breast, on which it moves. This joint is rendered secure by clastic liga- 
ments, which have a considerable degree of force. Some muscle® 
which arise within the breast are inserted into the first ring, and de- 
termine the extent of its motions. The partial motion of the ring >® 
produced by very simple muscles, consisting of fibres which extend 
from the anterior edge of one ring to the posterior edge of that which 
immediately precedes it. When the dorsal fibres contract, the superior 
part of the abdomen being shortened, it hints up towards the back! 
but when the contraction bikes place in the ventral or lateral fibres, th® 
abdomen is inflected towards the belly, or directed towards one of th® 
sides. The extent of the motion, however, depends on the number o' 
the rinws and their mode of junction. In the Coleoptcra, for example 
the rinas only touch each other by their edges, and the motion is ver,® 
limited ; but in the Si/menoptera they arc so many small hoops, which 
are incased one into another like the tubes of a telescope, so that scarcely 
half, and sometimes not above one-third, of their extent appears visibl® 
externally. 
The form, connexion, proportion, and appearance, of the surfa c ® 
of the annulations of the abdomen, afford numberless specific distin c 
