32 r.T.EvrxTS or ^^'TOM0I.OGY. 



Diptera, the triangular contour is still more obsen-ablc under various 

 jnodifications, and most commonly with the iiosteriur tip rouudcil oif. 

 Sometimes, as in several of the hymenopterous insects, the posterior 

 end is armed with spines or denticulations; this is, however, not usual. 

 The sctifcl in the far greater number of insects, whetluT terminaling 

 in a point or rounded, is commonly unarmed. In point of size the 

 scutcl is more variable than hi figure : in some it is so small as almost 

 to escape notice, merely forming a point at the extremity of the tho- 

 rax, as we observe in certain kinds of the beetle tribe; in others it is 

 very conspicuous, being sometimes so lai'ge as to cover the middle of 

 the back; and in otliers, as the scutellate kinds of Cinikcs and a few 

 of the genus Acridiwn, 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 (Ustance, by 

 means of a fillet. The abdomen is composed of annular joiiit'; or 

 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 is 

 the vent; and the extremity in most insects contains the organs of ge- 

 neration : there are exceptions to the latter. 



The total movement of the abdomen is not very obvious, except in 

 insects which have that portion of the body pediculated, as in many of 

 the hymenopterous genera. It has then a real joint, in which the first 

 annulation is indented above, and receives a projecting jirocess from the 

 breast, on wh'ch it moves. Tins joint is rendered secure by elastic liga- 

 ments, which have a considerable degree of force. Some muscles 

 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 is 

 produced by very simple uuiscles, consisting of fibres which extend 

 from the anterior edge of one ring to the posterior edge of that \\ hich 

 immediately precedes it. When the dorsal fibres contract, the superior 

 part of tlie abdomen being shortened, it turns up towards the back ; 

 but when the contraction takes place in the ventral or lateral fibres, the 

 abdomen is inflected towards the belly, or directed towards one of the 

 sides. The extent of the motion, however, depends on the number of 

 the rings and their mode of junction. In the Coleoptera, for example, 

 the rings only tuuch each oilier by their edges, and the motion is very 

 limited; but in the Hymenoptcra they are 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 visible 

 externally. 



The form, connexion, projwrtion, and appearance, of the surface 

 of the anuulations of the abdomen, afford numberless specific distiuc- 



