32 ELEMENTS OF EXTOMOLOGY. 



Dipfe?'a, the triangular contour is still more observable under various 

 modifications, and most commonly with the posterior tip rounded oif. 

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

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

 The sen/ el in ihc far greater number of insects, whether terminating 

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

 sculel is more variable than in 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 large as to cover the middle of 

 the back; and in otliers, as the scutellatc kinds of Ciniices and a few 

 of the genus Acrklhoii, it expands over the back, entirely conccahng 

 the wings and wing-cases, and covering the margin of the abdomen. 



ABDOMEN. Tiie 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 or 

 segments, the number of which \ary in difterent insects. The upper 

 part of the abdomen is called by entomologists, ta-guin ; the hiferior 

 or belly, WH^e?-. 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 arc exceptions to the latter. 



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

 insects which have that portion of the lj(j(ly pciliculated, as in many of 

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

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

 breast, on whch it moves. This joint is rendered secin"e bv elastic liga- 

 meats, 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 muscles, consisting of fibres which extend 

 from' the anterior edge of one ring to the posterior edge of that which 

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

 part of the 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 nmnber of 

 the rings and their mode of junction. In the Co/eoplera, for example, 

 the rings only touch each other by their edges, and the motion is very 

 limited ; but in the Hymcywplera they are so many small hoops, which 

 are incased one into another like the tidies of a telescope, so that scarcely 

 half, and sometimes not above one-third, of tlieir extent appears visible 

 externally. 



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

 of the annulalious of the abdomen, afford nunibcrlebs specific distinc- 



