110 PARTIAL ORISMOLOGY. 



With respect to the general form of the abdomen, it varies so 

 extremely, that we can scarcely suggest a universal type of construction. 

 It is sometimes ovate, longitudinal or cylindrical, sometimes compressed 

 and angular or broad and flat ; but its transverse section may always be 

 readily reduced to the form of a rectangular triangle, the base of which 

 lies above, the apex pointing downwards. It is not possible to give a 

 more definite determination to this triangle, for its sides are sometimes 

 straight, sometimes, chiefly the upper one, convex, sometimes the 

 opposite sides form an apparent semicircle, occasionally they bend 

 inwards or outwards, c. &c. 



The form of the abdomen depends much upon the mode of its 

 attachment to the thorax. In the majority of cases, for instance, 

 in the Coleoptera, Orthoptera, Dictyotopfera, Hemiptera, and in 

 many families of the other orders, the abdomen is conical, that is to 

 say, it commences with a broad base and gradually decreases towards 

 its apex; when this broad base is united by its whole circumference to 

 the metathorax, the abdomen is called SESSILE. But even a perfectly 

 conical abdomen, the base of which is sharply truncated, is sometimes 

 connected with the metathorax by means only of a small portion of its 

 margin ( Vespa). This mode of union between both parts of the body is 

 most perceptible in those insects whose first abdominal segment has the 

 form of a thin tube, which, towards its apex, distends more or less 

 trumpet-shaped. The succeeding broader and larger segments are 

 united to the first in the same way as among themselves, and by this 

 means the either ovate, conical, compressed, falcate, flat, or longitudinal 

 abdomen appears as if, like the leaf of a tree, it was supported by 

 a distinct stalk, whence it has been called by entomologists PETIOLATED 

 (ab. petiolatum\ The tubular first segment itself is called the 

 PEDICLE (petiolus). It is not always a direct tube, but occasionally 

 swollen into knots (pet. nodosus}, or distended upwards into a thin 

 scale (pet. squamatiis). If the second abdominal segment be of greater 

 compass than the following, so that its margin stands freely out, 

 and the succeeding segment completely received within it, the abdomen 

 is then called CAMPANULATE (ab. campanulatum, for example Zethus). 

 But if, on the contrary, the abdomen be constricted at its commencement, 

 and not perfectly petiolated, as in the Butterflies and most Diptera, 

 it is called COARCTATE (ab. coarctatum). 



In many instances, all or individual segments of the abdomen have 

 peculiar processes, which are found sometimes at their sides, and which 



