EXTERNAL STRUCTURE. 11 



and Camerou ; but its old collective name of nietathorax, 

 embracing both the true metatborax and the superincunibeut 

 abdominal segment, is here retained, for the sake of convenience 

 and in accordance with the usage of most authorities u])on 

 loHNEUMONiDiE. The scutellum is the centre of the insect, and 

 consequently the prothorax is at the ajiex of the mesothorax and 

 the abdomen at the apex of the metatborax. The metatborax is 

 divided up in most instances by a number of costae or carina), 

 whicb are nearly always constant in the same species and form 

 valuable characters for specific distinctions. The basal area 

 occupies the extreme base of the metatborax, immediately beyond 

 the postscutellum, and is often divided into three parts by longi- 

 tudinal costse, in which case the two lateral are termed the 

 external area3 ; beyond this may usually be seen a strong trans- 

 verse basal carina and across the metatborax a little beyond its 

 centre is more often a second apical transverse carina ; between 

 these two carina? in the centre of the disc is a usually well-defined 

 central area, known as the areola, divided from the areas on either 

 side by an often weaker longitudinal costa. The area? on either 

 side of the areola are known as the dentiparal, since their apices 

 are not infrequently produced into stout and elongate apophyses or 

 teeth ; these areaj are sometimes centrally bisected by weak trans- 

 verse costse, known as the costnla? and usually situated in a line 

 with the centre of the areola. Laterally, beyond the dentiparal, 

 are the longitudinal spiracular arete, bearing towards their base 

 the metathoracic spiracles, Mbich afford useful characters for 

 main divisions on account of their constanc}^ of shape. Laterally, 

 beyond these again, are the lateral and finally the juxta-coxal 

 areffi. Apically, beyond the apical transverse carina, the declivous 

 part of the metatborax is termed as a A\-hole the petiolar area, 

 because from its apex is emitted the abdominal petiole ; this apical 

 region is sometimes longitudinally divided into five or three area? 

 by longitudinal costte, in whicb case the petiolar area is said to be 

 discrete. Of the above areae, the areola is the most important 

 and constant, at least some part of it being ])resent when all 

 other area3 have become obsolete or entirely wanting, through the 

 razing of the costte and carina). Its apical carina has been 

 especially employed by Lurster in grouping the Pezomaclii ; 

 Thomson has used the basal and apical carina? in his main divisions 

 of the Cryptin.t:, resulting in a beautifully natural classification, 

 and the former carina is also a conspicuous feature of the 

 PiMPUNiE. Ashmead has founded genera upon the confluence 

 of the juxta-coxal with the pleural areae, and in most cases the 

 apical central costa) are wanting, though these exhibit good 

 specific features in the difficult groups of smaller Campoplegids. 



The abdomen is connected with the tliorax by a more or less 

 distinctly constricted stem, called the petiole. This is tlie base of 

 the first abdominal segment, which towards its apex becomes niore 

 or less explanate and is thence known as the postpetiole, usually 



