EXTERNAL STRUCTURE. 11 



and Cameron ; but its old collective name of metathorax, 

 embracing both the true metathoi'ax and the superincumbent 

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

 and in accordance with the usage of most authorities upon 

 ICHNEUMONID^E. The scutellum is the centre of the insect, and 

 consequently the prothorax is at the apex of the raesothorax and 

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

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

 which are nearly always constant in the same species and form 

 valuable characters for specific distinctions. The basal area 

 occupies the extreme base of the metathorax, immediately beyond 

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

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

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

 verse basal carina and across the metathorax a little beyond its 

 centre is more often a second apical transverse carina ; between 

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

 central area, known as the areola, divided from the area3 on either 

 side by an often weaker longitudinal costa. The arese 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 area? are sometimes centrally bisected by weak trans- 

 verse costa?, known as the costulae and usually situated in a line 

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

 are the longitudinal spiracular areas, bearing towards their base 

 the metathoracic spiracles, which afford useful characters for 

 main divisions on account of their constancy of shape. Laterally, 

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

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

 part of the metathorax is termed as a whole the petiolar area, 

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

 region is sometimes longitudinally divided into five or three arese 

 by longitudinal cost*, in which 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 present when all 

 other areae have become obsolete or entirely wanting, through the 

 razing of the costse and carinae. Its apical carina has been 

 especially employed by Forster in grouping the PezomacJ/i ; 

 Thomson has used the basal and apical carinae in his main divisions 

 of the CRYPTIX^B, resulting in a beautifully natural classification, 

 and the former carina is also a conspicuous feature of the 

 PIMPLIN^;. Ashmead has founded genera upon the confluence 

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

 apical central costae are wanting, though these exhibit good 

 specific features in the difficult groups of smaller Campoplegids. 



The abdomen is connected with the thorax bv a more or less 

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

 the first abdominal segment, which towards its apex becomes more 

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



