THE THORAX AND ITS APPENDAGES. 55 



internal pos.phra2^ni!i {Pph) which has no median tergal connection 

 at all. (4) The nietaterginn (tigs. '21 and 23 A, T..) consists of a 

 single narrow plate. (5) The nietapleurnni (fig. 21, Pl.^ and pl^) 

 shows no trace of a division into episternuni and epinieruni, but is 

 divided into an npper {Pl-^) and a lower (/^Z.,) pleural plate. (6) 

 The first abdominal tergum (fig. 21, IT) is solidly attached to the 

 metathorax and forms an intimate part of the thoracic mass. 



We shall now proceed with a more detailed account of the thorax, 

 and the reader should occasionally tnrn back to figure 4 (p. 19) in 

 order to keep clearly in mind the parts that make up a generalized 

 thoracic segment. 



The parts of the prothorax are so separated from each other that 

 they appear to belong to difl'erent segments. The protergnm (fig. 21. 

 7\) forms a collar completely encircling the front of the mesothorax. 

 On each side a large lobe {w) projects posteriorly as far as the base 

 of the front wing and constitutes a protective shield over the first 

 thoracic spiracle. The tergum ])resents a median transverse groove, 

 dividing it into an anterior and a posterior part, which parts may 

 be called the scutum (fig. 23 A, T^, Set) and scutellum {Scl). The 

 propleurum (figs. 20, 21, Eps^) consists of a large plate presenting 

 both a lateral surface (fig. 21) and a ventral surface (fig. 20). On 

 account of the position of the coxal articulation (fig. 21) this plate 

 would seem to be the anterior pleural plate alone (see fig. 4), which 

 is the episternuni. In some Hymenoptera the epimerum is repre- 

 sented by a very small plate on the rear edge of the episternuni. 

 The anterior ends of the two episterna form knobs which loosely 

 articulate with the occipital region of the head (figs. 11 B, 20, and 

 21). Lying just ventrad of each is a slender cervical sclerite (fig. 21, 

 mi). The prosternum {S^) is shown by figure 20. It carries a large 

 entosternum (Fi/^), forming a bridge over the nervous system behind 

 the prothoracic ganglion (fig. 52). 



The mesotergum, as seen in its natural position (fig. 21, 7'.,), consists 

 of a large anterior scutum (Set.,) and of a smaller but very prominent 

 posterior scutellum (Scl,), separated by a very distinct suture (r). 

 The scutellum has two latero-anterior areas partially separated from 

 the median area by sutures. When the mesotergum is detached from 

 the rest of the thorax (fig. 22) it is discovered that there is attached 

 laterally to the scutellum a large posterior internal part, which does 

 not show on the surface at all. This is the representative of the 

 postscutellum {Pscl) and its phragma {Pph) constituting the post- 

 notum (PX) of our diagrammatic segment (fig. 4). The proof of 

 this, again, is to be derived from a study of the lower Hymenopteran 

 families. In some of the horntails (Siricida?) the postnotuni or 

 postscutellum is a prominent plate on the surface of the dorsum be- 

 hind the scutellum. In S/rcc (Siricida') this plate is sunken below 



