22 THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



below by the mesosternum and metastemum, behind by the lateral 

 plate of the postscutellum, and above by the epistemum and alar 

 membrane. The parapteron (pt.) is a sclerite situated at the top of 

 the mesopleural membrane. The greater portion of it is internal, 

 only a small triangular portion can be seen externally. Inter- 

 nally this is continued as a crurifijrm sclerite to which are attached 

 important muscles controlling the wings. The costa (ca.) is a small 

 sclerite situated on the dorsal margin of the epimeron. The 

 internal skeleton of the mesothorax consists of the entothorax, 

 entopleura, mesophragma, and the inflected edges of the epistema 

 and epimera. The entothorax is composed of a median vertical 

 plate subtriangular in shape, on the top of which a median plate 

 produced laterally into wing-like processes rests. On this structure 

 the thoracic nerve-centre lies. The entopleura and the inflected 

 edges of the episterna and epimera all serve for the attachment of 

 wing muscles. The mesophragma (mph.) is a convex sclerite fused 

 with the lower edge of the postscutellum. Its posterior edge is 

 incised in the middle and forms the dorsal arch of the thoraco- 

 abdominal foramen. 



The Metathorax. The largest sclerite of the greatly reduced 

 metathorax is the metasternum (mts.). It is a wing-shaped sclerite 

 with the narrow^ transverse portion situated between the coxal 

 foramina of the median posterior pairs of legs ; the expanded 

 lateral portions form the wall of the thorax above the insertion of 

 these legs. The edges of the narrow transverse strip are inflected, 

 and unite the lateral portions of the metastemum. A trough- 

 shaped longitudinal fold^the metafurca — rests on the narrow- 

 transverse portion of the metasteiTium. The posterior end of the 

 metafurca bends doA\Tiwards and articulates with the posterior 

 coxae on each side. The metafurca serves for the attachment of 

 the thoraco-abdominal muscles. The pleural region of the meta- 

 thorax is a narrow triangular space situated behind the lateral 

 portion of the metastemum and the posterior coxae. It is com- 

 posed of a narrow triangular episternum and epimeron. The 

 former (eps'".) is bounded in front by the metasternum, the 

 posterior thoracic spiracle and the base of the haltere, below by 

 the posterior coxal foramen, and behind by the epimeron. The 

 epimeron {ep".) is also bounded below by the coxal foramen and 



