18 THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



fulcrum is hinged. The fulcrum is therefore quadrilateral in sec- 

 tion at the upper proximal end, and trilateral at the lower distal 

 end. The basal portion (fig. 28, b.p.) forms the floor of the pharjTix ; 

 the roof of the pharynx is formed by another chitinous piece {r.p.) 

 with a median thickened raphe. This roof lies parallel with the 

 basal piece, and is fused with the sides of the fulcrum. On the 

 membranous wall of the pharynx, between the labium-hypopharjux 

 and the fulcrum, a small chitinous sclerite (fig. 3, k.) is developed, 

 which LoAvne terms the hyoid sclerite. It is U-shaped in section, 

 and serves to keep the lumen of the pharj-nx in this region 

 distended. 



The Thorax (fig. 6). 



As in all Diptera the possession of a single pair of wings has 

 resulted in the gi-eat development of the mesothorax at the expense 

 of the other thoracic segments, consequently the thorax is chiefly 

 made up of the sclerites composing the mesothorax. The prothorax 

 and metathorax compose very small portions on the anterior and 

 posterior faces respectively. Seen from above the thorax is oviform 

 with the blunt end anterior and slightly flattened. Three trans- 

 verse sutures on the dorsal side mark the limits of the prescutum, 

 scutum and scutellum of the mesothoracic segment; the meso- 

 thoracic scutellum forms the pointed posterior end, and slightly 

 overhangs the anterior end of the abdomen. 



The Prothorax. The prothoracic segment has been reduced to 

 such an extent that it is hopeless to attempt to homologise all the 

 separate sclerites with those of a typical thoracic segment. To 

 obtain a complete view of the prothorax it is necessary to examine 

 it from the anterior end after the removal of the head. The follow- 

 ing sclerites can then be recognised. The presternum is a median 

 ventral plate, quadrilateral in shape, having the anterior end rounded 

 and broader than the posterior end. It does not occupy the whole 

 of the prosternal area, but is bounded by the prosternal membrane. 

 Internally, a ridge runs to the posterior end of the presternum and 

 bifurcates, each ridge running to the posterior corners, to which 

 two strong processes (the hypotremata of Lowne) are attached. In 

 front of the prosternum there is a small saddle-shaped sclerite 

 which, on account of its position, may be called the interclavicle 



