20 THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



median line, their dorsal sides being curved. From the ventral 

 side of the pronotum a pair of chitinoiis apodemes project into 

 the thoracic cavity. The lateral regions of the pronotum are 

 in contact with the humeri Qiu.) and the prothoracic episterna. 

 The humeri are a pair of strongly convexed sclerites situated in 

 the antero-lateral region of the thorax. They are bounded above 

 by the prescutum of the mesothorax, internally and below by the 

 episterna of the pr-othorax, and externally by the lateral plate of 

 the mesostemum and the anterior thoracic spiracle. Its inner 

 concave surface serves for the attachment of the muscle of the 

 prothoracic coxa. The episterna {eps'.) (epitrochlear sclerites of 

 Lowne) are comparatively large sclerites forming the lateral 

 regions of the prothorax. They overhang the attachments of the 

 prothoracic limbs. The internal skeleton of the prothorax consists 

 of the two stout hollow apodemes — the hypotremata mentioned 

 previoush'. They arise from the postero-lateral edges of the pro- 

 stemum, and run obliquely across the ventral edge of the anterior 

 thoracic spiracle where the hypotreme divides, the posterior branch 

 runs up the posterior margin of the spiracle, between the lateral 

 plate of the mesosternum and the peritreme (the chitinous ring 

 surrounding the spiracle), the anterior branch fuses with the 

 prothoracic episternum. 



The Mesothorax. The notum of the mesothorax occupies the 

 whole of the dorsal side of the thorax. It is composed of the four 

 sclerites to which Audouin (1824) gave the names of prescutum, 

 scutum, scutellum, and postscutellum. The prescutum (pj^s.) forms 

 the anterior part of the dorsal region of the thorax. Its anterior 

 portion bends down almost vertically to unite with the pronotum. 

 The anterior edge of the prescutum is inflected after the pronotal 

 suture, and is reduced in the median line to a small bifurcating 

 process. The prescutum is bounded laterally by the humerus and 

 a membranous strip — the dorso-pleural membrane. The scutum 

 (sc.) is the largest of the mesonotal plates. It occupies the whole 

 of the median dorsal region of the thorax. Anteriorly it is bounded 

 by the prescutum, laterally by the alar membrane and the lateral 

 plate of the postscutellum, and posteriorly by the scutellum. From 

 the lateral region of the scutum a process projects forwards and 

 downwards and articulates with the posterior portion of the 



