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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 409 



the scutellura. From the lateral region of the scutum a pro- 

 cess projects forwards and downwards, and articulates with 

 the posterior portion of the wing-base (the rnetapterygium). 

 The scutellum (sctl.) is a triangular pocket-shaped sclerite 

 which overhangs the postscutellum and the base of the abdo- 

 men. The posterior surface of the thorax is chiefly composed 

 of the large postscutellum. This is made up of three pieces, 

 a median escutcheon-shaped plate (mpsc.) strongly convex to 

 the exterior, and two convex lateral plates (lp.sc.). The lateral 

 plates are bounded below by the metasternum and spiracles, 

 and anteriorly by the pleura! region of the mesothorax. 



The mesosternum is a sclerite of considerable size and forms 

 the keel of the thorax. It consists of a median ventral por- 

 tion (ms.) which is produced laterally to form two large 

 lateral plates (lp.). The median portion is bounded in front 

 by the prosternum and the foramina of the anterior coxaa, and 

 behind by the median coxal foramina. A short distance 

 behind the anterior end a depression in the mid-ventral line 

 extending to the posterior edge indicates a median inflection 

 forming the entothorax. The lateral regions of the posterior 

 margins of the mesosternum are inflected on each side to form 

 the entopleura. The lateral plates of the mesosternum form 

 the whole of the anterior portion of the pleura! region ; each 

 is bounded in front by the humerus, spiracle, and prothoracic 

 episternum, and above by the dorso-pleural membrane, and 

 behind by the mesopleural membrane. The ventral side of 

 the lateral plate is continuous in front with the median plate 

 of the mesosternum, and behind is united by means of a 

 suture. The remaining portion of the mesopleural region is 

 made up of the episternum, epimeron, and two small sclerites 

 connected with the wing-base the parapteron and costa. 

 The episternum (eps.") is situated behind the mesopleural 

 membrane and below the alar membrane, below and behind 

 it is bounded by the epimeron. Its surf ace is marked by two 

 convexities, the ampullas, the upper of the two corresponding 

 to Lowne's great ampulla of the blowfly. The dorsal side of 



