SCLERITES OF MESOTHORAX 21 



wing-base (the inetapterygium). The scutelhiin (sc</.) is a triangular 

 pocket-shaped sclerite which overhangs the postsciitelium and the 

 base of the abdomen. 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 (Ip.sc). The lateral 

 plates are bounded below by the metasternum and spiracles, and 

 anteriorly by the pleural region of the mesothorax. 



The uiesosternum is a sclerite of considerable size and forms 

 the keel of the thorax. It consists of a median ventral portion 

 (771.9.) which is produced laterally to form two large lateral plates 

 (Ip.). The median portion is ))oun(l('d in tVont by the prosternum 

 and the foramina of the anterior coxae, and behind by the median 

 coxal foramina. A short distance behind the anterior end a de- 

 pression 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 pleural 

 region ; each is bounded in front by the humerus, spiracle, and 

 prothoracic episternum, 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 

 surface is marked by two convexities, the ampullae, the upper of 

 the two corresponding to Lowne's great ampulla of the blow-fly. 

 The dorsal side of the episternum is intimately connected with the 

 sclerites' of the anterior portion of the wing- base. 



The epimeron (ep.) is a triangular sclerite, and is bounded 



1 In this account the individual sclerites which compose the wing-base will not 

 be described. Lowne has described them at gi-eat length for the blow-fly, and 

 although the wing-base sclerites of ill. domestica differ slightly in shape from those 

 of Calliphom, Lowne's description of the relations hold good for the fomier insect. 



