84 THE LARVA OF THE BLOW- FLY. 



the ridges which surround it become the other tarsal joints, 

 except the outermost, which is the rudiment of the femoro- 

 tibial part of the leg. The most external portion of the epiblast 

 becomes the sternal region of the thorax. 



The leg discs, in an early stage of development, exhibit only 

 the central papillae, and the number of concentric rings increases 

 as development progresses. 



The superior prothoracic disc (Fig. ii, j) is formed by an 

 involution of the hypoblast at the base of the anterior spiracle ; 

 it appears first towards the end of larval life, and, according 

 to Weismann, it is the only thoracic disc not present in the 



Fig. i6.— Group of tracheal discs from an adult larva : Ti', wing disc ; //, superior, 

 and /, inferior metathoracic discs ; tr, main trachea ; tr , modified tracheal 

 vessel, seen with an inch-objective. 



young larva. At the end of the resting stage, or early in the 

 pupa stage, the mesoblast of this disc surrounds the main 

 tracheal trunk, and unites it with the mesoblast of the superior 

 mesothoracic disc. 



The superior mesothoracic, or wing, disc (Figs. 14, j, and 16) 

 lies above and in front of the great cephalic disc. It is the 

 largest of all the imaginal discs. Its epiblast is corrugated, so 

 that it presents numerous concentric lines when seen in optical 

 section. The wing projects from the surface of the disc as a 

 conical papilla. 



