IMAGINAL THORACIC DISCS 



149 



the jinterior vnd ot'thr gani^lion ;ui(l slope oblnjudy toi'wards ; the 

 distal end of each is attached to the body-wall on the ventral side 

 between segments three and four. These discs develop into the 

 prothoracic legs, and probably also into the much-reduced pro- 

 thoracic segment, as I was unable to discover any other rudiments 

 corresponding to the dorsal imaginal discs of the mesothijracic 

 and metathoracic segments. Arising from the sides of the 

 ganglion immediately behind the attachment of the prothoracic 

 rudiment are the imaginal rudiments of the mesothoracic legs 

 and sternal region {v.ms.) ; the distal stalks of this pair of imaginal 

 discs are attached to the body-wall at the posterior border of the 

 fourth segment. The dorsal mesothoracic imaginal discs, from 



Fig. 67. Transverse section of a portion of the ventriculus of mature larva. 

 (Camera lucida drawing.) 



e.v. Epithelial cell of ventriculus showing large active nucleus and striated peri- 

 pheral region of cell, fix. Probable gland cells. i.e. Group of imaginal 

 cells. 



which originate the mesonotal region and the wings, may be 

 termed the alar or wing discs. They form a pair of flattened 

 pyriform sacs (fig. 64, d.ms.), lying one on each side of the ventral 

 side of the fifth segment and slightly external to the lateral 

 tracheal trunk (fig. 66, d.ms.), to a ventral branch of which each is 

 attached. The metathoracic discs consist of two pairs of small 

 pyriform masses (fig. 64) h^ing immediately behind the alar discs 

 in the intersegmental line. They are attached to a ventral branch 

 of the lateral tracheal trunk. The anterior rudiment (v.mt.) is the 

 larger, and forms the imaginal metathoracic leg and sternal region ; 

 in the blow-fly and in Volucella it is interesting to note that 

 this pair of imaginal discs is situated further forward, and is in 



