(94) 



534 0. GORDON HEWITT. 



three pairs of major cephalic discs, which will form the 

 cephalic capsule. 



In addition to these two other pairs of cephalic discs are 

 found. A pair of small flask-shaped maxillary rudiments are 

 situated one at the base of each of the oral lobes; a second 

 pair of imaginal discs, similar in shape to the maxillary discs, 

 is found adjacent to the hypostomal sclerite ; the latter, I 

 believe, are the labial rudiments, and will form almost the 

 whole of the proboscis of the fly. 



The thoracic discs. In M. domestica there are five 

 pairs of thoracic discs. The pro-thoracic imaginal discs (figs. 

 24 and 28, pr. d.) are attached to the anterior end of the gan- 

 glion and slope obliquely forwards; the distal end of each is 

 attached to the body-wall on the ventral side between seg- 

 ments three and four. These discs develop into the pro- 

 thoracic 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 

 meso-thoracic and meta-thoracic segments. Arising from the 

 sides of the ganglion immediately behind the attachment of 

 the pro-thoracic rudiment are the imaginal rudiments of the 

 meso-thoracic legs and sternal region (v. 7ns.) ; 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 meso- 

 thoracic imaginal discs, from which originate the mesonotal 

 region and the wings, may be termed the alar or wing discs. 

 They form a pair of flattened pyriforrn saca (fig. 22, d.ms.), 

 lying one on each side of the ventral side of the fifth segment 

 and slightly external to the lateral tracheal trunk (fig. 28, 

 d.ms.), to a ventral branch of which each is attached. The 

 meta-thoracic discs consist of two pairs of small pyriform 

 masses (fig. 22) lying 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 meta-thoracic leg 

 and sternal region ; in the blow-fly and Volucella it is interest- 

 ing to note that this pair of imaginal discs is situated further 



