(93) 



STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HO USE-FLY. 533 



of M. doinestica are similar in some respects to those of the 

 blow-fly, as described by Lowne, there are several important 

 differences, chief of which is the position of the iinaginal discs 

 of the meta-thoracic legs. 



During the resting period of the larva the cephalic and 

 thoracic discs can be distinguished, but the abdominal discs 

 are small and not so obvious except in sections. 



The cephalic discs. The chief cephalic discs are con- 

 tained in what at first appears to be a pair of cone-shaped 

 structures in front of each of the cerebral lobes of the gan- 

 glion (fig. 24, m. c. d.) ; the cone, however, is not complete. 

 The outer sheath of each of these major cephalic iinaginal 

 rudiments is continued dorsally, and joins the cellular struc- 

 ture mentioned previously (see fig. 28), thus enclosing a 

 triangular space which is a portion of the ventral sinus. 

 These sheaths are continued anteriorly and are connected to 

 the pharyngeal mass, and it is through this connecting strand 

 of tissue that the discs are everted to form the greater part 

 of the head of the nymph. Immediately in front of the 

 cerebral lobe is the so-called optic disc (fig. 27, o. d.), which 

 in its earlier stages is cup-shaped, but later it assumes a 

 conical form, having a cup-shaped base adjacent to the cere? 

 bral lobe. The optic disc is connected to the cerebral lobe 

 laterally by a stalk of tissue, the optic stalk (0.5.), which 

 becomes hollow later, and it is through this stalk that the 

 optic ganglion and associated structures contained in the 

 cerebral lobe appear to evaginate when the final metamorphosis 

 and eversion of the imaginal rudiments takes place. The optic 

 discs form the whole of the lateral regions of the head of the 

 fly. The remaining portion of the head-capsule of the fly is 

 formed from two other pairs of imaginal rudiments, the 

 antennal and facial discs. The antennal disc (an. d.) lies in 

 front of, arid internal to, the optic discs. Each consists of an 

 elongate conical structure, in which at a later stage the 

 individual antennal joints can be distinguished. The facial 

 discs (f.d.) are anterior to the antennal discs and extend to 

 the anterior end of the conical structure containing these 



