STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 533 
of M. domestica 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 imaginal 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- 
olion (fig. 24, m.c.d.); the cone, however, is not complete. 
The outer sheath of each of these major cephalic imaginal 
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 dise (fig. 27, 0. 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.s.), 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, and internal-to, the optic discs. Hach 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 
