CHAPTER IV 



THE INTERNAL STRUCTURE OF THE HEAD AND PROBOSCIS 



OF MUSCA DOMESTICA 



The exo-skeleton and tracheal system of the head and pro- 

 boscis have already been described. An account will now be given 

 of the internal structure and musculature of the head and pharynx 

 and also of the oral lobes. 



The posterior region of the head (fig. 3) not occupied by 

 tracheal sacs is usually filled up with small multinucleate fat-cells 

 (f-c), which are also occasionally fijund on the proboscis. The 

 frontal sac or ptilinum (Pt.) fills up the anterior portion of the 

 head not occupied by the air-sacs. Its crescentic opening, the 

 lunule, has already been described. It is attached to the wall of 

 the cephalic capsule by muscles which vary considerably in the 

 extent of their development. In recently emerged flies the muscle- 

 supply of the ptilinum is considerable, as they have served to 

 retract the sac after it has been inflated to assist the exclusion of 

 the imago, but in older specimens it becomes less. The walls 

 of the ptilinum are muscular and lined by a chitinous intima 

 covered with small broad spines. 



The Musculature of the Proboscis. 



The chief muscles controlling the movements of tlie pharynx 

 and proboscis are as follows : 



The Dilators of the Pharynx (Figs. 3, 28, d.pk). This pair of 

 muscles occupies the interior of the fulcrum. Each muscle is 

 attached to the antero-lateral regions of the fulcrum and inserted 

 into the dorsal plate of the pharynx (r:p.). These muscles are the 

 chief agents in pumping the liquid food into the oesophagus, and 

 in drawing it up through the pharyngeal tube. 



