46 ON THE ANATOMY OF THE FLY. 



joints with their apodSmes, which give insertion to two pairs of muscles 

 both arising from the pharynx. Plate II, Fig. 13, shows the points of origin 

 at c and d ; both are inserted into the free extremity of the apodeme ; they 

 are probably flexors of the second upon the first joint of the proboscis. 

 They act by drawing the head of the apodeme towards the dorsum 

 and pressing it forward ; as the apode'mes act upon the operculum 

 slightly behind the line of flexion, they press its superior extremity down- 

 wards and forwards, causing its anterior extremity to revolve in the 

 opposite direction. The whole of the interior of the operculum is lined 

 with epithelial cells containing bright red pigment. 



The canula consists below of the rnentum, a shield-like plate bearing a 

 pair of processes at its anterior extremity, which keep the lobes of the 

 proboscis closed by their elasticity ; a small triangular plate unites 

 them inferiorly. The whole superior surface of the mentum gives 

 origin to muscles. The largest pair being inserted into the terminal pro- 

 cesses serve to open the lips. A pair of thin muscular slips arising from 

 the margin of the occipital foramen is inserted into the posterior margin 

 of the mentum. The latter are retractors of the whole proboscis as well as 

 extensors of the second joint. The edges of the mentum are everywhere 

 united to the membranous sheath of the proboscis. 



Tracing the membranous covering of the canula from the edges 

 of the mentum, it is found to become chitinous in the groove on 

 its upper surface and round the anterior extremity of the mouth, 

 forming the base of the great lobes ; these parts represent the 

 labium or lower lip of which the last named are lateral appen- 

 dages. Perhaps the labium is the most difficult organ to under- 

 stand in the mouth of insects. It consists of five distinct parts, if 

 the tongue be considered as a portion of it, and these seem to be 

 very constant. They are 1 , the floor of the mouth united to the 

 upper side of the mentum ; 2, the tongue united to the posterior 

 extremity of the floor of the mouth, but more or less free in front ; 

 3, a plate attached to the anterior border of the floor of the mouth 

 in front of the mentum ; 4, a pair of palpiborne by the base of the 

 last part on its inferior surface which are turned downwards ; 5, a 

 pair of lobes turned upwards and borne by the extremity of the same 

 plate : these are the modified limbs which belong to the most an- 

 terior segment. All these parts may be distinguished in the labium 

 of the fly, except the palpi. Much confusion exists as to their 

 nomenclature especially between the last-named and the tongue, 

 both having been called the ligula I shall call them respectively 



