THE EXO-SKELETON OF THE PROBOSCIS. 147 



integument of the proboscis will be more fully considered in 

 another section of this work. 



The Ligula (hypopharynx) is the only lancet-like mouth organ 

 in the Muscidse. It is situated in the groove of the pseudo- 

 labrum, and arises from its proximal end. 



It consists of a prolongation of the salivary (lingual) duct, 

 and in transverse sections, through the middle region of the 

 labrum and theca, the ligula exhibits a flange-like projection on 

 either side, which articulates with a groove in the side of the 

 labrum, so that the tube through which the food passes is 

 formed entirely by the ligula and the labrum (Fig. 29). 



Further back the two flanges unite on the dorsal surface of 

 the salivary tube, and the latter becomes separated from the 

 plate produced by the union of the flanges. This plate is 

 concave towards the mouth, and its edges unite with the edges 

 of the oral sclerites of the labrum and surround the posterior 

 or proximal portion of the mouth. This I term the ' pre- 

 pharyngeal tube.' Kraepelin names the distal end of the 

 prepharyngeal tube 'the true mouth orifice.' The labrum, or 

 upper lip, projects from it dorsally, and the ligula is a 

 prolongation of its ventral margin. 



If my contention is correct, that the proboscis consists mainly of 

 the first pair of maxillce, the true labium would lie at the base of 

 the ligula, and form the lower boundary of the true mouth 

 orifice. I think it probable that the flanges are the rudiments 

 of the paraglossae. 



In the larva (Fig. 7) I have already shown that the true 

 mouth lies between the labrum and the ligula, and is concealed 

 by the great maxillae. The flanges on the ligula of the imago 

 originate from two rows of cells, which first appear in the 

 root of the ligula in the larva, and lie in a true labium 

 developed from the second pair of maxillae (Fig. 5, lb). In 

 Fig. 6 it is easy to see that a retraction of the ligula (/) and 

 a great enlargement of the maxillae (mx) would give rise to a 

 condition similar to that seen in the fly larva, and precisely the 

 same relations appear to exist in the imago of the true flies and 

 in the Diptera generally. 



10 



