ORAL LOBES 61 



edges of the labiiiiii-li\ popluiryngeal sclerite. By tlnir (■..Mtiac-iioii 

 they will widen the channel of the labium-hypophaiynx. 



The Dilators of the Lahrum-epiphm^ynx (fig. 29, di.L). These 

 form a series of shoit muscles attached to the anterioi- and pos- 

 terior walls of the lahniin-epipharynx. The size of the phaiyngeal 

 channel will be regulatt'd by these muscles. 



Fig. 29. Transverse section through the lower half of the haustellum, where the 

 hypopharynx {hp.) has become free from the labium. (Camera lucida draw- 

 ing.) 



di.l. Dilator muscles of the labrum-epipharynx. (/•. Trachea. Other lettering 



as in fig. 3. 



The Oral Lobes. 



The external structure of the oral lobes has already been 

 described. Their internal structure and histology will now be 

 given. 



The setigerous cuticle and the pseudo-tracheae lie on a hypo- 

 dermis of cubical cells (fig. 30, hy.). Beneath the hypodermis of 

 the aboral surface is another layer of cells containing a large 

 amount of dark pigment. Each of the large marginal sensory 

 bristles {g.s.) of the aboral surface has a fine channel running down 

 the whole length of the seta. This channel communicates with 

 the cavity of a pyriform mass of nerve-end cells {s.p.), consisting of 

 five or six cells. These masses of cells occupy a large part of the 

 interior of the oral lobes. As these gustatory bristles are exposed 

 and directed ventrally when the proboscis is retracted, they may 

 assist the fly in testing the nature of its food before extending its 

 proboscis. On the oral side of the oral lobes the nipple-like 



