(44) 



438 0. GORDON HEWITT. 



of the f urea, their contraction causing the divergence of the 

 discal sclerites, and the consequent opening of the oral pit. 



The Dilators of the Labium-hypopharynx (di.L). 

 These fan-shaped muscles arise in the middle region of 

 the theca on either side the median line, and diverging 

 are inserted in the lateral edges of the labium-hypopharyngeal 

 sclerite. By their contraction they will widen the channel of 

 the labium-hypopharynx. 



The Dilators of the Labium-epipharynx (di.L) 

 These form a series of short muscles attached to the anterior 

 and posterior walls of the labium-epipharynx. The size of 

 the pharyngeal channel will be regulated by these muscles. 



The Oral Lobes. The external structure of the oral lobes 

 has already been described. Their internal structure and 

 histology will be given here, as it seemed preferable to do so 

 rather than postpone it to a future communication. 



The setigerous cuticle and the ps-eudo-tracheaa lie on a 

 hypodermis of cubical cells (fig, 18, hy.). Beneath thehypo- 

 dermis of the aboral surface is another layer of cells contain- 

 ing 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 pro- 

 boscis. On the oral side of the oral lobes the nipple-like gus- 

 tatory papillae (figs 1 and 18, gp.) have already been descrided. 

 The aperture at the end of the papilla leads into a fine duct, 

 which ends in a pyriform sensory bulb (s.g.p.). The tracheae 

 (tr.) can be seen running through the cells, some of which con- 

 tain several nuclei, and from their appearance are probably 

 derived from the fat-body. No tracheal sacs could be found 

 either in the oral lobes or at their bases, but the annular 

 tracheae are continuous with those of the proboscis. The 



