THE EXO-SKELETON OE THE PROBOSCIS. 



135 



The Clypeus (Figs. 23,5 c, and 25, h) forms the external dorsal 

 portion of the fulcrum, or cephalo-pharyngeal skeleton. This 

 supports the haustellum at its distal extremity. 



The Palpigerous Scales and Palpi, — On each side of the rostrum, 

 near its apex, there is a convex scale (PI. VI., ps), which sup- 

 ports a single-jointed clavate palpus — the maxillary palpus — at 

 its inner proximal angle. These scales are united in many of 

 the Diptera by a curved plate of chitin, which embraces the 

 posterior surface of the rostrum. 



Sesamoid Selerites. — I apply this term to two small nodulated 

 fusiform rods of chitin close to the apex of the rostrum on its 



h 



Fig. 27. — A portion of the Proboscis of the Blow-fiy, showing the relations of the 

 prelabrum with the theca ligula and pharynx : ap, apodeme of the labrum ; 

 ///, the pharynx ; Iiy, hyoid sclerite ; ////, prepharyngeal tube ; ;' r, part of the 

 rostrum ; ///, the prelabrum ; lig, the ligula ; // // h, the haustellum ; ss, sesamoid 

 sclerite ; tk s, thyroid sclerite ; m m, muscles ; /r, trachea ; /d, lingual, and sd, 

 salivary ducts ; sv, salivary valve. 



ventral aspect. They give insertion to the long retractor 

 muscles of the rostrum (Fig. 27, ss). 



The Haustellum, or middle joint of the proboscis, consists of 

 two valves, a large ventral or posterior valve, which I shall 

 term the theca, and a narrow styliform dorsal or anterior valve, 

 the prelabrum. 



The prelabrum lies in a groove on the anterior face of the 

 theca, the edges of which overlap it. The theca ends distally 

 in the great two-lobed oral disc, or sucker. 



The Oral Sucker is a fleshy oval disc, deeply cleft at its anterior 



