136 THE INTEGUMENT A L SKELETON OE THE LMAGO. 



margin. The edges of the cleft are continuous with the 

 margins of the groove in the theca, and are united as far as the 

 edge of the disc by a remarkable bead and channel joint — the 

 thick edge of one lobe of the disc fits into a corresponding 

 cylindrical channel in the other (Fig. 31). The distal 

 surface of the disc is channeled by the well-known 'false 

 tracheae.' In the centre there is a deep longitudinal fissure, 

 which extends into the tubular mouth situated between the 



Fig. 28. — /, The fulcrum : a, proximal, and b, distal cornua ; c, median raphe ; i/, 

 chitinous plate, uniting the clypeus with the epistome ; 2, side view of the same : 

 ph, pharyngeal tube ; //, lateral plate ; j, transverse section through the 

 rostrum : ///, pharynx ; /;//, mesolabrum ; ap, apodeme of the prelabrum ; iiix^ 

 maxilla ; ///, muscles ; ii\ trachea; ; 7, a vertical section of the mouth : /<■, hyoid ; 

 //, prepharyngeal tube ; c, inner plate of the prelabrum ; /, ligula ; sd, salivary 

 (lingual) duct ; st^ stomal or hypoglossal plate ; d s, discal sclerite ; //, nodulus : 

 ch, cochleariform sclerite ; s s, sesamoid sclerite ; /, the furca. 



labrum and the theca. The proximal surface of the sucker is 

 convex and covered by setae ; those near its margin are ver}" 

 long and form a fringe. 



Morphology of the Rostrum. — Transverse sections of the 

 rostrum show that it consists of two parts (Fig. 28, j), a median 

 anterior or dorsal tube (ml), which is continuous with the 

 cavity of ihe labrum — this represents a mesolabrum, and 



