164 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



MUSCLES ORIGINATING IN THE ROSTRUM 



Extensors of the haustellum (figs, i, 2, 3 ExHst). — Three distinct 

 bundles of fibers comprise this short, stout muscle which originates 

 on the distal cornua and adjacent regions of the fulcrum and is in- 

 serted into the outer sides of the heads of the apodemes. When these 

 muscles contract, they force the apodemes downward away from the 

 head and cause the haustellum to extend. They are innervated by 

 small branches leaving the labial nerve in the region of the nerve of 

 the palpus. 



Adductors of the apodemes (figs, i, 2, 3, Ad A). — These muscles, 

 consisting of two bundles each, originate on the anterior distal margins 

 of the lateral plates of the fulcrum. They are inserted into the inner 

 sides of the heads of the apodemes. By their contraction they draw 

 the heads of the apodemes toward the median line. In so doing they 

 cause the haustellum to be extended. Thus, together with the exten- 

 sors of the haustellum, they work to extend this section of the probos- 

 cis. The nerves serving these muscles are small fibers leaving the 

 labial nerve in the region of the branch to the palpus. 



Flexors of the labriim (figs, i, 2, 3, FlxLm). — These long, flat 

 muscles have their origins on the distal edge of the anterior arch of 

 the fulcrum and from the adjacent areas of the lateral plates. They 

 are inserted into the sides of the lateral cornua of the labrum in close 

 proximity to the articulations of the apodemes. They are innervated 

 by the labral nerve. 



Gracilis muscles (fig. 3, Gr). — These thin, delicate muscles have 

 their origins at the bases of the proximal cornua of the fulcrum. 

 They are inserted into the anterior wall of the salivary pump. They 

 control the flow of saliva into the salivary canal of the hypopharynx. 



Dilators of the cibarial pump (fig. i, DChP^). — According to 

 Graham-Smith these powerful muscles (which he termed pharyngeal 

 muscles) arise from the internal surfaces of the anterior arch and 

 lateral plates of the fulcrum. In Fhormia there appear to be three 

 distinct sets of muscles. Two of these {DChP^) originate at a flexure 

 in the outside surface of the lateral plate in the region of the anterior 

 arch. The remaining massive muscle originates on the internal sur- 

 faces of the anterior arch and lateral plates. All are inserted into the 

 dorsal plate of the oesophagus. These are the muscles the action of 

 which pumps fluid into the oesophagus. They act by drawing the 

 dorsal plate of the oesophagus away from the ventral plate of the ful- 

 crum. They are innervated by both branches of the labrofrontal 

 nerve. 



