60 INTERNAL STRUCTURE OF HEAD AND PROBOSCIS 



{ap.) and serve to flex the haustelluiu on to the anterior face of 

 the rostrum. 



The Extensors of the Hamtellum (ex.k). Each of these 

 muscles arises from the distal cormi of the fulcrum, and is inserted 

 into the head of the labral apodeme. 



The Accessory Flexors of the Haustellum {a fa.) are attached 

 to the lower (distal) anterior margin of the fulcrum, and inserted 

 with the extensors into the head of the labral apodeme. 



The Flexors of the Labrum-epipharynx (ft.). These muscles 

 have their origin on the anterior and upper edge of the fulcrum, 

 and are inserted into the proximal end of the labrum-epi- 

 phar\Tix. 



The first pair of the last three sets of muscles serve to extend 

 the haustellum in the extension of the proboscis, and the remaining 

 two pairs assist in the retraction of the proboscis by flexing the 

 haustellum on to the rostrum. 



A pair of very fine muscles (s.m.) have their origin at the base 

 of and internal to the posterior cornua of the fulcrum. They are 

 inserted into the dorsal side of a small valve (s.v.) on the common 

 salivary duct which regulates the flow of the secretion of the 

 lingual salivary glands. 



The muscles of the haustellum are : 



The Retractors of the Furca (rfu.). A pair of muscles having 

 their origin on the upper part of the theca. Each is inserted 

 along the upper proximal half of the lateral process of the furca. 

 When the muscles contract the lateral processes of the furca, 

 which, in a state of repose, are brought together by the elasticity 

 of the ventral cornua of the theca, are diverged, and thus cause 

 the divergence and opening of the oral lobes. 



The Retractors of the Discal Sclerites (r.d.s.). These muscles 

 have their origin on the lateral edges of the upper part of the 

 theca, and are inserted upon the sides of the discal sclerites. They 

 work together with the retractors of the furca, their contraction 

 causing the divergence of the discal sclerites, and the consequent 

 opening of the oral pit. 



The Dilators of the Lahium-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 



