STRUCTUREjDEVELOPMENTjAND BIONOMICS OP HOUSE-FLY. 437 



sides of the occipital foramen. They are inserted into the 

 base of the labral apodeme (ap.), and serve to flex the 

 haustellum on to the anterior face of the rostrum. 



The Extensors of the Haustellum (ex.h.). — Each of 

 these muscles arises from the distal cornu of the fulcrum, and 

 is inserted into the head of the labral apodeme. 



The Accessory Flexors of the Haustellum {a.f.li.) 

 are attached to the lower (distal) anterior margiu of the ful- 

 crunij and inserted with the extensors into the head of the 

 labral apodeme. 



The Flexors of the Labium-epipharynx [f.l.). — 

 These muscles have their origin on the anterior and upper 

 edge of the fulcrum, and are inserted into the proximal end 

 of the labium-epipharynx. The first pair of the last three 

 sets of muscles serve to extend the haustellum in the exten- 

 sion 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.ni.) 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 [r.fu.). — 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 



VOL. 51, PART 8. NEW SERIES. 33 



