PROBOSCIS OF PHORMIA REGINA — DETHIER 169 



the accessory retractors of the rostrum. The principal branch then 

 continues ventrally to the labelKim. It innervates the retractors of 

 the furca, the retractors of the paraphyses, and the tranverse muscle 

 of the haustellum. It is the principal sensory trunk from the labellum. 



The second principal branch of the labial nerve, the lateral branch, 

 immediately subdivides. The more dorsal branch sends a small twig 

 to the accessory retractors of the rostrum, then continues up into 

 the cranial cavity to the tracheae and fat body. 



The remaining branch passes anterior to the retractors of the 

 rostrum, then curves posterior to the flexors of the haustellum and 

 the accessory retractors of the rostrum, thence anterior to all three 

 muscles and ventrally down the proboscis. Near the proximal cornua 

 of the fulcrum it gives off a small branch to the retractors of the 

 rostrum. Shortly thereafter it subdivides to send sensory fibers to 

 the maxillary palpi and motor fibers to the extensors of the haustellum 

 and the adductors of the apodemes. To this point at least the main 

 nerve is demonstrated to be a compound maxillary-labial nerve. 



EXTENSION OF THE PROBOSCIS 



Available evidence suggests that extension of the rostrum is ac- 

 complished by distension of the large air sacs contained within it. 

 Lowne (1892) stated that control of the passage of air from the 

 thoracic cavity is exercised by "... a slender rod of chitin in its 

 (the tracheal trunk) anterior wall, which closes the tube by pressing 

 against the jugum. This forms a valve capable of being opened by 

 a small bundle of muscle fibers, which arise from the front edge of 

 the gena; their contraction opens the valve and permits the passage 

 of air from the thoracic cavity into the tracheae of the proboscis." 

 In Phormia extension of the rostrum can be prevented by puncturing 

 the air sacs in question or by ligaturing the neck so that no air can 

 be passed into the head. However, the particular muscular arrange- 

 ment described by Lowne has not yet been observed and further in- 

 vestigation is necessary. If such muscles exist, it is probable that 

 they derive their innervation from branches of the labial nerve aris- 

 ing close to those which innervate the accessory retractors of the 

 rostrum. 



Extension of the more distal portions of the proboscis is accom- 

 plished by direct muscular action. The haustellum is extended by 

 contraction of the extensors of the haustellum and the adductors of 

 the apodemes. The oral disc is extended by contraction of the retrac- 

 tors of the paraphyses and the transverse muscles of the haustellum. 



