148 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



The Hyoid Sclerite (Fig. 28, h). The last sclerite which I have to 

 describe in the proboscis of the Blow-fly I term the hyoid. It is 

 developed in the wall of the alimentary tube, between its pre- 

 pharyngeal portion and the pharynx. It is U-shaped, with its 

 convexity backwards, and it keeps this section of the alimentary 

 tube open when the haustellum is flexed on the rostrum. 



c. The Proboscis of the Immature Imago. 



When the imago first emerges from the pupa case, the 

 proboscis is immovable, and lies on the ventral surface of 

 the thorax between the legs, like the proboscis of an hemi- 

 pterous insect. 



The rostrum at this period consists of two very distinct parts, 

 a proximal portion in front of, and a distal portion behind, the 

 insertion of the palpi. 



The proximal portion of the rostrum (Fig. 25, a b) closes the 

 peristomal cleft of the head capsule, and consists of a central 

 part, the epistome clypeus and metalabrum, and of two lateral 

 parts, the maxillary folds (Mihi). The maxillary folds are 

 separated from the metalabrum by two deep inflections of the 

 integument, which form the lateral plates of the fulcrum. 



The distal portion consists of the mesolabrum, continuous 

 with the metalabrum, and of two large sub-cylindrical palpi- 

 gers, covering and concealing a portion of the theca (Fig. 



25). 



The prelabrum is seen at the distal extremity of the meso- 

 labrum, and lies chiefly upon the lobes of the oral sucker. 

 Between these lobes and the theca there is a distinct ovoid 

 swelling, from the wall of which the body of the furca is 

 developed. I shall term this ovoid swelling the ' discophore ' 

 (Figs. 25, d and 32). A second smaller swelling intervenes 

 between the pre- and mesolabrum ; it is apparently nothing 

 more than a protuberance of the syndesmosis at the base of the 

 prelabrum. 



The ventral (at this period upper) surface of the proboscis is 

 much shortened by two deep infoldings of the rostrum, whilst 



