54 MEDICAL ENTOMOLOGY 



The articulations between the labium and the labella are strong and 

 well defined. The anterior articulation, between the discal sclerite and 

 the labial gutter, is a close one, and permits of only a very limited range 

 of movement, while the posterior one permits of a wider, though much 

 more defined, excursion than that of Musca or Philaematomyia. 



The discal sclerite (Plate XII, fig. 3 and Plate XIII, fig. 7) is 

 completely altered in form from the condition seen in Musca. It now 

 forms a collar around the prestomum, articulating with the end of the 

 labial gutter behind, and bearing the teeth at the distal end. The parts 

 representing the labellar rods and the axial apophysis are well distin- 

 guished from one another. The labellar rods are flattened plates, parallel 

 with one another, but concave on their opposing surfaces, arranged 

 vertically in the long axis of the proboscis, and narrower at the proximal 

 end than distally. The distal margins are concave, to correspond with 

 the curvature of the ridge formed by the united bases of the teeth, to 

 which they are attached on each side by a strong band of fibrous tissue. 

 The narrow proximal ends are moulded to fit the end of the labial gut- 

 ter ; the dorsal posterior angle of each is hollowed out to an L-shaped 

 notch, into which the lateral rod of the gutter, produced distally and 

 separated from the median ventral portion, is fitted. The middle portion 

 of the gutter is also produced to a blunt point on the ventral aspect, and 

 this fits between the two plates of the sclerite, the whole producing an 

 articulation which can permit of only a limited amount of movement. 



The axial apophysis is a thick rounded mass of chitin, situated be- 

 tween and behind the labellar rods, to which it is closely fused ; it thus 

 binds them together and forms the floor of the groove. The dorsal 

 side of the groove is open, and is continuous with the groove on the 

 dorsal surface of the labium. 



The posterior joint (Plate XII, fig. 1, and Plate XIII, fig. 7), be- 

 tween the fork of the mentum and the furca, is the one on which the 

 most important muscles of the cutting apparatus act. The parts bear 

 little superficial resemblance to those of Philaematomyia, for they are 

 moulded to form a strong and compact joint which is capable of bear- 

 ing a considerable strain. The distal end of the mentum is cut away 

 obliquely upwards and backwards, so that the posterior surface extends 

 considerably beyond the anterior. On the ventral side there are two 

 thick wedge-shaped rods, the broad ends of which are directed towards 

 the labella ; these are the homologues of the proximal portions of the fork 

 of the mentum. The furca is a thick chitinous arch, embracing the 

 posterior and lateral aspects of the labella, and so moulded as to 



