50 MEDICAL ENTOMOLOGY 



separated from one another, and are also broadened out. The last pair, 

 which are situated opposite one another, are enlarged into broad leaf-like 

 blades, set one on each side of the termination of the channel. 



Philaematomyia insignis, Austen (Plate X, fig. 1), represents a much 



more advanced condition, although the outward form of the proboscis 



remains practically the same. There are four teeth 



on each side (Plate VIII, fig. 8), of the same shape 



as those of gurnei, but larger and stouter, and more 



closely approximated at their bases. The two middle teeth are rather 



larger than the lateral ones, but on account of the curvature of the 



recess into which they fit the apices of all are on the same level. 



Between the teeth there is a well-developed and complex interdental 

 armature. This consists of two sets of processes between each two 

 adjacent teeth. Each set consists of five or six small leaf-like blades 

 with deeply serrated edges, arising from a common stalk at different 

 levels and partly superimposed upon one another. The two sets of 

 each pair diverge distally so as to enclose the termination of one of the 

 pseudotracheal channels. 



The discal sclerite (Plate IX, fig. 6) in this species is very much 

 modified from that of Musca, and one would hardly be able to recognize 

 in them the same structure were it not for the intermediate forms. The 

 two parts are quite distinct. The labellar rods are stout pigmented 

 bars of chitin, flattened from side to side, and almost parallel to one 

 another, but diverging a little posteriorly. About the middle point of 

 each there is a prominent tubercle on the ventral surface; the tubercles 

 of the two sides articulate with the end of the labial gutter, which is 

 moulded to fit them. At the distal end the rods are dilated to form 

 shallow receptacles, in which the bases of the teeth rest (Plate VIII, 

 fig. 8). The axial apophysis is a thick shield-shaped piece of chitin, 

 situated between and behind the labellar rods, binding them firmly to- 

 gether. 



The other structures are correspondingly increased in strength. The 

 labial gutter (Plate X, figs. 1 and 2) is thicker than in the last species, 

 and its articulation with the discal sclerite is more definite. The 

 posterior joint, between the fork of the mentum and the furca, is stronger, 

 and the furca itself thicker and rounder than in Musca. 



The mechanism of the proboscis in these flies appears to be as follows. 

 The rostrum and the haustellum are extended exactly as in Musca. The 

 labella are erected by the contraction of the anterior set of muscles 

 attached to the proximal ends of the labellar rods, until, in the case at 



